Network Working Group P. Psenak
Internet-Draft S. Mirtorabi
Expires: August 5, 2006 A. Roy
L. Nguyen
P. Pillay-Esnault
Cisco Systems
February 1, 2006
Multi-Topology (MT) Routing in OSPF
draft-ietf-ospf-mt-06.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This draft describes an extension to OSPF in order to define
independent IP topologies called Multi-Topologies (MTs). The MT
extension can be used for computing different paths for unicast
traffic, multicast traffic, different classes of service based on
flexible criteria, or an in-band network management topology.
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[M-ISIS] describes a similar mechanism for ISIS.
An optional extension to exclude selected links from the default
topology is also described.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Differences with RFC 1583 TOS Based Routing . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1 Requirements notation . . . . . . . . . . . . . . . . . . 5
2.2 Terms . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Base MT Functional Specifications . . . . . . . . . . . . . . 6
3.1 MT Area Boundary . . . . . . . . . . . . . . . . . . . . . 6
3.2 Adjacency for MTs . . . . . . . . . . . . . . . . . . . . 6
3.3 Sending OSPF control packets . . . . . . . . . . . . . . . 6
3.4 Advertising MT Adjacencies and the Corresponding IP
Prefixes . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.4.1 Advertising MT Adjacencies and the Corresponding
IP Prefixes . . . . . . . . . . . . . . . . . . . . . 6
3.4.2 Inter-Area and External Routing . . . . . . . . . . . 7
3.5 Flushing MT Information . . . . . . . . . . . . . . . . . 7
3.6 MT SPF Computation . . . . . . . . . . . . . . . . . . . . 7
3.7 MT-ID Values . . . . . . . . . . . . . . . . . . . . . . . 8
3.8 Forwarding in MT . . . . . . . . . . . . . . . . . . . . . 8
4. Default Topology Link Exclusion Functional Specifications . . 9
4.1 Exclusion of Links in the Default Topology . . . . . . . . 9
4.2 New Area Data Structure Parameter . . . . . . . . . . . . 9
4.3 Adjacency Formation with Link Exclusion Capability . . . . 10
4.4 OSPF Control Packets Transmission Over Excluded Links . . 11
4.5 OSPF LSA Advertisement and SPF Computation for
Excluded Links . . . . . . . . . . . . . . . . . . . . . . 11
5. Interoperability between MT Capable and Non-MT Capable
Routers . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.1 Demand Circuit Compatibility Considerations . . . . . . . 12
6. Migration from non-MT-Area to MT-area . . . . . . . . . . . . 13
7. Security Considerations . . . . . . . . . . . . . . . . . . . 14
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
9.1 Normative References . . . . . . . . . . . . . . . . . . . 16
9.2 Informative References . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 16
A. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18
B. OSPF data formats . . . . . . . . . . . . . . . . . . . . . . 19
B.1 Router-LSAs . . . . . . . . . . . . . . . . . . . . . . . 19
B.2 Network-LSAs . . . . . . . . . . . . . . . . . . . . . . . 20
B.3 Summary-LSAs . . . . . . . . . . . . . . . . . . . . . . . 20
B.4 AS-External-LSAs . . . . . . . . . . . . . . . . . . . . . 21
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B.5 NSSA-LSAs . . . . . . . . . . . . . . . . . . . . . . . . 22
Intellectual Property and Copyright Statements . . . . . . . . 23
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1. Introduction
OSPF uses a fixed packet format, therefore it is not easy to
introduce any backward compatible extensions. However, the OSPF
specification [OSPF] introduced TOS metric in an earlier
specification [RFC1583] in order to announce a different link cost
based on TOS. TOS based routing as described in [RFC1583] was never
deployed and was subsequently deprecated.
We propose to reuse the TOS based metric fields. They have been
redefined as MT-ID and MT-ID Metric and are used to advertise
different topologies by advertising separate metrics for each of
them.
1.1 Differences with RFC 1583 TOS Based Routing
Multi-topology routing differs from RFC 1583 TOS based routing in the
following ways:
1. With RFC 1583 TOS routing, the TOS or DSCP in the IP header is
mapped directly to the the corresponding OSPF SPF calculation and
routing table. This limits the number and definition of the
topologies to the 16 TOS values specified is section 12.3 of RFC
1583 [RFC1583]. With multi-topology routing, the classification
of what type of traffic maps to which topology is not within the
scope of the document.
2. With RFC 1583 TOS routing, traffic which is unreachable in the
routing table associated with the corresponding TOS will revert
to the TOS 0 routing table. With multi-topology routing, this is
optional.
