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Versions: (draft-mirtorabi-ospf-multi-area-adj) 00 01 02 03 04 05 06 07 08 09 RFC 5185

Network Working Group                                       S. Mirtorabi
Internet-Draft                                             Nuova Systems
Intended status: Standards Track                               P. Psenak
Expires: October 25, 2008                                  Cisco Systems
                                                      A. Lindem (Editor)
                                                                A. Oswal
                                                        Redback Networks
                                                          April 23, 2008


                       OSPF Multi-Area Adjacency
                 draft-ietf-ospf-multi-area-adj-09.txt

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   Copyright (C) The IETF Trust (2008).










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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 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].













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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 point-to-point links.  Once
   the router's multi-area adjacency reaches the FULL state it will be



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   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 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.

   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
   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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