Network Working Group                                      Manav Bhatia
Internet Draft                                      Riverstone Networks
Expires: January 2006                                    Vishwas Manral
Informational                                              SiNett Corp.
                                                         Yasuhiro Ohara
                                                        Keio University

                   IS-IS and OSPF Difference Discussions

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Copyright Notice

   Copyright (C) The Internet Society (2005).


   The increasing popularity of IS-IS [IS-IS] and OSPF [OSPF] over
   the years has drawn significant attention to the relative merits and
   de-merits of one with respect to the other. This draft presents an
   elaborate comparison between the two routing protocols to explain how
   the features and functionalities of one differs from the other.
   Wherever applicable the differences between OSPFv2 and OSPFv3[OSPFv3]
   have also been pointed out.

Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",

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   document are to be interpreted as described in RFC 2119 [KEYWORDS]

Table of Contents

   1. Terminologies..................................................3
   2. Acknowledgements...............................................4
   3. Evolution of the protocols.....................................4
   4. Interface Types Supported......................................5
      4.1 Support for NBMA Networks..................................5
      4.2 Point-to-Multipoint model..................................6
      4.3 Unnumbered broadcast.......................................7
   5. Encapsulation..................................................7
      5.1 IP Fragmentation...........................................8
      5.2 ATM Encapsulation..........................................8
   6. Designated Router (DR) concept.................................9
      6.1 DR election deterministic/non-deterministic................9
      6.2 Backup Designated Router/Intermediate System..............10
   7. Areas/Hierarchy...............................................10
   8. Checks on Hellos for adjacency formation......................12
   9. Database Exchange and Flooding................................13
      9.1 Initial Database Exchange.................................14
      9.2 Asynchronous Flooding.....................................15
   10. Flushing LSA/LSP.............................................16
   11. SPF Calculation..............................................16
   12. Area Types...................................................17
      12.1 Area Partitions..........................................17
      12.2 Level 2 Partitions (Backbone Area Connectivity)..........18
      12.3 Injection of Level 2 Information.........................19
      12.4 Stub Area................................................20
      12.5 Not So Stub Area (NSSA)..................................20
   13. Architectural Values.........................................21
      13.1 Architectural Constants..................................21
      13.2 Synchronized Parameter Setting...........................21
   14. Virtual Links................................................22
   15. Packet Alignment/Extensibility...............................23
   16. MTU Limitations..............................................24
   17. Security/Authentication Issues...............................25
   18. IS-IS/OSPF for IPv6..........................................26
   19. Current Deployments..........................................28
   20. Metrics Size.................................................28
   21. Database Granularity.........................................29
   22. Separation of TE and topology information....................32
   23. Convergence and Scalability Issues...........................33
   24. Area Id Change Functionality.................................35
   25. Backward Compatibility.......................................35
   26. Hitless Restart Mechanisms...................................36
   27. Demand Circuits..............................................37
   28. IANA Considerations..........................................38
   29. References...................................................38

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   30. Author's Addresses...........................................40
   31. Appendix.....................................................41
   32. Intellectual Property Notice.................................42
   33. Disclaimer of Validity.......................................42
   34. Full Copyright Notice........................................43
   35. Acknowledgment...............................................43

1. Terminologies

   Since both these routing protocols originated in different standard
   bodies, IS-IS in ISO and OSPF in the IETF, there exists some
   difference in the terminologies used.


   End System - Host
   Intermediate System - Router
   Circuit - An adjacency on one link
   SNPA Address - Data link Address
   Protocol Data Unit (PDU) - Packet
   Designated Intermediate System (DIS) - Designated Router (DR)
   IS to IS Hello PDU (IIH) - Hello Packet
   Not Applicable - Backup Designated Router (BDR)
   Link State Packet(LSP) - Link State Advertisement (LSA)
   Link State Packet - Link State Update
   Complete Sequence Number Packet(CSNP) - Database Description packet
   Partial Sequence Number Packet(PSNP) - Link state ACK or Request
   Routing Domain - AS
   Level 2 Subdomain - Backbone Area
   Level 1 Area - Non Backbone Area
   Level 1/2 IIH PDU - Simple Hello Packet
   Level 1/2 LSP - No Distinction
   L1L2 router - ABR
   System ID - Router ID
   Link State Packet ID(LSPID) - Link State ID
   Pseudonode LSP - Network LSA

   Router LSAs, Summary LSAs, Network LSAs, ASBR Summaries, AS-external
   LSAs are equivalent of TLVs carried in LSPs in IS-IS. The difference
   is that each LSA has its own header whereas the TLVs share a common

   IS-IS Terms with no OSPF equivalent:
   TLV - Type-Length-Value tuple. These carry most of the information in
   IS-IS PDUs.

   OSPF Terms with no IS-IS equivalent:

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   Advertising Router - Router that originated the advertisement. In IS-
   IS, this is the LSP's originator.

   Backup Designated Router - Router which takes over in case the DR
   goes down. In IS-IS, there is no Backup DIS and the DIS election
   takes place again in case the former goes down or is no more

   Backbone Area - In IS-IS, L2 routers appear in all areas, but must
   all be interconnected to form a backbone (the L2 subdomain)..

2. Acknowledgements

   This document is a result of the extensive discussions in the diff-
   ospf-isis list and the following people have co-authored and
   contributed to this draft, either directly or indirectly:

   Danny McPherson, Jeff Learman, Jonathan Sadler, Radia Perlman, Philip
   Christian, J.J. Syed, Satish Dattari, Sina Mirtorabi, Nabendu Das,
   Russ White, Alex Zinin and Venkata Naidu.

3. Evolution of the protocols

   Both Integrated IS-IS and OSPF were specified in the latter part of
   the 1980s.

   In 1987 OSI adopted DECnet Phase V's routing algorithm with some
   modifications and named it IS-IS. Around 1988, the NSFnet deployed an
   IGP loosely based on an early draft of IS-IS. Around the same time,
   development on OSPF started which took most of the basic concepts
   from this early version of IS-IS but was designed to support only
   IPv4. In October 1989 the version 1 of OSPF was released as RFC 1131
   and around the same time in December 1990, Integrated IS-IS was
   released and published as RFC 1195.

   Version 2 of OSPF was first published in July 1991 as RFC 1247 and
   CISCO started shipping it. It released its implementation for Dual
   IS-IS in 1992. Till now numerous ISPs had deployed OSPF and very few
   IS-IS. In 1994 there were significant improvements done to CISCO's
   IOS implementation for in conjunction with support for Network Link
   Service Protocol (Novell's IPX protocol).

   These enhancements improved the performance, resilience and
   robustness of CISCO's implementation which made a lot of ISPs to
   shift to IS-IS.

   By 1995 most of the major ISPs had started deploying IS-IS. What
   helped this further was US government's interest in ISO CLNS suite,
   which was reflected in a requirement for CLNP routing support in the

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   NSFnet project by the NSF. Interest in Dual IS-IS continued to grow,
   and most ISPs that sprung up in Europe chose to deploy ISO standards
   based on IS-IS instead of OSPF.

   Unlike IS-IS which started as an ISO protocol, OSPF was inherently
   designed to support only IPv4 and was promoted by IETF as the
   referred IGP for IP networks. Additionally, because IS-IS support was
   not available on some major routers (noticeably Bay and 3com routers),
   OSPF automatically became the standard de-facto IGP for the
   reasonably large sized networks with multi-vendor platforms. An
   active IETF WG and evolving specifications also went a long way to
   help promote OSPF; and thus it started becoming more popular and more
   widely adopted compared to IS-IS [MARTEY].

   There has been no major standardization effort in the ITU for a while,
   so ISO 10589 and RFC 1195 still remain the authoritative complete
   standards for IS-IS. The IETF IS-IS WG has been opened recently which
   is now working on standardizing newer applications like MPLS, Traffic
   Engineering, IPv6, etc for IS-IS.

   To summarize, both the protocols have prevailed through the test of
   time and have established themselves as the IGPs of choice for ISPs.
   New extensions such as, MPLS TE, IPv6, have been deployed over the
   past 3 years, and with active working groups for either protocol in
   IETF, they continue to evolve in lock-step fashion.

4. Interface Types Supported

   OSPF models networks as
          - Broadcast links
          - Point to Point (P2P)
          - Point to Multi-Point (P2MP)
          - Non-Broadcast multi-access Networks (NBMA)

   IS-IS models networks as
          - P2P
          - Broadcast
          - Unnumbered Broadcast

   The key differences are the way OSPF provides support for NBMA
   networks and inherent protocol support for unnumbered broadcast by

4.1 Support for NBMA Networks

   IS-IS has no direct support for connecting ISs over a NBMA network
   and it must be modeled as a LAN or treated as a set of P2P links.
   Modeling it as the latter involves a lot of configuration and if full

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   connectivity is not configured, multiple hops might be required for
   traversing the NBMA cloud.

   Experience with ATM LAN emulation has proven un-scalable and
   insufficiently reliable because of the single point where replication
   takes place to emulate multicast.

   The best alternative for IS-IS is thus to treat each PVC as a point-
   to-point link. All PVC failures are handled by the protocol since
   each PVC is visible to the protocol. IS-IS mesh groups [MESH] may be
   used to address the scaling issues which may result from redundant
   flooding in the highly meshed environments.

   In OSPF there is a "NBMA mode" in the original specification which
   makes the protocol aware that it is on a NBMA network.

