OSPF Multi-Area Adjacency
RFC 5185

Network Working Group                                       S. Mirtorabi
Request for Comments: 5185                                 Nuova Systems
Category: Standards Track                                      P. Psenak
                                                           Cisco Systems
                                                          A. Lindem, Ed.
                                                                A. Oswal
                                                        Redback Networks
                                                                May 2008

                       OSPF Multi-Area Adjacency

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

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.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.2.  Possible Solutions  . . . . . . . . . . . . . . . . . . . . 3
     1.3.  Proposed Solution . . . . . . . . . . . . . . . . . . . . . 4
     1.4.  Requirements Notation . . . . . . . . . . . . . . . . . . . 4
   2.  Functional Specifications . . . . . . . . . . . . . . . . . . . 4
     2.1.  Multi-Area Adjacency Configuration and Neighbor
           Discovery . . . . . . . . . . . . . . . . . . . . . . . . . 4
     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 . . . . . . . . . . . . . . . . . . . . . . . . . 7
     3.1.  Adjacency Endpoint Compatibility  . . . . . . . . . . . . . 7
   4.  OSPFv3 Applicability  . . . . . . . . . . . . . . . . . . . . . 7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     6.1.  Normative References  . . . . . . . . . . . . . . . . . . . 8
     6.2.  Informative References  . . . . . . . . . . . . . . . . . . 8
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . . . 9

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1.  Introduction

1.1.  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 of this requirement is to use a
   high-speed link between two Area Border Routers (ABRs)in multiple
   areas.

   Consider the following topology:

                          R1-------Backbone------R2
                           |                      |
                         Area 1                 Area 1
                           |                      |
                          R3--------Area 1--------R4

                            Multi-Link Topology

   The backbone area 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 [OSPF], 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
   area and Area 1.

1.2.  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, it doesn't apply to unnumbered interfaces, and
   advertising secondary addresses will result in a larger overall
   routing table.

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   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 [OSPF]
   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 by pinging
   its IP address.

1.3.  Proposed Solution

   ABRs will simply establish multiple adjacencies belonging to
   different areas.  Each multi-area adjacency is announced as a point-
   to-point link in the configured area.  However, unlike numbered
   point-to-point links, no type 3 link is advertised for multi-area
   adjacencies.  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].

1.4.  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 RFC 2119
   [RFC-KEYWORDS].

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.

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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 a multi-area adjacency, the packet is
          accepted and is from now on associated with the multi-area
          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 ABR, 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.

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   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 of [OSPF]).

   o  [...]  The remainder of Section 8.2 of [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 Finite State Machine (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 point-to-point links.  Once
   the router's multi-area adjacency reaches the FULL state, 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).

   Unlike numbered point-to-point links, no type 3 link is advertised
   for multi-area 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 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.

4.  OSPFv3 Applicability

   The mechanisms defined in this document also apply to OSPFv3
   [OSPFV3].  As in OSPF, a multi-area adjacency is advertised as a
   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 to Section 3.4.3.1 of
   [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 to Section 3.8.1.1 of [OSPFV3]).

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.  References

6.1.  Normative References

   [OSPF]          Moy, J., "OSPF Version 2", STD 54, 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 RFCs to Indicate
                   Requirement Levels", BCP 14, RFC 2119, March 1997.

6.2.  Informative References

   [P2PLAN]        Shen, N. and A. Zinin, "Point-to-point operation over
                   LAN in link-state routing protocols", Work
                   in Progress.

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Appendix A.  Acknowledgments

   The authors wish to acknowledge Pat Murphy for convincing the OSPF WG
   to address 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.

   Thanks to Jari Arkko for comments during the IESG 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
   Mlynske nivy 43
   821 09 Bratislava
   Slovakia

   EMail: ppsenak@cisco.com

   Acee Lindem (editor)
   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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