3. With RFC 1583 TOS routing, individual links or prefixes could not
be excluded from a topology. If the LSA options T-bit was set,
all links or prefixes were either advertised explicitly or
defaulted to the TOS 0 metric. With multi-topology routing,
links or prefixes that are not advertised for a specific topology
do not exist in that topology.
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2. Terminology
2.1 Requirements notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [RFC2119].
2.2 Terms
We define the following terminology in this document:
Non-MT router
Routers that do not have the MT capability
MT router
Routers that have MT capability as described in this document
MT-ID
Renamed TOS field in LSAs to represent multitopology ID.
Default topology
Topology that is built using the TOS 0 metric (default metric)
MT topology
Topology that is built using the corresponding MT-ID metric
MT
Shorthand notation for MT topology
MT#0 topology
Representation of TOS 0 metric in MT-ID format
Non-MT-Area
An area that contains only non-MT routers
MT-Area
An area that contains both non-MT routers and MT routers or only
MT routers
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3. Base MT Functional Specifications
3.1 MT Area Boundary
Each OSPF interface belongs to a single area and all MTs sharing that
link need to belong to the same area. Therefore the area boundaries
for all MTs are the same but each MT's attachment to the area is
independent.
3.2 Adjacency for MTs
Each interface can be configured to belong to a set of topologies. A
single adjacency will be formed with neighbors on the interface even
if the interface is configured to participate in multiple topologies.
Furthermore, adjacency formation will be independent of the
topologies configured for the interface or neighbors on that
interface.
3.3 Sending OSPF control packets
Sending OSPF control packets is unchanged from RFC2328. For OSPF
control packets sent to the remote end of a virtual link, the transit
area path MUST be composed solely of links in the default topology
and the OSPF control packets MUST be forwarded using the default
topology.
3.4 Advertising MT Adjacencies and the Corresponding IP Prefixes
We will reuse the TOS metric field in order to advertise a topology
and prefixes belonging to that topology. The TOS field is redefined
as MT-ID in the payload of Router-LSAs, Summary-LSAs, NSSA-LSAs, and
AS-External-LSAs (see Appendix A).
MT-ID metrics in LSAs SHOULD be in ascending order of MT-ID. If an
MT-ID exists in an LSA or router link multiple times, the metric in
the first MT-ID instance MUST be used.
3.4.1 Advertising MT Adjacencies and the Corresponding IP Prefixes
When a router establishes a FULL adjacency over a link that belongs
to a set of MTs, it will advertise the corresponding cost for each
MT-ID.
By default, all links are included in default topology and all
advertised prefixes belonging to the default topology will use the
TOS0 metric the same as in standard OSPF [OSPF].
Each MT has its own MT-ID metric field. When a link is not part of a
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given MT, the corresponding MT-ID metric is excluded from the LSA.
The Network-LSA does not contain any MT information since the DR is
shared by all MTs. Hence, there is no change to the Network-LSA.
3.4.2 Inter-Area and External Routing
In Summary-LSAs, NSSA-LSAs, and AS-External-LSAs, the TOS metric
fields are defined as MT-ID metric fields and are used in order to
advertise prefix and router reachability in the corresponding
topology.
When a router originates a Summary-LSA, NSSA-LSA, or AS-External-LSA
that belongs to a set of MTs, it will include the corresponding cost
for each MT-ID. By default, the router participates in the default
topology and uses the TOS0 metric for the default topology the same
as in standard OSPF [OSPF].
Setting the P-bit in NSSA-LSAs is topology independent and pertains
to all MT-ID advertised in the body of the LSA.
3.5 Flushing MT Information
When a certain link or prefix that existed or was reachable in a
certain topology is no longer part of that topology or is unreachable
in that topology, a new version of the LSA must be originated
excluding metric information representing the link or prefix in that
topology.
The MT metric in the Router-LSA can also be set to the maximum
possible metric to enable the router to become a stub in a certain
topology [STUB].
3.6 MT SPF Computation
By considering MT-ID metrics in the LSAs, OSPF will be able to
compute multiple topologies and find paths to IP prefixes for each MT
independently. A separate SPF will be computed for each MT-ID to
find independent paths to IP prefixes. Each nexthop computed during
the MT SPF MUST belong to the same MT.
Network-LSAs are used by all topologies during the SPF computation.
During the SPF for a given MT-ID, only the links and metrics for that
MT-ID will be considered. Entries in the Router Routing table will
be MT-ID specific.
During the SPF computation for the default topology only the TOS0
metric is considered during the SPF computation.