   Neighbours are discovered initially through configuration which is
   restricted to the ones eligible for the DR election. To make
   administration easier and to reduce the HELLO traffic, most of the
   other routers attached to the NBMA subnet are assigned a router
   priority of zero. It thus involves quite a bit of administration
   overhead and is prone to mis-configuration. Also the network will
   malfunction if one of the nodes loses its link to the DR.

   In this mode, each node in the NBMA must have a PVC to the DR and BDR.
   Since adjacencies between non-DR nodes is not mandated, the order of
   the number of adjacencies is O(2n), rather than O(n^2) as required
   when running OSPF without NBMA mode.

   NBMA networks are thus only as robust and reliable as the underlying
   data-link service. If for example, a PVC fails or is mis-configured
   or if an SVC cannot be established, due to capacity or policy reasons,
   routing over NBMA subnet will fail. And, unfortunately, often the
   reason for the failure will not be immediately obvious to the network

   The P2MP can be applied to rectify these problems, although at some
   loss of efficiency.

4.2 Point-to-Multipoint model

   This model can be used on any data link technology that the NBMA
   model can be used on. In addition, the P2MP model doesn't require all
   the participating routers to be able to communicate directly to model
   a partial PVC mesh as a single P2MP networks. Dropping the full mesh
   requirement also allows the modeling of more exotic data link
   technologies, such as packet radio, as P2MP networks [Moy].

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   So if an Operating system can't support virtual interfaces or if
   there's too much overhead involved in generating separate sub
   interfaces to each of the 500 ATM circuits then P2MP is good and can
   be handy that way.

   However, when operating a full mesh Frame Relay or ATM network in
   P2MP mode, the work involved in neighbor maintenance, flooding, and
   database representation increases as O(n^2), where n is the number of
   OSPF routers attached to the subnet, instead of O(n)behavior that can
   be achieved with the original NBMA model.

4.3 Unnumbered broadcast

   IS-IS supports unnumbered broadcast interfaces; however, most
   implementations do not.  The protocol provides all necessary routing
   information without the aid of ARP [ARP], but doing this requires
   that each FIB entry contain a next-hop (circuit, SNAP address) pair
   for each path to a destination, and many routers are designed with
   FIB entries that contain only next-hop IP addresses instead, to
   reduce the size of the FIB and perhaps as a simplification.

   For this reason, many implementations won't interoperate with an
   unnumbered broadcast interface, and won't interoperate with an
   implementation that doesn't support ARP.

5. Encapsulation

   IS-IS runs directly over the data link alongside IP. On Ethernet, IS-
   IS packets are always 802.3 frames, with LSAP value 0xFEFE while IP
   packets are either Ethernet II frames or SNAP frames identified with
   the protocol number 0x800. OSPF runs over IP as protocol number 89.

   IS-IS runs directly over layer 2 and hence

   - cannot support virtual links unless some explicit tunneling is

   - packets are intentionally kept small so that they don't require
   hop-by-hop fragmentation

   - uses ATM/SNAP encapsulation on ATM but there are hacks to make it
   use VcMux encapsulation

   - some operating systems that support IP networking have been
   implemented to differentiate Layer 3 packets in kernel. Such OSs
   require a lot of kernel modifications to support IS-IS for IP routing.

   - can never be routed beyond the immediate next hop and hence
   shielded from IP spoofing and similar Denial of Service attacks.

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   - need to provide code points of access for each data link protocol
   types (Frame Relay, Ethernet, ATM, PPP [PPP], etc.)

   - don't need to rely on network layer protocols (like ARP) to
   communicate with the neighboring systems. Some implementations
   however, do rely on ARP or static routing to communicate with
   neighbors on LAN.

   OSPF runs over IP and hence

   - can support virtual links
   - can use IP fragmentation services
   - can use VcMux encapsulation on ATM
   - if an OS already supports IP, no changes are necessary to support
   - can be routed to a destination multiple hops away and thus
   vulnerable to Denial of Service attacks and IP spoofing
   - transmitted with additional IP header information, thereby
   increasing some packet overhead

5.1 IP Fragmentation

   LSPs in IS-IS, unlike as in OSPF, are not regenerated hop-by-hop and
   so they must be small enough that they are guaranteed to be able to
   cross *any* media in the network and the value of the maxsized LSP
   should thus not be greater than the minimum link MTU size in the area.
   If a router has more than maxsized LSP bytes of information to
   advertise into IS-IS, then this originating router must fragment its
   LSP before flooding.

   One area of the concern regarding the scalability of the link state
   routing protocols is the flooding and it is believed that preventing
   fragmentation during flooding is the reason why IS-IS fragments only
   at the originating router.

   OSPF does not provide any explicit fragmentation/reassembly support.
   When fragmentation is necessary, IP fragmentation/reassembly is used.
   OSPF protocol packets have been designed so that large protocol
   packets can be generally be split into several smaller protocol

5.2 ATM Encapsulation

      OSPF can run over ATM using VcMux encapsulation (which essentially
   assumes that all the packets carried are IP) while IS-IS requires
   LLC/SNAP encapsulation where ATM layer can distinguish between
   multiple Layer 3 protocols over the same VC. The disadvantage of

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   using the LLC/SNAP encapsulation is that it has some additional bytes
   for the LLC-SNAP header which results in a packet size > 40 bytes.
   Thus a simple TCP ACK message of 40 bytes along with the LLC-SNAP
   header adds enough bytes so that a single TCP ACK won't fit into one
   ATM cell.

   Much bandwidth is thus wasted because now each TCP ACK requires 2 ATM
   cells. An IETF draft proposes a workaround to this issue in which
   both IS-IS and IP packets can be sent over an ATM VC using Vc Mux
   encapsulation by reading into the first byte of the L3 header to
   distinguish between IP and ISO family packets, such as  IS-IS, CLNS
   and ES-IS. However this did not gain popularity because of the demise
   of ATM cores in the largest ISPs (which were also among the few
   running IS-IS).

   [*] The first two fields in the IP header are the 4-bit version
   number and the 4-bit header length. The value of the first byte is
   normally 0x45. If there are IP header options attached to the IP
   header, the first byte can be between 0x46 and 0x4F. The first byte
   in an IS-IS packet is always 0x83. Thus by looking at the first byte
   of an incoming packet, the receiver can separate IP and IS-IS packets.
   Because of this feature one does not need to depend on the ATM layer
   anymore to help with the de-multiplexing. Routers an now send and
   receive both IS-IS and IP packets using Vc Mux encapsulation and thus
   avoid the ATM cell tax.  [*]

6. Designated Router (DR) concept

   The DR concept is used by both IS-IS and OSPF on the broadcast media
   to limit the amount of LS information exchanged between the routers
   on such media. It helps to reduce the number of adjacencies formed on
   broadcast media to O(n) instead of O(n^2), where n is the number of


   - DR election is deterministic
   - No concept of backup DIS
   - A new DIS is elected when the current goes down.


   - DR election is non-deterministic.
   - Elects DR and BDR to conduct flooding on a LAN.
   - All routers on the LAN are only synchronized with the DR and BDR.
   - DRship is sticky

6.1 DR election deterministic/non-deterministic

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   In IS-IS, deterministic DIS election makes the possibility of
   predicting the router that will be elected as DIS from the same set
   of routers. The router advertising the numerically highest priority
   wins, with numerically highest MAC address breaking the tie. In IS-IS,
   DIS can be pre-empted at any time by a router with higher priority
   coming alive.

   In OSPF, the DR election is sticky meaning that after a router has
   been elected, no other router can take over the position unless the
   original DR goes down. When a router comes up, it accepts the DR
   regardless of its own priority if a DR is already there. Otherwise
   the router itself becomes DR if it has the highest priority on the
   network. The above scheme makes it harder to predict the identity of
   the DR, but ensures that DR changes less often.

   The rationale behind this sticky nature of DRship in OSPF is that it
   is disruptive to have DR changes as DR keeps track of which nodes
   have acknowledged which link state information and it would require a
   lot of time and protocol messages for another router to take over in
   case the DR went down.

   Both the sticky and deterministic mechanisms of DR/DIS elections in
   OSPF and IS-IS can be modified to provide the functionality of the
   other with some simple modifications in the implementations.

6.2 Backup Designated Router/Intermediate System

   A backup DIS is redundant in IS-IS because all the routers are
   synchronized with each other and also because the shorter Hello
   interval used by the DIS allows for faster detection of failures and
   subsequent replacement of the DIS.

   The presence of BDR in OSPF makes the replacement of the DR
   transparent in case the DR goes down. All routers on the LAN are only
   adjacent and synchronized with DR and BDR; and backup DR is fully
   synchronized with the DR. Forming adjacencies with only the DR/BDR is
   done to reduce the complexity of data exchange and minimize flooding.

7. Areas/Hierarchy

   This is required primarily for scalability issues wherein
   instabilities inside one small section of the network are hidden from
   the rest of the network. This also helps in reducing the size of the
   routing tables, etc. Both the protocols establish a two level
   hierarchy among the areas.


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   - Divides the whole routing domain into small areas and uses logical
   hierarchy based on routing levels called Level 1 and Level 2

   - Level 1 routing is within the area and L2 is between the areas.

