Network Working Group S. Mirtorabi
Internet-Draft Nuova Systems
Intended status: Standards Track P. Psenak
Expires: September 2, 2008 Cisco Systems
A. Lindem (Editor)
A. Oswal
Redback Networks
March 2008
OSPF Multi-Area Adjacency
draft-ietf-ospf-multi-area-adj-08.txt
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Abstract
This document describes an extension to the Open Shortest Path First
(OSPF) protocol to allow a single physical link to be shared by
multiple areas. This is necessary to allow the link to be considered
an intra-area link in multiple areas. This would create an intra-
area path in each of the corresponding areas sharing the same link.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements notation . . . . . . . . . . . . . . . . . . 3
1.2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Possible Solutions . . . . . . . . . . . . . . . . . . . . 4
1.4. Proposed Solution . . . . . . . . . . . . . . . . . . . . 4
2. Functional Specifications . . . . . . . . . . . . . . . . . . 5
2.1. Multi-Area Adjacency Configuration and Neighbor
Discovery . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Multi-Area Adjacency Packet Transmission . . . . . . . . . 5
2.3. Multi-Area Adjacency Control Packet Reception Changes . . 5
2.4. Interface Data Structure . . . . . . . . . . . . . . . . . 6
2.5. Interface FSM . . . . . . . . . . . . . . . . . . . . . . 6
2.6. Neighbor Data Structure and Neighbor FSM . . . . . . . . . 6
2.7. Advertising Multi-Area Adjacencies . . . . . . . . . . . . 6
3. Compatibility . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1. Adjacency Endpoint Compatibility . . . . . . . . . . . . . 8
4. OSPFv3 Applicability . . . . . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1. Normative References . . . . . . . . . . . . . . . . . . . 12
7.2. Informative References . . . . . . . . . . . . . . . . . . 12
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . . . . 15
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1. Introduction
1.1. 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
[RFC-KEYWORDS].
1.2. Motivation
It is often a requirement to have an Open Shortest Path First (OSPF)
[OSPF] link in multiple areas. This will allow the link to be
considered as an intra-area path in each area and be preferred over
higher cost links. A simple example is to use a high-speed backbone
link between two Area Border Routers (ABRs) to create multi-area
adjacencies belonging to different areas.
Consider the following topology:
R1-------Backbone------R2
| |
Area 1 Area 1
| |
R3--------Area 1--------R4
Multi-Link Topology
The backbone link between R1 and R2 is a high-speed link and it is
desirable to forward Area 1's traffic between R1 and R2 over that
link. In the current OSPF specification, intra-area paths are
preferred over inter-area paths. As a result, R1 will always route
traffic to R4 through Area 1 over the lower speed links. R1 will
even use the intra-area Area 1 path though R3 to get to area 1
networks connected to R2. An OSPF virtual link cannot be used to
solve this problem without moving the link between R1 and R2 to area
1. This is not desirable if the physical link is, in fact, part of
the network's backbone topology.
The protocol extension described herein will rectify this problem by
allowing the link between R1 and R2 to be part of both the backbone
and Area 1.
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1.3. Possible Solutions
For numbered interfaces, the OSPF (Open Shortest Path First)
specification [OSPF] allows a separate OSPF interface to be
configured in each area using a secondary address. The disadvantages
of this approach are that it requires additional IP address
configuration, doesn't apply to unnumbered interfaces, and
advertising secondary addresses will result in a larger overall
routing table.
Allowing a link with a single address to simply be configured in
multiple areas would also solve the problem. However, this would
result in the subnet corresponding to the interface residing in
multiple areas that is contrary to the definition of an OSPF area as
a collection of subnets.
