Network Working Group S. Mirtorabi
Request for Comments: 5185 Nuova Systems
Category: Standards Track P. Psenak
A. Lindem, Ed.
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.
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.
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
Consider the following topology:
Area 1 Area 1
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
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
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
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 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.
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
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)
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.
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 22.214.171.124 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-
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 126.96.36.199 of [OSPFV3]).
5. Security Considerations
This document does not raise any security issues that are not already
covered in [OSPF] or [OSPFV3].
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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