   - Original spec called for Level 1 routers to know only the topology
   inside their area and they were unaware of routers/destinations
   outside of their area. They simply forwarded all their traffic for
   outside their area to the nearest Level 2 router

   - Level 2 routers knew only the Level 2 topology and didn't know any
   topology inside the area. This forced strict hierarchal routing
   between the areas where all inter-area data traffic originating from
   one area followed a default route to the Level 2 sub-domain, where it
   was forwarded by L2 routing to the destination area.

   - This has now changed and a recent draft in IETF allows leaking L2
   information inside L1 for more optimal routing.

   - There was some work done in IS-IS for multi-level hierarchies but
   it wasn't all that useful and was dropped in between. The idea was
   that if the networks use IDRP as well along with IS-IS then the 2
   levels may not be enough.

   - IS-IS routers are associated with a single area and the whole
   router then belongs to that particular area.

   - Area boundaries intersect on links

   - can be extended to support higher levels of hierarchy based on the
   way routes are leaked in between the levels by setting the up/down
   bit, when routes are propagated down the hierarchy.


   - Divides the routing domain into regular areas and a backbone area
   that is designated as area and all packets going from one
   area to the other must traverse through this backbone.

   - The spec calls for the backbone to be contiguous and to be
   connected to all the areas through an ABR. There is however a
   provision to work with disconnected physically disparate backbone
   areas using virtual links [Refer to section 13 for more details]

   - Can be attached to multiple areas as its designed around links and
   uses a links based addressing scheme. It's the links which are
   assigned to the areas and not the routers themselves.

   - Areas intersect on routers.

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8. Checks on Hellos for adjacency formation

   The HELLO protocol is responsible for formation of adjacencies.
   Forming adjacencies is an integral part of link state routing
   protocols as all protocol packets other than hellos are flooded only
   over these adjacencies. The rules for formation of such adjacencies
   however differ between IS-IS, OSPF v2 and OSPF v3. The main points
   are: -


   Besides the basic checks to verify the integrity of the packet, IS-IS
   has a few checks to verify before formation of adjacencies when
   receiving hellos.

   - The IS-IS protocol allows multiple area-address to be configured on
   a router. During the hello exchange the adjacency is formed only if
   atleast one of the area address matches. The advantage of having
   multiple areas is given in section 22. However Level 2 only
   adjacencies can be formed even if the area addresses are not matching.

   - Besides to prevent the LSP's and CSNP's being dropped due to
   different values for originatingLSPBufferSize and
   ReceiveLSPBufferSize, all HELLOs are padded till the adjacency comes
   up again. This check verifies consistent settings between the
   adjacent routers. This is however not a sufficient check.

   - Adjacencies are formed without regard to interface addressing or
   asymmetric in HOLD timer values. Values of HELLO interval are not
   sent in HELLO packets. While the IS-IS protocol provides sufficient
   routing information for relaying packets between adjacent routers,
   many implementations nonetheless require ARP support to do this.
   These implementations typically refuse to form an adjacency unless
   the neighbour interface IP address is on the local interface's IP

   - IS-IS can carry addressing information of different protocols inT
   TLV's. However, the protocol supported field must be sent in
   Dual[RFC1195] and IP-Only routers. RFC1195 specifies no checks for
   the protocol supported field for adjacency formation. It places
   topology restrictions on multi-protocol networks.  In networks that
   conform to these restrictions, neighboring routers will always have a
   protocol in common. Therefore, it does not state whether adjacency
   formation should take protocols supported into account. Many
   implementations however, do not form an adjacency with a neighbor
   unless they have at least one protocol in common [as described in
   ITU-T G.7712 and draft-ietf-IS-IS-auto-encap-02.txt.]

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   - Not matching hold timer values has advantages wherein the
   administrator can set different hold times for different routers.
   This helps in cases where the going down of a DIS or some router
   needs to be detected faster. For such routers the hold timer can be
   set to a lower value.


   The checks for formation of adjacencies are stricter in OSPFv2 than

   - The area-id of the received packet should always match the incoming
   interface (with the exception of virtual links). Area type is
   strictly checked by checking the E-bit (not set for non-default
   areas) and the N- bit (not-set for non-NSSA areas).

   - The values of the HELLO interval, the Router Dead Interval and
   network mask received in HELLOs are matched with those on the
   configured interface. Any mismatch in the values causes the HELLO
   packet to be dropped and hence prevents formation of adjacencies. The
   disadvantages of this approach is that Hello  Interval and Router
   Dead Interval changes need to be done within  the Router Dead
   Interval, to prevent breaking adjacencies. The advantage is we would
   not form adjacency in case there is a router that has been mis-
   configured with a large value and which could cause problems later.
   The network mask check however does not apply to point to point links.
   That allows the two ends of a Point-to-Point link to have different

   - MTU check is not done in the hellos. It is done in the during  the
   DB Exchange process.


   Most of the checks for OSPFv3 are similar to that of OSPFv2. The main
   points of differences are: -

   - OSPFv3 runs on a per link basis instead of a per subnet basis.  The
   check for network mask is not done.

   - Instance ID field (non-existent in OSPFv2) on the link is matched
   with the incoming ID in Hellos. The adjacency is formed only if the
   Instance-ID matches.  This allows multiple instances of OSPF to run
   on a single link.

9. Database Exchange and Flooding

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9.1 Initial Database Exchange

   For the SPF algorithm to work properly, all routers in the area
   should have the same database information on which the SPF algorithm
   works. The process of synchronization includes the "Initial Database
   Exchange" which is done when the adjacency is coming up and the
   asynchronous flooding when the Adjacencies are up.


   - A master-slave relation is established to do the database exchange.
   Besides the MTU is exchanged in the database description packets
   before any database exchange starts.

   - The database exchange begins once the adjacency state reaches
   Exstart. On a broadcast links, the DR and BDR form adjacencies with
   all other routers on the network.

   - Only one DB Description packet can be unacknowledged at a time that
   is, the window size is 1. Each DB Description packet from the master
   is acknowledged by the slave. The slave sends its own DB Description
   packet with similar identifiers as the masters.

   - DB description packets containing the summary of LSA's at each end
   are exchanged. Only when the entire summary is received by the
   neighbour can it tell which instance of the LSA is not there in the
   senders database.

   - An adjacency in OSPF is declared FULL/UP, when the entire database
   exchange is completed.

   - OSPF does not allow routers to resynchronize their link state
   database in the steady state. It is only done during the initial
   database synchronization or when network topology changes. However,
   there are techniques to do that. One such way is described in "OSPF
   Out-of-band LSDB resynchronization" [OOB]


   - The MTU check is done at the hello exchange time itself.

   - CSNP's are sent by the DIS on a broadcast link. On a point-to-point
   link both the neighbours exchange CSNP's with each other.

   - On point-to-point link all the LSP's SRM flag is also set for the
   circuit, to indicate the LSP's have to be sent over the circuit.

   - The CNSP's are sent to reduce the actual flooding of all the LSP's
   between the neighbours.

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   - Multiple CSNP's can be sent together. CSNP's unlike DB Descriptions
   in OSPF are not acknowledged.

   - As the CSNP's have a range of LSP-ID's, and contain all the LSP's
   in the database falling in that range. A neigbour on receiving a CSNP
   can know which LSP's in the neighbour are newer, which older and
   which are absent. Based on this the neighbour can send newer LSP's to
   the neighbour.

   - Link state database is continuously refreshed and synchronized
   because of the periodic CSNPs that are announced.

9.2 Asynchronous Flooding

   Whenever any information in an the database changes, the information
   is to be exchanged with all other routers in the network. This is
   done by the flooding process: -


   - Uses reliable flooding mechanism for all link types.

   - Changed LSA's are packed in LS Update packets and send over
   adjacencies to the neighbour, which unpacks the LSA's. LS
   Acknowledgement packets are sent by the receiver, which informs the
   sender that the receiver has received the LSA.

   - The sender retransmits the LSA's after the re-transmission interval
   if it does not get acknowledgements for them.

   - On a broadcast link LSUpdate packets are sent only to all-DR
   routers multicast address. The DR floods the LSUpdate packets to All-

   - Whenever a new DR/BDR is elected, it has to form adjacencies with
   all other routers in the network.

   - There is no difference in the asynchronous flooding procedures
   between OSPFv2 and OSPFv3.


   - LSP's are flooded as is across the area. They are not packed inside
   any other packet.

   - On broadcast links flooding is not done reliably. A changed LSP is
   flooded to all IS-IS routers, however no retransmissions occur.

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   - The reliability in database exchange on a broadcast link is
   achieved by periodic database exchange. This is done as CSNP's are
   sent periodically by the DIS, which initiates the entire database
   exchange process all over again.

   - As the DIS sends periodic CSNP, nothing different needs to be done
   when a new DIS is exchanged.

   - On a point-to-point link flooding is done reliably. LSP's are
   flooded to the neighbour and if CSNP entry for the LSP is not
   received in a particular time interval, the LSP is re-flooded to that

10. Flushing LSA/LSP

   An LSA/LSP is flushed (purged) when the contents carried by the
   LSA/LSP are no longer valid. In OSPF when an LSA is flushed the age
   is set to MaxAge and the LSA is flooded. In IS-IS when an LSP is
   purged (flushed) the header alone is flooded with the Remaining
   Lifetime set to 0, and the value of checksum set to 0. OSPF only
   allows self originated LSA to be flushed, IS-IS spec allows in
   certain cases for non-self originated the LSP to be purged (though
   new implementations don't support this and the update draft has
   changed it) which can lead to problems.