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3.7 MT-ID Values
Since AS-External-LSAs use the high order bit in the MT-ID field (E
bit) for the external metric-type, only MT-IDs in the range [0-127]
are valid. The following MT-ID values are reserved:
0 - Reserved for advertising the metric associated with the
default topology (see Section 4.2)
1 - Reserved for advertising the metric associated with the
default multicast topology
MT-IDs [128-255] SHOULD be ignored.
3.8 Forwarding in MT
It's outside of the scope of this document to specify how the
information in various topology specific forwarding structures are
used during packet forwarding or how incoming packets are associated
with the corresponding topology. For correct operation, both
forwarding behavior and methods of associating incoming packets to a
corresponding topology must be consistently applied in the network.
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4. Default Topology Link Exclusion Functional Specifications
The multi-topologies imply that all the routers participate in the
default topology. However, it can be useful to exclude some links
from the default topology and reserve them for some specific classes
of traffic.
The multi-topologies extension for default topology link or prefix
exclusion is described in the following subsections.
4.1 Exclusion of Links in the Default Topology
OSPF does not have the notion of an unreachable link. All links can
have a maximum metric of 0xFFFF advertised in the Router-LSA. The
link exclusion capability requires routers to ignore TOS0 metrics in
Router-LSAs in the default topology and to alternately use the MT-
ID#0 metric to advertise the metric associated with the default
topology. Hence, all routers within an area MUST agree on how the
metric for default topology will be advertised.
The unused T-bit is defined as the MT-bit in the option field in
order to assure that a multi-topology link-excluding capable router
will only form an adjacency with another similarly configured router.
+---+---+---+---+---+---+---+---+
|DN |O |DC |EA |NP |MC |E |MT |
+---+---+---+---+---+---+---+---+
MT-bit: If DefaultExclusionCapability is enable, this bit MUST
be set in Hello packets and SHOULD be set in Database
Description packet (see Section 4.2).
4.2 New Area Data Structure Parameter
We define a new parameter in the Area Data Structure:
DefaultExclusionCapability
This configurable parameter ensures that all routers in an area
have this capability enabled before the default topology can be
disabled on a router link in the area without causing backward
compatibility problems.
When an area data structure is created the DefaultExclusionCapability
is disabled by default.
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If DefaultExclusionCapability is disabled:
o The MT-bit MUST be cleared in Hello packet and SHOULD be cleared
in Database Description packets.
o If a link participates in a non-default topology, it is
automatically included in the default topology to support backward
compatibility between MT and non-MT routers. This is accomplished
through advertisement via the TOS0 metric field the same as in
standard OSPF [OSPF].
If DefaultExclusionCapability is enabled:
o The MT-bit MUST be set in Hello and SHOULD be set in Database
Description packets
o The router will only accept a Hello packet if the MT-bit is set
(see Section 4.3)
When DefaultExclusionCapability is set to enabled a router is said to
be operating in DefaultExclusionCapability mode.
4.3 Adjacency Formation with Link Exclusion Capability
In order to have a smooth transition from a non-MT area to an MT-
area, an MT router with DefaultExclusionCapability disabled will form
adjacencies with non-MT routers and will include all links as part of
default topology.
A link may cease participating in default topology if
DefaultExclusionCapability is set to enabled. In this state, a
router will only form adjacency with routers that set the MT-bit in
their Hello packets. This will ensure that all routers have
DefaultExclusionCapability enabled before the default topology can be
disabled on a link.
Receiving OSPF Hello packets as defined in section 10.5 of [OSPF] is
modified as follows:
o If the DefaultExclusionCapability in the Area Data structure is
set to enabled, Hello packets are discarded if the the received
packet does not have the MT-bit set in the header options.
Receiving OSPF Database Description packets as defined in section
10.6 of [OSPF] is unchanged. While packet options are validated in
hello packets, the only option checking performed for Database
Description packets is assuring that the options do not change during
the database exchange process.
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4.4 OSPF Control Packets Transmission Over Excluded Links
If DefaultExclusionCapability is enabled, the default topology can be
disabled on an interface. Disabling the default topology on an
interface does not impact the installation of connected routes for
the interface in the default topology. It only affects what a router
advertises in its Router-LSA.
This allows OSPF control packets to be sent and received over an
interface even if the default topology is disabled on the interface.
4.5 OSPF LSA Advertisement and SPF Computation for Excluded Links
When DefaultExclusionCapability is enabled and the link does not
participate in the default topology, the MT-ID#0 metric is not
advertised. The link's TOS0 metric is ignored during the default
topology SPF computation.
When DefaultExclusionCapability is enabled and a link participates in
the default topology, MT-ID#0 metric is used to advertise the metric
associated with the default topology. The link's TOS0 metric is
ignored during the default topology SPF computation.