Another approach is to simply allow unnumbered links to be configured
in multiple areas. Section 8.2. of the OSPF specification already
specifies that the OSPF area ID should be used to de-multiplex
received OSPF packets. One limitation of this approach is that
multi-access networks are not supported. Although this limitation
may be overcome for LAN media with support of "Point-to-Point
operation over LAN in link-state routing protocols" [P2PLAN], it may
not be acceptable to configure the link as unnumbered due to network
management policies. Many popular network management applications
individually test the path to each interface and an IP address
facilitates this task.
1.4. Proposed Solution
ABRs will simply establish multiple adjacencies belonging to
different areas. Each multi-area adjacency is announced as a point-
to-point unnumbered link in the configured area. This point-to-point
link will provide a topological path for that area. The first or
primary adjacency using the link will operate and advertise the link
in a manner consistent with RFC 2328 [OSPF].
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2. Functional Specifications
2.1. Multi-Area Adjacency Configuration and Neighbor Discovery
Multi-area adjacencies are configured between two routers having a
common interface. On point-to-point interfaces, there is no need to
configure the neighbor's address since there can be only one
neighbor. For all other network types, the neighbor address of each
multi-area adjacency must be configured or automatically discovered
via a mechanism external to OSPF.
2.2. Multi-Area Adjacency Packet Transmission
On point-to-point interfaces, OSPF control packets are sent to the
AllSPFRouters address. For all other network types, OSPF control
packets are unicast to the remote neighbor's IP address.
2.3. Multi-Area Adjacency Control Packet Reception Changes
Receiving protocol packets is described in section 8.2 of [OSPF].
The text starting with the second paragraph and continuing through
the third bullet beneath that paragraph is changed as follows:
Next, the OSPF packet header is verified. The fields specified in
the header must match those configured for the receiving interface.
If they do not, the packet should be discarded:
o The version number field must specify protocol version 2.
o The Area ID found in the OSPF header must be verified. If all of
the following cases fail, the packet should be discarded. The
Area ID specified in the header must either:
1. Match the Area ID of the receiving interface. In this case,
the packet has been sent over a single hop. Therefore, the
packet's IP source address is required to be on the same
network as the receiving interface. This can be verified by
comparing the packet's IP source address to the interface's IP
address, after masking both addresses with the interface mask.
This comparison should not be performed on point-to-point
networks. On point-to-point networks, the interface addresses
of each end of the link are assigned independently, if they
are assigned at all.
2. Indicate a non-backbone area. In this case, the packet has
been sent over a multi-area adjacency. If the area-id matches
the configured area for multi-area adjacency, the packet is
accepted and is from now on associated with the multi-area
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adjacency for that area.
3. Indicate the backbone. In this case, the packet has been sent
over a virtual link or a multi-area adjacency.
o For virtual links, the receiving router must be an area border
router, and the Router ID specified in the packet (the source
router) must be the other end of a configured virtual link. The
receiving interface must also attach to the virtual link's
configured transit area. If all of these checks succeed, the
packet is accepted and is from now on associated with the virtual
link.
o For multi-area adjacencies, if the area-id matches the configured
area for the multi-area adjacency, the packet is accepted and is
from now on associated with the multi-area adjacency for that
area.
o Note that if there is a match for both a virtual link and a multi-
area adjacency then this is a configuration error that should be
handled at the configuration level.
o Packets whose IP destination is AllDRouters should only be
accepted if the state of the receiving interface is DR or Backup
(see Section 9.1 [OSPF]).
o [...] The remainder of section 8.2 [OSPF] is unchanged.
2.4. Interface Data Structure
An OSPF interface data structure is built for each configured multi-
area adjacency as specified in section 9 of [OSPF]. The interface
type will always be point-to-point.
2.5. Interface FSM
The interface FSM will be the same as a point-to-point link
irrespective of the underlying physical link.
2.6. Neighbor Data Structure and Neighbor FSM
Both the neighbor data structure and neighbor FSM are the same as for
standard OSPF, specified in section 10 of [OSPF].