   In OSPF a flushed LSA is not removed unless the LSA is not on any of
   the retransmit lists and none of the adjacencies on the router are in
   state Exchange or loading. This ensures that an LSA that an LSA is
   flooded to all its neighbors before it is removed from the domain. In
   IS-IS an LSP purged is kept for ZeroAge lifetime if the LSP purged is
   a self originated LSP and the LSP is kept for MaxAge if the LSP is
   non self-originated before the LSP is deleted.

   When purging an IS-IS LSP the header and authentication data is kept
   while purging (certain OSPF implementations do the same). However for
   those LSP's that don't support authentication, because the checksum
   is set to 0 for purged LSP's, the integrity of the contents cannot be
   verified. In OSPF the entire content of the LSA is intact while
   flushing leading to unnecessary data sending.

11. SPF Calculation

   Both the protocols use Shortest Path First (SPF) algorithm to
   calculate the best path to all known destinations based on the
   information in their link state database. The SPF algorithm works by
   building the shortest path tree from a specific root node to all
   other nodes in the area and thereby computing the best route to every
   known destination from that particular source/node.

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   - SPF for a given level is computed in a single phase by taking all
   IS-IS LSP's TLV's together.

   - IP routing is integrated into IS-IS by adding some new TLVs which
   carry IP reachability information in the LSPs. All IP networks are
   considered externals, and they always end up as leaf nodes in the
   shortest path tree when IS-IS does a SPF run.

   - Performs only the less CPU intensive Partial Route Calculation
   (PRC) when network events do not affect the basic topology but only
   the IP prefixes.

   - Used narrow (6 bits wide) metrics which helped in some SPF
   optimization. However such small bits proved insufficient for
   providing flexibility in designing IS-IS networks and other
   applications using IS-IS routing (MPLS-TE). "IS-IS extensions for
   Traffic Engineering" [X] draft introduced new TLVs which defined
   wider metrics to be used for IS-IS thus taking away this optimization.
   But then CPU are fast these days and there are not many very big
   networks anyway.


   - SPF is calculated in three phases. The first is the calculation of
   intra-area routes by building the shortest path tree for each
   attached area. The second phase calculates the inter-area routes by
   examining the summary LSAs and the last one examines the AS-External-
   LSAs to calculate the routes to the external destinations.

   - Is built around links, and any IP prefix change in an area will
   trigger a full SPF.

   - Only changes in interarea and external routes result in partial SPF
   calculations and thus IS-IS's PRC is more pervasive than OSPF's
   partial SPF. This difference allows IS-IS to be more tolerant of
   larger single area domains whereas OSPF forces hierarchical designs
   for relatively smaller networks. However with the route leaking from
   L2 to L1 [RFC 2966] incorporated into IS-IS the apparent motivation
   for keeping large single area domains too goes away.

12. Area Types

    IS-IS: Leaking between levels/areas(how it is controlled)  OSPF:

12.1 Area Partitions

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   With hierarchical routing (look at Areas/Hierarchy), it is possible
   for an area to partition so that level 1 routing cannot connect the
   partitions. If both partitions contain level 2 routers, and the level
   2 network is connected, the network as a whole is not physically
   partitioned. There is a path between the partitions of the area. The
   path is level 2 path.

   The symptoms of a partitioned area can be difficult to diagnose and
   annoying for the users. Not only is communication impossible between
   nodes that should be in the same area, but are currently in different
   partitions of the area, but communication between members of the area
   and nodes outside the area can be disrupted since the traffic into
   the area might enter the wrong partition and be undeliverable.

   IS-IS has mechanisms in which level 2 routers residing in a
   partitioned area automatically detect and repair the partition by
   utilizing the level 2 path as a level 1 link. Routing control
   messages as well as data packets are encapsulated with a network
   layer header and transmitted over the virtual link. To the rest of
   the nodes in the area, the area is no longer partitioned and level 1
   routing proceeds normally within the area.

   OSPF does not have any standard explicit area repair mechanisms. If
   an area splits in such a way that a ABR in one partition announces an
   address summary that includes an address reachable in a different
   partition, then routing will not work, since a packet may be
   delivered to the incorrect partition.

   There are two methods by which OSPF can accomplish this:
   - Someone might notice that the area has partitioned, and manually
   reconfigures the ABR in the area, so ABRs in each partition do not
   contain summary addresses for addresses reachable in other partitions.

   - No summary address were used, and each ABR reports each IP address
   individually. If summary addresses are not used, areas do not become
   partitioned, they merely break into multiple areas.

   However an on demand tunnel [TUNNEL] adjacency mechanism has been
   recently proposed in the IETF which solves this problem by choosing
   an inter-area path over an intra-area path.

12.2 Level 2 Partitions (Backbone Area Connectivity)

   IS-IS requires a connected level 2 network. This means there must be
   a path from every level 2 router to every other level 2 router that
   traverses only level 2 routers [RADIA].

   OSPF similarly requires a connected backbone (level 2) area, but
   allows a link between a pair of backbone routers to consist of a

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   manually configured ┬čvirtual link" that consists of a path through a
   non-backbone area. Communication over a virtual link between backbone
   routers A and B can be done in two ways:

   - A can encapsulate traffic being forwarded to B in a network layer
   header giving B as the destination.
   - A can assume all non-backbone routers on the path towards B know
   enough to forward traffic to the destination towards B.

   Virtual link uses the second approach, this requires that all non-
   backbone routers in the transit area know about all destinations in
   the backbone area, so they will be able to forward backbone traffic
   in case they windup in the path of a virtual link. In other words
   summarization of backbone area into the transit area is ignored.

   Tunnel adjacency uses the first approach, further it can used for on
   demand partition so that the adjacency will be established
   dynamically once the backbone is partitioned.

   Because of the possibility of manually configured virtual links in
   OSPF, IS-IS has a topological restriction that OSPF does not.

12.3 Injection of Level 2 Information

   In IS-IS, level 1 routers only know information about their own area.
   If a level 1 router R receives a packet with an address not reachable
   within the area, R forwards the packet to the level 2 router nearest
   to R. In OSPF, level 2 information is fed into the non-backbone areas.
   Suppose there is an area A in some AS such that:

   - n IP destination addresses are reachable within the AS, but outside
   the area A
   - m IP destinations are reachable outside the AS
   - k ABRs in area A
   - j ASBRs in the AS

   Each of the "k" ABRs reports their own distance to the "n" IP
   destination addresses and the "j" ASBRs. This information is
   O(k*(j+n)). Each of the "j" border routers also reports its distance
   to each of the "m" IP destinations reachable outside the AS. This
   information is O(j*m).

   Giving level 2 information to level 1 routers enables the routers to
   choose the exit level 2 router that will give the best path to the

   Thus, OSPF yields more optimal interarea routes than IS-IS. The cost
   of providing more optimal routing is increased bandwidth usage by the
   routing algorithm and increases memory and CPU requirements in level

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   1 routers. Aside from increased bandwidth, CPU, and memory usage,
   there is an additional issue raised as a result of the OSPF
   requirement for level 1 routers to store level 2 information. In IS-
   IS where an area is independent of the rest of the network, database
   sizes in level 1 routers can be calculated based on the size of the
   area. If the area never changes, the level 1 routers will continue to
   function. In contrast, as the entire network grows in OSPF, demand on
   level 1 routers increases. One small area with small routers, cannot
   be sheltered from the growth of the rest of the network.

12.4 Stub Area

   There is an option in OSPF, called "Stub Area." If an area is a stub
   area, the information concerning destinations outside the AS is not
   flooded into the area, saving O(j*m). Information about destinations
   within the AS, but outside the area are still flooded within an area,
   even if the area is configured as a stub area.

   In other words, an OSPF stub area is a compromise between a nonstub
   OSPF and an IS-IS area. OSPF stub areas require significantly less
   storage than nonstub OSPF areas. Like IS-IS, OSPF does not attempt to
   optimize the route from a stub area to a destination outside the AS,
   but unlike IS-IS, OSPF does attempt to optimize routes from a stub
   area to destinations within the AS, but outside the area.

   In IS-IS, none of this information is seen by the level 1 routers.
   The cost of not storing, propagating, and computing this information
   in IS-IS is that some routes to other ASs will be less optimal than
   those used in OSPF.

12.5 Not So Stub Area (NSSA)

   "not-so-stubby" area (or NSSA), which has the capability of importing
   external routes in a limited fashion.

   The OSPF specification defines two general classes of area
   configuration. The first allows Type-5 LSAs to be flooded throughout
   the area.  In this configuration, Type-5 LSAs may be originated by
   routers internal to the area or flooded into the area by area border
   routers.  These areas are distinguished by the fact that they can
   carry transit traffic. The backbone is always a Type-5 capable area.
   The second type of area configuration, called stub (described in
   section 10.4) does not allow Type-5 LSAs to be propagated
   into/throughout the area and instead depends on default routing to
   external destinations.

   NSSAs are defined in much the same manner as existing stub areas.
   Type-7 LSAs provide for carrying external route information within an
   NSSA. Type-7 LSAs have virtually the same syntax as Type-5 LSAs with

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   the obvious exception of the link-state type. Both LSAs are
   considered a type of OSPF AS-external-LSA.  There are two major
   semantic differences between Type-5 LSAs and Type-7 LSAs.