Independent of the DefaultExclusionCapability setting, the TOS0
metric is used for Summary-LSAs, NSSA-LSAs, and AS-External-LSAs.
o If the prefix or router does not exist in the default topology,
the TOS0 metric is set to infinity (0xFFFFFF).
o If the prefix or router exists in default the topology, the TOS0
metric is used to advertise the metric in the default topology.
During the summary and external prefix calculation for the default
topology the TOS0 metric is used for Summary-LSAs, NSSA-LSAs, and AS-
External-LSAs.
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5. Interoperability between MT Capable and Non-MT Capable Routers
The default metric field is mandatory in all LSAs (even when metric
value is 0). Even when a link or prefix does not exist in the
default topology, a non-MT router will consider the zero value in the
metric field as a valid metric and consider the link or prefix as
part of the default topology.
In order to prevent the above problem, an MT capable router will
include all links as part of the default topology. If links need to
be removed from the default topology, an MT capable router MUST be
configured in DefaultExclusionCapability mode. In this mode,
routers will assure that all other routers in the area are in the
DefaultExclusionCapability mode before considering the MT-ID#0 metric
in the SPF calculation. Only then can the TOS0 metric field in
Router LSAs be safely ignored during the default topology SPF
computation.
Note that for any prefix or router to become reachable in a certain
topology, a contiguous path inside that topology must exist between
the calculating router and the destination prefix or router.
5.1 Demand Circuit Compatibility Considerations
A change to an area's DefaultExclusionCapability requires additional
processing for area neighbors that are suppressing hellos as
specified in "Extending OSPF to Support Demand Circuits" [DEMAND].
When the DefaultExclusionCapability for an area is changed, hello
suppression must be disabled for these neighbors for a period of
RouterDeadInterval seconds. This implies that hello packets are sent
with the DC bit clear as specified in section 3.2.1 of [DEMAND]
during this period. After RouterDeadInterval seconds, either the
adjacency will be taken down due to rejection of hellos with a
conflicting MT-bit or hello suppression will be renegotiated.
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6. Migration from non-MT-Area to MT-area
Introducing MT-OSPF into a network can be done gradually to allow MT
routers and non-MT routers to participate in the default topology
while MT routers participate in other topologies.
If there is a requirement to exclude some links from the default
topology in an area, all routers in the area MUST be in
DefaultExclusionCapability mode. In this section we describe the
migration steps to consider while transitioning from a non-MT network
to an MT network.
Consider a network with a backbone area and a set of non-backbone
areas functioning in standard OSPF mode. We would like to migrate to
an MT network either partially or completely.
1. As required, part of an area is upgrade to be MT capable. The MT
routers will interact with non-MT routers in the default topology
and participate in other topologies as required.
2. If a new non-backbone area is created for MT routers, it may be
configured in DefaultExclusionCapability mode since there is no
interaction required with non-MT routers. In this mode, the
default topology can be excluded on links as required.
3. If there is more than one non-backbone areas where MT is being
used, it is desirable that the backbone area first be upgraded to
be MT capable so that inter-area routing is assured for MT
destinations in different areas.
4. Gradually the whole network can be made MT capable.
Note that inter-area routing for the MT-area still depends on the
backbone area. Therefore, if different areas configured for a given
topology need to communicate, the backbone area also needs to be
configured for this topology.
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7. Security Considerations
This document does not raise any security issues that are not already
covered in [OSPF].
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8. IANA Considerations
The T-bit as defined in [RFC1583] for a router's TOS capability is
redefined as the MT-bit in this document. Similarly, the TOS field
for Router-LSAs, Summary-LSAs, NSSA-LSAs, and AS-External LSAs as
defined in [OSPF] is redefined as MT-ID in this document.
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9. References
9.1 Normative References
[DEMAND] Moy, J., "Extending OSPF to Support Demand Circuits",
RFC 1793, April 1995.
[NSSA] Murphy, P., "The OSPF Not-So-Stubby Area (NSSA) Option",
RFC 3101, January 2003.
[OSPF] Moy, J., "OSPF Version 2", RFC 2328, April 1998.
[RFC1583] Moy, J., "OSPF Version 2", RFC 1583, March 1994.
[RFC2119] Bradner, S., "Key words for use in RFC's to Indicate
Requirement Levels", RFC 2119, March 1997.
9.2 Informative References
[M-ISIS] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
Topology (MT) Routing in IS-IS",
draft-ietf-isis-wg-multi-topology-07.txt (work in
progress).
[STUB] Retana, A., Nguyen, L., White, R., Zinin, A., and D.
McPherson, "OSPF Stub Router Advertisement", RFC 3137,
June 2001.