2.7. Advertising Multi-Area Adjacencies
Multi-area adjacencies are announced as unnumbered point-to-point
links. Once the router's multi-area adjacency reaches the FULL state
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it will be added as a link type 1 to the Router Link State
Advertisement (LSA) with:
Link ID = Remote's Router ID
Link Data = Neighbor's IP Address or IfIndex (if the underlying
interface is unnumbered).
This will announce a topological path through the corresponding area.
While advertising the neighbor's IP address in the link data isn't
consistent with the unnumbered link model, it is required to
eliminate ambiguity when there are parallel point-to-point
adjacencies.
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3. Compatibility
All mechanisms described in this document are backward compatible
with standard OSPF implementations [OSPF].
3.1. Adjacency Endpoint Compatibility
Since multi-area adjacencies are modeled as unnumbered point-to-point
links, it is only necessary for the router at the other end of the
adjacency to model the adjacency as a point-to-point link. However,
the network topology will be easier to represent and troubleshoot if
both neighbors are symmetrically configured as multi-area
adjacencies.
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4. OSPFv3 Applicability
The mechanisms defined in this document also apply to OSPFv3
[OSPFV3]. As in OSPF, a multi-area adjacency is advertised as an
unnumbered point-to-point link in the advertising router's router-
LSA. Since OSPFv3 router-LSA links are independent of addressing
semantics and unambiguously identify OSPFv3 neighbors (refer section
3.4.3.1 [OSPFV3]), the change to router-LSA links described in
Section 2.7 is not applicable to OSPFv3. Furthermore, no prefixes
corresponding to the multi-area adjacency are advertised in the
router's intra-area-prefix-LSA.
A link-LSA SHOULD NOT be advertised for a multi-area adjacency. The
neighbor's IPv6 link local address can be learned in other ways,
e.g., it can be extracted from the IPv6 header of Hello packets
received over the multi-area adjacency. The neighbor IPv6 link local
address is required for the OSPFv3 route next-hop calculation on
multi-access networks (refer section 3.8.1.1 [OSPFV3]).
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5. Security Considerations
This document does not raise any security issues that are not already
covered in [OSPF] or [OSPFV3].
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6. IANA Considerations
This document does not require any IANA assignments or action.
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7. References
7.1. Normative References
[OSPF] Moy, J., "OSPF Version 2", RFC 2328, April 1998.
[OSPFV3] Coltun, R., Ferguson, D., and J. Moy, "OSPF for IPv6",
RFC 2740, December 1999.
[RFC-KEYWORDS]
Bradner, S., "Key words for use in RFC's to Indicate
Requirement Levels", RFC 2119, March 1997.
7.2. Informative References
[P2PLAN] Shen, N. and A. Zinin, "Point-to-point operation over LAN
in link-state routing protocols",
draft-ietf-isis-igp-p2p-over-lan-06.txt (work in
progress).
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Appendix A. Acknowledgments
The authors wish to acknowledge Pat Murphy for bringing focus to the
requirement.
Thanks to Mitchell Erblich's for his last call review and comments.
Thanks to Padma Pillay-Esnault for her last call review and comments.
Also thanks to Padma for comments on the OSPFv3 applicability section
that was last called separately.
Thanks to Nischal Seth for pointing out that the document
inadvertently precluded point-to-point over LAN interfaces.
Thanks to Ben Campbell for performing the General Area Review.
The RFC text was produced using Marshall Rose's xml2rfc tool.
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Authors' Addresses
Sina Mirtorabi
Nuova Systems
3 West Plumeria Drive
San Jose, CA 95134
USA
Email: sina@nuovasystems.com
Peter Psenak
Cisco Systems
Apollo Business Center
Mlynaki nivy 43
821 09 Bratislava, Sovakia
Slovakia
Email: ppsenak@cisco.com
Acee Lindem
Redback Networks
102 Carric Bend Court
Cary, NC 27519
USA
Email: acee@redback.com
Anand Oswal
Redback Networks
300 Holger Way
San Jose, CA 95134
USA
Email: aoswal@redback.com
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