   - Type-7 LSAs may be originated by and advertised throughout an NSSA;
   as with stub areas, Type-5 LSAs are not flooded into NSSAs and do not
   originate there.

   - Type-7 LSAs are advertised only within a single NSSA; they are not
   flooded into the backbone area or any other area by border routers,
   though the information that they contain may be propagated into the
   backbone area.

   In order to allow limited exchange of external information across an
   NSSA border, NSSA border routers will translate selected Type-7 LSAs
   received from the NSSA into Type-5 LSAs.  These Type-5 LSAs will be
   flooded to all Type-5 capable areas.  NSSA border routers may be
   configured with address ranges so that multiple Type-7 LSAs may be
   aggregated into a single Type-5 LSA.  The NSSA border routers that
   perform translation are configurable. In the absence of a configured
   translator one is elected.

   IS-IS does not have such capability of an area being a Not-So-Stubby
   Area (NSSA).

13. Architectural Values

13.1 Architectural Constants

   OSPF does have a large number of tunable parameters that can make
   configuration seem complicated. However, most of these parameters
   should be set to default values in an OSPF implementation.

13.2 Synchronized Parameter Setting

   In OSPF, there are several parameters that must be configured
   identically in routers, or else the router will refuse to communicate
   with each other. This creates a problem because it is virtually
   impossible to change the parameter setting via network management.
   Once a router's parameter setting is changed, it is cut off from the
   rest of the network since no other routers will be able to
   communicate with it. In contrast, there is always a way in IS-IS to
   migrate from one setting to another by configuring routers one at a
   time while the network is running.

   The parameters in OSPF that must be set identically in neighboring
   routers are the HelloTime and the DeadTime

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   IS-IS reports only DeadTime in its Hello messages (not HelloTime). As
   a result, the ratio between DeadTime and HelloTime is fixed in IS-IS,
   but can be configured in different ways by OSPF. IS-IS uses the
   information solely to determine how long to wait between receipt of
   Hellos from a particular neighbor before declaring the link to that
   neighbor down. There is no necessity for neighboring nodes to have
   the same value.

   Being able to change these timers in a running network is important.
   As a LAN becomes larger it might be decided that the overhead from
   hellos is too great. It also might be important in some
   configurations to be able to run with different hello timers for
   different routers. There might be some routers for which quick
   deletion of failure would be very desirable, whereas for other
   routers quick deletion of failure might not be as important. To lower
   overhead these routers might be configured with a longer HelloTime.
   This cannot be done in OSPF since all routers must have identical

   - Stub Area Flag:

   OSPF requires every router in an area to be configured with a flag
   indicating whether the area is a stub area. If a level 2 router has a
   stub area flag set, it will not flood type 5 LSPs into the area. The
   "Stub Area" flag is reported in OSPF Hello messages. If a router
   disagrees with a neighbor as to the setting of the "stub area" flag,
   it will bring the link to the neighbor down. IS-IS has no such

   - Authentication Password:

   Both OSPF and IS-IS have the optional feature of providing
   authentication. In OSPF, there is a single password per link. The
   password a router transmits is the same as the password it will
   accept on the link. IS-IS allows configuration of multiple receive
   passwords so it is possible to migrate from one password to another
   without disrupting the operation.

14. Virtual Links


   - IS-IS allows a Level-1 Area which is partitioned to be
   automatically repaired, by electing Partition Designated Level 2
   routers and having a virtual link between them. The mechanism is not
   often implemented and requires an explicit tunnelling mechanism."

   - Used in ISO IS-IS for connecting partitions of Level 1 Area over
   the Level 2 backbone.

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   - Used for connecting physically separate area zeroes ( to
   maintain contiguity of the backbone

   - Used for connecting remote areas to the backbone through other
   areas if direct physical connectivity is not possible. This enables
   an OSPF packet to be sent from one part of an remote isolated site to
   the main OSPF network.

   - For Virtual links to work, OSPF accepts packets which are have
   originated more than one hop away. This can lead to security concerns
   if the packets at the edge of the domain are not properly filtered.

15. Packet Alignment/Extensibility


   - Does not require any particular alignment of packet fields.

   - Uses TLV (Tag-Length-Value) encoded packets to advertise routing

   - TLVs not supported/recognized are ignored by IS-IS routers

   - LSPs are flooded intact with unrecognized TLV information making it
   very extensible. Ipv6 support is provided by simply adding a few more

   - TLVs can be nested as sub-TLVs providing even more flexibility for
   future extensions. Though the base spec does not use them but the
   newer drafts have started using them (TE extensions, etc).


   - Uses fixed format packets with all fields aligned at 32-bit
   boundaries for faster processing of the OSPF packets (doesn't really
   matter anymore as the CPUs are really fast these days!). This was
   also primarily done because OSPF was meant to be an IPv4 only

   - The downside is that the packet formats are not at all extensible.

   - It uses LSAs for advertising the routing information and the
   original spec called for dropping any unrecognized LSA type.

   - LSAs of type 9, 10 and 11 (Opaque LSAs) have been introduced for
   advertising other application-specific information and enough vendors

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   now support this so that they are likely to get from one side of the
   network to the other.

   - Since the unrecognized LSA types are not flooded to neighbors it
   makes it very difficult to extend. It in turn means that all the OSPF
   routers must be upgraded network-wide to make the new extensions work.

   - The new drafts (TE, GMPLS extensions, etc) written for OSPF now
   support TLV encoding.


   - Exhibits implicit opaque LSA behaviour i.e. unrecognized LSA types
   are flooded to the neighbors making it more extensible that OSPFv2

   - Designed in a way which makes it easily extensible to any other
   layer 3 protocol suite.

16. MTU Limitations

   The MTU of a sub-network is the largest size packet or frame,
   specified in octets that can be sent over it. Both OSPF and IS-IS
   require communicating routers to have matching MTU sizes in order to
   form adjacencies. This is needed so that routers will not advertise
   packets larger than a neighbor can receive and process. However, each
   protocol uses a different mechanism to check against MTU mismatch.
   For this discussion the term MTU is used for a links Maximum Receive
   Unit (MRU) too.


   - IS-IS works over the link layer, which does not provide for
   fragmentation and reassembly.

   - Hello's are sent padded to MTU size till an adjacency comes up. If
   there is an MTU mismatch, the side having the lesser MTU would drop
   the bigger than MTU hello. This would not allow adjacencies to be
   formed between interfaces having different MTU's.

   - The hello MTU match is an insufficient condition for IS-IS as LSP's
   are flooded as is and not packed into any other packets. For the
   LSP's to be successfully synchronized across the subdomain, all LSP's
   need to be of a size lesser than the smallest link MTU in the
   subdomain, else the flooding of the LSP on the link will fail
   resulting in inconsistent routing tables.

   - Mis-configuration of the maximum packet size that a router sends
   out can cause problems across the subdomain as there is no way to
   check the value between routers that are not adjacent.

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   - OSPF works over IP, so the fragmentation and reassembly of any OSPF
   packet is taken care by the IP layer. However for some link
   technologies where MTU is configurable but not negotiated, we can
   have packet black-holes whenever packets larger than the receiving
   sides MTU are sent.

   - The MTU is exchanged in the database description packets. If the
   value of MTU received in the first DB description packet is greater
   than that can be accepted on an interface, the packet is rejected and
   the adjacency is not formed. Retransmissions of DB description
   packets occur because the packets are never acknowledged. The
   adjacency therefore gets stuck in EXstart state.

   - As LS Update's are assembled in each router, the MTU of another
   link does not affect the size of the LS Update packet.

   - As the MTU match is done at the database exchange state after the
   DR election has been completed, in case the DR itself cannot form
   adjacencies with the rest of the routers, it can cause the network to
   become a stub.

17. Security/Authentication Issues

   OSPF: Replay protection/KeyId field

   IS-IS: HMAC MD5/checksum not in all PDU's(optional)/ need to dig into
          PDU's to find TLV/ LSP's checksum does not cover length field/
          purging done with 0 checksum (contents can't be verified)

   Both protocols have a field indicating the "type" of authentication.
   There are however differences in the two protocols. In IS-IS, the
   data associated with the authentication is a variable length. In OSPF
   it is fixed at 64 bits. 64 bits is sufficient for a password scheme,
   but would not suffice for a public key signature scheme, which would
   need a field several hundreds of bits long.

   In OSPF there is a single password per link. A router is configured
   with a password for each link to which it is attached. It transmits
   that password when it transmits OSPF messages on that link. It
   expects all OSPF messages it receives on that link to have that
   password. In IS-IS, a router is configured with a transmit password
   on a link, which is the password it uses when it transmits IS-IS
   messages, as well as a set of acceptable receive passwords.

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   On a P2P link a password scheme in which the receive and transmit
   passwords are different offers some security. If the passwords are
   the same, the intruder need only wait for the other router to
   transmit first, and the intruder will find out the password. Even
   with two passwords, an intruder can, with effort, discover the

   The reason IS-IS configures routers with a set of acceptable receive
   passwords, rather than a single receive password, is so that a link,
   such as a LAN, can be migrated from one password to another without
   disrupting the network. Since OSPF has single password per link, it
   is not possible to change the password in an operational network. The
   routers would all have to be brought down and locally reconfigured.