Authors' Addresses
Peter Psenak
Cisco Systems
Parc Pegasus, De Kleetlaan 6A
1831 Diegem
Belgium
Email: ppsenak@cisco.com
Sina Mirtorabi
Cisco Systems
225 West Tasman Drive
San Jose, CA 95134
USA
Email: sina@cisco.com
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Abhay Roy
Cisco Systems
225 West Tasman Drive
San Jose, CA 95134
USA
Email: akr@cisco.com
Liem Nguyen
Cisco Systems
7025 Kit Creek Road
Research Triangle Park, NC 27709
USA
Email: lhnguyen@cisco.com
Padma Pillay-Esnault
Cisco Systems
225 West Tasman Drive
San Jose, CA 95134
USA
Email: ppe@cisco.com
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Appendix A. Acknowledgments
The authors would like to thank Scott Sturgess, Alvaro Retana, David
Kushi, Yakov Rekhter, Tony Przygienda, and Naiming Shen for their
comments on the document. Special thanks to Acee Lindem for editing
and to Tom Henderson for an extensive review during the OSPF Working
Group last call.
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Appendix B. OSPF data formats
LSA content defined in [OSPF] is modified to introduce the MT-ID.
B.1 Router-LSAs
Router-LSAs are the Type 1 LSAs. Each router in an area originates a
router-LSA. The LSA describes the state and cost of the router's
links (i.e., interfaces) to the area. All of the router's links to
the area must be described in a single router-LSA. For details
concerning the construction of router-LSAs, see Section 12.4.1
[OSPF].
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|*|*|*|N|W|V|E|B| 0 | # links |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | # MT-ID | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MT-ID | 0 | MT-ID metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MT-ID | 0 | MT-ID metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
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B.2 Network-LSAs
Network-LSAs are the Type 2 LSAs. A network-LSA is originated for
each broadcast and NBMA network in the area which supports two or
more routers. The network-LSA is originated by the network's
Designated Router. The LSA describes all routers attached to the
network, including the Designated Router itself. The LSA's Link
State ID field lists the IP interface address of the Designated
Router.
The distance from the network to all attached routers is zero. This
is why metric fields need not be specified in the network-LSA. For
details concerning the construction of network-LSAs, see Section
12.4.2 [OSPF].
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attached Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
Note that network LSA does not contain any MT-ID fields as the cost
of the network to the attached routers is 0 and DR is shared by all
topologies.
B.3 Summary-LSAs
Summary-LSAs are the Type 3 and 4 LSAs. These LSAs are originated by
area border routers. Summary-LSAs describe inter-area destinations.
For details concerning the construction of summary- LSAs, see Section
12.4.3 [OSPF].
Type 3 summary-LSAs are used when the destination is an IP network.
In this case the LSA's Link State ID field is an IP network number
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(if necessary, the Link State ID can also have one or more of the
network's "host" bits set; see Appendix E [OSPF] for details). When
the destination is an AS boundary router, a Type 4 summary-LSA is
used, and the Link State ID field is the AS boundary router's OSPF
Router ID. (To see why it is necessary to advertise the location of
each ASBR, consult Section 16.4 of [OSPF]). Other than the
difference in the Link State ID field, the format of Type 3 and 4
summary-LSAs is identical.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 3 or 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MT-ID | MT-ID metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MT-ID | MT-ID metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
B.4 AS-External-LSAs
AS-external-LSAs are the Type 5 LSAs. These LSAs are originated by
AS boundary routers, and describe destinations external to the AS.
For details concerning the construction of AS-external-LSAs, see
Section 12.4.3 [OSPF].
AS-external-LSAs usually describe a particular external destination.
For these LSAs the Link State ID field specifies an IP network number
(if necessary, the Link State ID can also have one or more of the
network's "host" bits set; see Appendix E [OSPF] for details). AS-
external-LSAs are also used to describe a default route. Default
routes are used when no specific route exists to the destination.
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When describing a default route, the Link State ID is always set to
DefaultDestination (0.0.0.0) and the Network Mask is set to 0.0.0.0.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 5 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E| 0 | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Forwarding address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| External Route Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E| MT-ID | MT-ID metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Forwarding address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| External Route Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E| MT-ID | MT-ID metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Forwarding address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| External Route Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
B.5 NSSA-LSAs
NSSA-LSAs are the Type 7 LSAs. These LSAs are originated by AS
boundary routers local to an NSSA, and describe destinations external
to the AS. The changes to NSSA-LSAs are identical to those for
External-LSAs (Appendix A.4.5). For details concerning the
construction of NSSA-LSAs see Section 2.4 [NSSA].
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