Network Working Group S. Mirtorabi
Internet-Draft Force10 Networks
Intended status: Standards Track P. Psenak
Expires: September 27, 2007 Cisco Systems
A. Lindem (Editor)
A. Oswal
Redback Networks
March 26, 2007
OSPF Multi-Area Adjacency
draft-ietf-ospf-multi-area-adj-07.txt
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Copyright (C) The IETF Trust (2007).
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Abstract
This document describes an extension to OSPF 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 to 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 . . . . . . . . . . . . . . . . . . . . 3
1.4. Proposed Solution . . . . . . . . . . . . . . . . . . . . 3
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
There could be a requirement to have a link in multiple areas in
order to allow the link to be considered as an intra-area link in
multiple areas and be preferred over high cost intra-area paths. 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.
1.3. Possible Solutions
For numbered interfaces, the OSPF 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.
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
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primary adjacency using the link will operate and advertise the link
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 physical point-to-point networks, 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 physical point-to-point networks, 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] and
is changed as follow:
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
adjacency for that area.
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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
it will be added as a link type 1 to the Router Link State
Advertisement (LSA) with:
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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,
it will be cleaner from a deployment standpoint for both neighbors to
be configured as multi-area adjacencies.
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4. OSPFv3 Applicability
All 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 no prefixes are associated with a multi-area adjacency,
it will have no impact on the advertising 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.
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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.
The RFC text was produced using Marshall Rose's xml2rfc tool.
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Authors' Addresses
Sina Mirtorabi
Force10 Networks
350 Holger Way
San Jose, CA 95134
USA
Email: sina@force10networks.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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