   One of the brought up issue with IS-IS proponents is apparently the
   big advantage that IS-IS has over OSPF from a security point of view
   as IS-IS protocol packets cannot be routed beyond the immediate next
   hop or can never be sourced by non-border routers. This is claimed,
   can prevent a variety of potential DoS attacks as anyone can launch
   OSPF packet bombs in the others network. This apparent vulnerability
   to DoS attacks is because OSPF rides over IP rather than directly
   running on the link layer.

   Since all OSPF packets can be authenticated using MD5, all spurious
   OSPF packets can be dropped. But there can be times when MD5 can
   itself be a part of a problem because it takes significant CPU to
   check signatures and discard the packets. This is partly true but it
   is to be noted however that even if OSPF encapsulation is changed to
   L2, we would still have to support IP encapsulation for virtual links,
   so we would still have to do MD5.

   Moreover the system administrator can filter on the edges of the
   network to pry away all the OSPF messages coming from the edges. This
   will of course be done in addition to cryptography.

18. IS-IS/OSPF for IPv6


   - Designed to be protocol-agnostic using TLV encoding.

   - Distinct TLVs used to encode topology information and reachability
   (address prefix) information. As a direct consequence, extending ISIS
   to support IPv6 is just a matter of introducing some new TLVs. The
   existing TLVs continue to be used to advertise topology information

   - An extension to ISIS has been proposed that calculated Ipv4 and
   IPv6 topologies separately. This would still use a single instance of
   ISIS for each network protocol. There are proposals to extend ISIS to

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   enable multiple instances for each network layer protocol, thereby
   applying the "Ships in the Night" model for ISIS.


   - All routing information is advertised using LSAs, which are
   identified by the LS Type, LS Identifier and the advertising Router.

   - Adapting this to support IPv6 was difficult for the following

   Many fields (LS Identifier, the DR/BDR field in the HELLO Message,
   etc) in the OSPF packets are IPv4 specific. Thus adapting OSPFv2 to
   support IPv6, which has an expanded address space, becomes impossible.

   - OSPFv2 inherits IPv4's "subnet" restriction. Thus an OSPFv2 Router
   denies to form an adjacency if the neighboring router's IPv4 address
   does not match the router's IPv4 subnet. Further, OSPFv2 can
   calculate only one IPv4 prefix for a LAN segment. These "subnet"
   restrictions were removed in IPv6 specification, which makes OSPFv2
   even more difficult to adapt to IPv6.

   - Presents a "ship in the night" solution during the IPv6 migration.
   This means that the operator needs to run OSPFv2 for IPv4 routing and
   OSPFv3 for IPv6, as against an integrated solution provided by ISIS.
   If using OSPF, then OSPFv2 and OSPFv3 will independently calculate
   their network topology, routes, etc. This can lead to some redundancy
   and duplication when IPv4 network topology is identical to the IPv6
   topology. This leads to greater CPU, memory and bandwidth utilization
   because of double computation and advertisement.

   ISIS on the other hand, presents an integrated solution in the
   presence of IPv4 and IPv6 network protocols. Since ISIS can calculate
   IPv4 and IPv6 routes simultaneously it is relatively efficient with
   respect to the utilization of resources.

   However, most of the networks deploying IPv4 and IPv6 simultaneously
   typically have different topologies and IPv4 and IPv6 networks are
   constructed separately. This avoids a breakdown of one network
   because of the failure in the other.


   - Instead of putting hacks in OSPFv2 to support IPv6, OSPFv3 (also
   referred to as "OSPF for IPv6") was laid out by the OSPF WG.

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   - The packet format was changed, calculation and representation of
   address prefix information was separated from the topology

   - OSPFv3 provides native support for opaque LSAs

   - Other fundamental mechanisms of OSPF, like database synchronization,
   etc remain unchanged. The DR/BDR field in the Hello packet described
   above was simply changed to contain Router-ID of the DR/BDR.

   - Extensions have been proposed to adapt OSPFv3 for an "Integrated
   model" where OSPFv3 would be extended to calculate IPv4 routes

19. Current Deployments

   Both the protocols have been currently deployed in large scale IP


   - used in most Tier 1 ISP networks and in single area configurations

   - initally most large ISPs adopted IS-IS as it had a stable
   implementation, coupled with U.S. government's mandate to support ISO
   CLNS alongside IP.


   - more widespread from medium to large IP networks.

   - deployed in most IP based enterprise networks

20. Metrics Size

   Each interface in the link state protocols in given a metric, which
   is advertised with the link state information in LSP/LSA. The SPF
   algorithm uses this metric to calculate the cost and the nexthop to a
   destination. Metrics used are generally the inverse of bandwidth. A
   larger bandwith capacity link would have a lesser metric.


   - ISO10589 specifies metric 6 bit in size. Therefore the metric value
   can range from 0-63. The information is carried in neighbor
   reachbility TLV and the IP reachability TLV. This is called the
   Narrow metric. The maximum path metric MaxPathMetric supported is
   1023. This in theory brought the complexity of the SPF from O(nlog n)
   to O(n). But this isn't significant any more as the CPUs are really
   fast these days. The metric size was kept small to optimize search

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   while doing SPF. It also allows two types of metrics External and

   - The Narrow metric range was however found to be too small for
   certain networks. New TLV's(Extended IP and Extended neighbor
   reachability TLV's) to carry larger metrics was added as part of the
   traffic engineering document[IS-IS-TE]. This is called Wide Metrics.
   The MaxLinkMetric value is 0xFFFFFFand the MaxPathMetric       is

   The Extended IP reachability TLV allows for a 4 byte metric, while
   the Extended Neighbor reachability TLV allows for 3 bytes metric size.
   This is to enable the metric summarized across levels/domains to be
   as large as 0xFFFFFFFF while the link metric itself is no larger than
   0xFFFFFE. If a metric value of 0xFFFFFF is used the prefix is not
   used in SPF calculations.

   - Four kinds of narrow metrics are defined however only the default
   metric is used in networks.


   - OSPFv2 allows a link to have a 2 byte metric feild in the Router
   LSA. This implies the maximum metric of 0xFFFF.

   - The Summary, Summary-ASBR, AS-External and NSSA LSA's have a 3 byte
   metric value. A cost of 0xFFFFFF (LSInfinity) is used to tell the
   destination described in the LSA is unreachable.

   - AS-External and NSSA LSA's allow two metric types, Type-1 and Type-
   2 which are equivalent to IS-IS Internal and External metrics. The
   type 1 considers the cost to the ASBR in addition to the advertised
   cost of the route while the latter uses just the advertised cost
   while calculating the routes.

   - The scheme thus allows for links to be configured with a metric no
   larger than 0xFFFF, while allowing cost of destinations injected
   across areas/levels to be as large as 0xFFFFFE.


   - OSPFv3 allows similar metric size for the Router LSA's as in OSPFv2.

   - OSPFv3 allows similar metric sizes for Intra Area Prefix LSA, Inter
   Area Prefix LSA, AS-External LSA and NSSA LSA as in OSPFv2. The value
   and significance of LS Infinity is valid here.

21. Database Granularity

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   This section compares how the two protocols hold their routing
   information in their link state databases. The way these protocols
   encode the routing information in their database, affects their
   behavior in how they flood/distribute the change of routing


   - Organization of Routing Information

   OSPF encodes the routing information into small chunks, which it
   calls Link State Advertisement (LSA). Each LSA has its own 20-byte
   header in order to be identified uniquely. This header is called the
   LSA Header. There is no limitation on the size of a LSA, though the
   actual LSA size is limited by IP packet size limitation: 65,535 bytes
   minus the LSA Header size and IP packet header size. The database
   access in OSPF is per LSA basis.

   In OSPF routing, the information within an area is described by type
   1 and type 2 LSAs (known as Router-LSA and Network-LSA respectively).
   These LSAs can become big depending upon the number of adjacencies to
   be advertised and prefixes to be carried inside an area. In other
   words, the routing information with respect to a single node (either
   router or network node) is encoded inside a single LSA. On the other
   hand, each inter-area or external prefix is advertised in a separate
   LSA (AS-External LSA).

   An OSPFv2 router may originate only one Router-LSA for itself, while
   in OSPFv3, a router is allowed to originate multiple Router-LSAs. A
   router may originate a Network-LSA for each IP subnet on which the
   router acts as a DR. A router may originate one LSA for each inter-
   area and external prefix, with no limitations on the number of LSAs
   that it may originate.

   - Consequences

   Originating a new and a unique LSA for each inter-area route and  an
   external prefix implies that there is a LSA Header overhead involved
   while the information is kept in the database or is flooded to the
   neighbors. There is thus some extra memory and bandwidth consumed in

   - Carrying Routing Information

   LSAs are carried in Link State Update packets (called LS Updates or
   LSUs). Each LS Update packet has its own header, consists of a 24-
   byte OSPF protocol header, and a 4-bytes field indicating the number
   of LSAs contained in the packet. Thus multiple LSAs can be packed

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   into a single LS Update packet. Some implementations may not do this
   as its considered difficult achieving this during flooding.

   - Consequences

   In the face of network changes, OSPF floods only the updated LSAs.
   Therefore, even if an implementation does not pack multiple LSAs into
   a single LS Update packet (and so bandwidth is consumed by LS Update
   header for each update of a single LSA), the bandwidth consumption
   for each network change can be considered adequately small.


   - Organization of the Routing Information

   In IS-IS, protocol packets are called Protocol Data Units or PDUs.
   IS-IS encodes the link state information into the set of Type-Length-
   Value tuples (called TLVs), and packs these TLVs into one or more
   Link State PDUs (LSPs). The size limit of a LSP is configurable. The
   Routing database consists of these PDUs and the access to the
   database is per PDU basis. The original IS-IS specification places an
   upper bound on the number of LSPs a router can originate to 255.
   There are however techniques which enable a router to originate more
   than 255 LSPs, by using multiple system-id's for itself.

   - Consequences

   Since routing information in IS-IS for each router is packed in fewer
   LSPs, the memory consumed for bookkeeping of the routing data within
   the database is less and is more efficient.

   - Carrying Routing Information

   Each LSP is flooded independently, without being modified all the way
   from the originator through the routers till the very end. This
   results in all the routers having the same LSPs as that originated by
   the first router.

   - Consequences

   Since LSPs are not modified in any way and are not allowed to be
   fragmented, in order to be flooded successfully over all links
   existing in the IS-IS network, great care must be ensured when
   configuring the size limit of LSP that routers can originate and
   receive. [INTEROP] If the size limit of the LSP is set without taking
   into account the minimum value of the MTUs throughout the network, or
   if the size limit of LSPs conflict among some the routers in the
   network, the database synchronization may not be achieved, and this
   can result in routing loops and/or blackholes.

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   When a change occurs to a LSP, the whole LSP needs to be flooded, and
   therefore the bandwidth usage can be non-optimal. There is however a
   solution which exists in theory. If an implementation finds some of
   the entities to be flapping, then they may be packed into smaller
   LSPs or may be isolated from the other stable entities. This way one
   needs to only advertise the unstable LSP/LSPs.

   Database granularity also affects when two routers need to
   synchronize their databases. In OSPF, because of its high database
   granularity there are a lot of items which it needs to synchronize
   and that process is somewhat complicated with a lot of DBD packets
   being exchanged back and forth. This is simpler in case of IS-IS and
   there isn't any FSM that the neighbors need to go through to
   synchronize their databases. It just uses it regular flooding
   mechanism (a couple of CSNPs describe their entire topology
   information) to exchange its entire database.

22. Separation of TE and topology information

   Traffic Engineering (TE) is defined as the aspect of Internet Network
   Engineering concerned with the performance optimization of traffic
   handling in operational networks. The Link State Routing protocols
   transport traffic engineering information reliably by flooding
   mechanisms, thus helping in TE.


   - TE information is carried in Extended IS reachability TLV's which
   are also used in normal routing table calculations. TE information is
   carried as subTLV's.

   - A new Router-Id TLV is defined for TE purposes.

   - The Value field of the TLV length can only be 255 bytes, because of
   the limitations SRLG is defined in a seperate TLV.


   - TE extensions information is carried in TE LSA's. A TE LSA is an
   opaque type-10 LSA [OPAQUE], with the first 8 bits of the LSA-ID
   field value being 1 and the remaining 24-bits being used for type-
   specific data [OSPF-TE].

   - The payload of the TE LSA consists of TLV's. There are two top
   level TLV's defined though any LSA can carry only one TLV. The TLV's
   defined are Router address TLV and Link Address TLV.

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   - The length of the value field is 16 bits, hence the maximum length
   of the Value field in the TLV can be 2^16.

   - The Router-Id field used for OSPF is used to identify the other end
   of a point-to-point link. This Router-Id field is the same field used
   for normal SPF calculations.

23. Convergence and Scalability Issues


   - Is limited by the maximum number of LSPs that each IS-IS router can
   issue. This is 256 as its LSP ID is 1 octet long.  The total number
   of IP prefixes carried by IS-IS can be easily computed which comes to
   O(31000). For actual calculations refer to the [APPENDIX]

   This seems to be a reasonable number for any sane IS-IS deployment
   and it will not run out of space unless someone actually injects the
   entire BGP feed into the IGP. In that case we will run out of space
   at about 20% of the way into redistribution and not be able to
   advertise the rest. It is for this reason that this practice has now
   been deprecated and the RFC 1745 which lays down the rules for BGP-
   OSPF interaction moved to the HISTORICAL status [RFC1745].

   - 8 bits are used for defining a pseudonode number in the LSPID which
   means that a router can be  DIS for only 256 LANs. Additionally there
   is also a limitation on the number of routers that can be advertised
   in pseudonode LSP of the DIS.

   - There is however a recent IETF draft [256LSP] which describes a
   mechanism that allows an IS-IS router to originate more than 256 LSP
   fragments and RFC 3373 [3WAY] which proposes a method for new TLV
   HELLO packets that increase the number of p2p adjacencies.

   - The "Remaining lifetime" field which gives the number of seconds
   before LSP is considered expired is 16 bits wide.

   This gives the life time of the LSP as 2^16/60/60 Hrs = 18.7 Hrs

   Thus each LSP needs to be refreshed after every 18.7 Hrs.


   - In theory, OSPF topology is limited by the number of links that can
   be advertised in the Router LSA as each router gets only one Router
   LSA and it cant be bigger than 64K which is the biggest an IP packet
   can be. The same constraint applies to the Network LSA also.

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   Each link in the router can take up at most 24 bytes. Thus, number of
   links which can be supported is given by (64 * 1024) / 24 =  2370

   However, if we take the minimum link size per link (12 bytes) then
   the maximum is about 2 * 2370 = O(5000) links

   To be more specific, we can have O(2300) p2p and p2mp links (not
   considering virtual links, etc) and O(5000) broadcast/NMBA links

   Thus each Router LSA can carry some 5000 links information in it. It
   is hard to imagine a router having 5000 neighbors but there are
   already routers with 400 neighbors in some ISPs, and it doesn't take
   long to reach the order of the magnitude limited by OSPF.

   - Network LSAs are generated by the DR for each broadcast network it
   is connected to. To have scaling problems it should have 2730 * 6
   times neighbors on that interface. This is even less probable and
   hence there are no scalability problems with OSPF per se.

   - All other LSAs apart from Type 1 and Type 2 hold single prefixes.
   Because there is no limit to the number of such LSAs, a large number
   of inter-area and externals can be generated depending upon  the
   memory resources of the router.

   - Each LSA has an LS Age field which is counted upwards starting from
   zero. Its life is an architectural constant which says one hour. When
   an LSA's LS age field reaches MaxAge, it is reflooded in an attempt
   to flush the LSA from the routing domain. One hour seems like a long
   time but if one originates 50,000 LSAs then OSPF will be refreshing
   on an average of just 36ms

   Total number of LSAs to be refreshed = 50,000

   Time by which all the LSAs must be refreshed = LSRefreshTime =
   30mins = 1800 secs

   Rate at which the LSAs need to be refreshed = 1800/50000 = 36ms

   However, if the refreshes are perfectly spread out across time and
   perfectly batched, the actual update transmission rate may be on the
   order of one packet per second.

   There is however a "do-not-age" LSA [DEMAND] which in theory can be
   pressed into service and which never gets aged. However, such LSAs
   will be eventually purged from the LS database if they become stale
   after being held for at least 60 minutes and the originator not
   reachable for the same period. Moreover it is not backward compatible
   and if one deploys that in the network today with some routers not

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   supporting this then the network can really get weird. So there isn't
   really much that can be done using these unless the whole network is

   Both the routing protocols are scalable and there should not be any
   scalability issues with any one of them if implemented properly. Both
   have similar stability and convergence properties.

24. Area Id Change Functionality

   Changing area-id for an area is useful for link state routing
   protocols in order to merge two areas into one or to split an area
   into several areas.


   - An area address is a variable length quantity.

   - An area can have multiple area addresses. Neighboring IS's will not
   form an adjacency unless they have a single area address in common.
   This is quite useful for IP networks that are transitioning from one
   area address to another, merging two areas into one or even to split
   an area into several pieces.

   - Seamless transition of area addresses for an area is easier in IS-
   IS, e.g. initially an area can have area adress A, then the set {A,
   B} and when the new area address B is recognized by all the routers
   in the area, old area address A can be removed.


   - In OSPF each area has a single ID, a 4-byte quantity.

   - OSPF does not have the ability to merge and split areas dynamically
   as IS-IS has, though partitioned backbone can be repaired by using
   virtual link. But it should be ensured that the area through which
   virtual link is configured is having full routing information, i.e.
   it should not be a stub area.

   - Area-id can not be changed dynamically in case of OSPF.

25. Backward Compatibility

   For a protocol to be extensible, it should have mechanisms to allow
   changes in the protocol packets, without affecting backward
   compatibility. OSPFv2, OSPFv3 as well as IS-IS allow for extending
   the protocol in a backward compatible manner.

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   - All IS-IS packets contain TLV's. Unrecognized TLV's are ignored or
   receipt, this allows TLV types to be extended in a backward
   compatible manner.

   - TLV's can signal more information between neighbors than can option
   bits. It is for this reason IS-IS was able to allow IS-IS for IP
   extensions without any backward compatibility being lost.


   - OSPFv2 has options bit in the Hello, Database description packets
   as well as the LSA header filed, which can be used to signal to its
   capabilities of the neighbor. Any change of capability can be
   signaled and decision to form adjacency as well as the LSA's to
   exchange can be based on the option bits

   - There are only 8 bits in the options header most of which have
   already been utilized. To allow for further extensions OSPF allows
   the LLS option [LLS]. However this is not widely supported in
   commercial routers.

   - Any unrecognized LSA received is dropped. This does not allow new
   LSA types to be defined and prevents OSPFv2 to be really extensible.

   - Some fields in the OSPFv2 packets contain IPv4 specific information.
   It is for this reason a different protocol for OSPF for IPv6 was


   - OSPFv3 also allows options field like OSPFv2, however the options
   field have been expanded to 24-bits allowing for more options to be
   signaled. The options have been removed from LSA header and been
   added into LSA body for Router, Network, Inter-area-router and link

   - OSPFv3 LSA have a flooding scope in the upper three bits of the LSA
   type field. Unrecognized LSA's are not ignored but flooded based on
   the flooding scope of the 3 bits. This allows new LSA types to be
   flooded in the domain

26. Hitless Restart Mechanisms

   If the control and forwarding functions in a router can be separated
   independently, it is possible to maintain a router's data forwarding
   capability intact while the router's control software is

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   restarted/reloaded. This functionality is termed as "graceful
   restart" or "non-stop forwarding".


   - Restarting router does not re-compute its own routes until it has
   achieved database synchronization with its neighbors [GRACE-IS-IS].

   - IS-IS uses new type of TLV (restart TLV) in IIH to obtain the
   graceful restart functionality. Grace period is decided as the
   minimum of the Remaining times of received IIHs containing a restart
   TLV with RA bit set.

   - During grace period, restarting router does not transmit self-
   originated LSPs and self-LSPs are not purged or modified. These
   restrictions are necessary to prevent premature removal of an own LSP
   and hence churn in other routers.

   - Restart mechanism in IS-IS allows to establish adjacency without
   cycling through the normal operation of adjacency state machine.

   - Proper database synchronization is achieved in situations where the
   neighboring routers of the restarting router do not support the
   restart TLV.


   - OSPF routers can play either of two roles during graceful restart -
   as a restarting router or as a helper neighbor [GRACE-OSPF].

   - Restarting OSPF router originates new type of Grace-LSAs (link
   local Opaque-LSA) specifying the 'grace period'.

   - During graceful restart, the restarting router neither originates
   LSAs with LS types 1-5,7 nor does modify or flush received self-
   originated LSAs.

   - Router as helper neighbor advertises the restarting router in their
   LSAs as if it were fully adjacent during the grace period and also
   detects network topology changes.

   - OSPF automatically reverts back to standard OSPF restart from
   graceful restart if topological changes are detected or if one or
   more of the restarting router's neighbors do not support graceful

27. Demand Circuits

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   Demand circuits are network segments whose costs vary with usage;
   charges can be based both on connect time and on bytes/packets
   transmitted. Examples of demand circuits include ISDN links, X.25
   SVCs, dial-up lines,etc. It is thus desirable to use them only for
   the user traffic and minimal control traffic.


   - ISO 10589 provides very limited support for demand circuits called
   "dynamically assigned circuits" wherein it supports sending data
   traffic over them, but does not support running the routing protocol
   over them. Thus there are no HELLO suppression/DNA schemes in IS-IS
   for such circuits.


   - A new optional capability is described in RFC 1793 which modifies
   OSPF for supporting such circuits. In this, a router will set the DC
   bit in the options field if it supports this capability. Routers that
   support the capability will also set the high bit (known as the do-
   not-age bit), of the LS age field to indicating that the LSA should
   not be aged. OSPF running on such circuits suppresses periodic HELLOs
   and LSAs, but a topology change will still activate the demand
   circuit since LSA updates will be sent which are required to keep the
   LS database accurate [DEMAND].

   - Demand circuits are generally defined in stub areas which have
   limited topology database thus shielding them from frequent topology

   - There is however a problem in detecting inactive OSPF neighbors
   over such links as HELLO exchange is suppressed on these circuits. To
   work out a solution for this there are solutions suggested in a
   recent IETF draft [PROBE] which addresses this problem by the using
   ┬čneighbor probing" mechanisms.

28. IANA Considerations

   This document introduces no new security concerns to either of the
   specifications referenced in this document.

29. References

   J. Moy, "OSPF Version 2", RFC 2328, April 1998


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   R. Coltun, D. Ferguson and J. Moy, "OSPF for IPv6", RFC 2740,
   December 1999

   A. Martey, "IS-IS Network Design Solutions", CISCO Publications,
   February 2002

   John T. Moy, "OSPF: Anatomy of an Internet Routing Protocol", Addison
   Wesley, February 1998

   R. Balay, D. Katz and J. Parker, "IS-IS Mesh Groups", RFC 2973,
   October 2000

   D. C. Plummer, "Ethernet Address Resolution Protocol: or Converting
   Network Protocol Addresses to 48.bit Ethernet Addresses for
   Transmission on Ethernet Hardware", RFC 826, November 1982

   W. Simpson, "The Point-to-Point Protocol (PPP)", RFC 1661, July 1994

   A. Zinin, A. Roy and L. Nyugen, "OSPF Out-of-band LSDB
   resynchronization", Work in Progress

   S. Mirtorabi, P. Psenak, "OSPF Tunnel Adjacency", Work in Progress

   R. Perlman, "A comparision between two routing protocols: OSPF and
   IS-IS", IEEE Network, vol. 5, no. 5, pp. 18, 24, September 1991

   R. Coltun, "The OSPF Opaque LSA Option", RFC 2370, July 1998

   D. Katz, K. Kompella and D. Yeung, "Traffic Engineering Extensions to
   OSPF Version 2", RFC 3630, September 2003

   J. Parker, "Recommendations for Interoperable Networks using
   Intermediate System to Intermediate System (IS-IS)", RFC 3719,
   February 2004

   H. Smit and T. Li, "Intermediate System to Intermediate System (IS-
   IS) Extensions for Traffic Engineering (TE)", RFC 3784, June 2004

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   A. Hermelin, S. Previdi and M. Shand, "Extending the Number of
   Intermediate System to Intermediate System (IS-IS) Link State PDU
   (LSP) Fragments Beyond the 256 Limit", RFC 3786, May 2004

   D. Katz and R. Saluja, "Three-Way Handshake for Intermediate System
   to Intermediate System (IS-IS) Point-to-Point Adjacencies", RFC 3373,
   September 2002

   [RFC 1745]
   K. Varadhan, S. Hares and Y. Rekhter, "BGP4/IDRP for IP---OSPF
   Interaction", RFC 1745, December 1994

   A. Zinin, B. Friedman, A. Roy, L. Nguyen and D. Yeung, "OSPF Link-
   local Signaling", Work in Progress

   M. Shand and L. Ginsberg, "Restart Signaling for Intermediate System
   to Intermediate System (IS-IS)", RFC 3847, July 2004

   J. Moy, P. Pillay-Esnault and A. Lindem, "Graceful OSPF Restart", RFC
   3623, November 2003

   J. Moy, "Extending OSPF to Support Demand Circuits", RFC 1793, April

   S. Rao, A. Zinin and A. Roy, "Detecting Inactive Neighbors over OSPF
   Demand Circuits (DC)", RFC 3883, October 2004

30. Author's Addresses

   Vishwas Manral
   SiNett Corp,
   Embassy Icon Annexe,
   2/1, Infantry Road,
   Bangalore, India


   Manav Bhatia
   Riverstone Networks,
   3/1, J.P. Techno Parks,

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   Millers Road,
   Bangalore, India


   Yasuhiro Ohara
   Keio University, Shonan Fujisawa Campus
   5322 Endo, Fujisawa
   Kanagawa, Japan 252-8520

   Phone: +81-(0)466-47-5111


31. Appendix

   The maximum size of an LSP is 1492 bytes.

   Available space = 1492 - 27 (Header) = 1465 bytes for TLVs.

   Thus an IS-IS router has theoretically up to 256*1465 of space to
   pack IP reachability TLVs.

   The following calculation enables us to determine the number of IP
   prefixes that can be advertised in an LSP.

   The following constraints are to be considered in the calculation:

   The maximum size (maxLSPsize) of an LSP is 1492 bytes.
   The LSP header (lspHeadersize) is 27 bytes.
   The maximum length of a TLV (maxTLVlength) is 255 bytes.

   Each TLV 128 consists of type (1 byte), length (1 byte), and IP
   prefixes (n x 12 bytes) up to total of 255 bytes. The maximum number
   of fragments of an LSP (maxLSPfragments) is 256.

   The number of fragments is determined from the 1-byte LSP Number
   field in the LSP identifier.

   The first fragment contains other TLVs, and the remaining 255
   fragments are packed with only TLV 128.

   The actual calculation is as follows:

   The total space available for TLVs in an LSP is
   TLVSpace = maxLSPsize - lspHeadersize = 1492 - 27 = 1465 bytes

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   The number of TLVs that can fit into TLVSpace is 1465/255 = 5.7,
   approximately 6

   Assuming a 1-byte Type field and 1-byte Length field, overhead for 6
   TLVs is 6 x 2 = 12 bytes.

   Actual space available for prefixes is 1465 - 12 bytes overhead =
   1453 bytes

   Number of prefixes, each 12 bytes (address + subnet mask + metric)
   that can fit into TLVSpace is 1453/12 = 121.08 (approximately 121 IP
   prefixes per LSP)

   Considering that few other TLVs can be generated by the router, the
   number of IP prefixes that can be supported per IS-IS router is 256
   fragments, each containing 121 prefixes, for a total of 30,976

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33. Disclaimer of Validity

   This document and the information contained herein are provided on an

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34. Full Copyright Notice

   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.

35. Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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