Network Working Group A. Zinin
Request for Comments: 3509 Alcatel
Category: Informational A. Lindem
Alternative Implementations of OSPF Area Border Routers
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright (C) The Internet Society (2003). All Rights Reserved.
Open Shortest Path First (OSPF) is a link-state intra-domain routing
protocol used for routing in IP networks. Though the definition of
the Area Border Router (ABR) in the OSPF specification does not
require a router with multiple attached areas to have a backbone
connection, it is actually necessary to provide successful routing to
the inter-area and external destinations. If this requirement is not
met, all traffic destined for the areas not connected to such an ABR
or out of the OSPF domain, is dropped. This document describes
alternative ABR behaviors implemented in Cisco and IBM routers.
An OSPF routing domain can be split into several subdomains, called
areas, which limit the scope of LSA flooding. According to [Ref1] a
router having attachments to multiple areas is called an "area border
router" (ABR). The primary function of an ABR is to provide its
attached areas with Type-3 and Type-4 LSAs, which are used for
describing routes and AS boundary routers (ASBRs) in other areas, as
well as to perform actual inter-area routing.
Zinin, et al. Informational [Page 1]RFC 3509 OSPF ABR Behavior April 20031.2 Motivation
In OSPF domains the area topology is restricted so that there must be
a backbone area (area 0) and all other areas must have either
physical or virtual connections to the backbone. The reason for this
star-like topology is that OSPF inter-area routing uses the
distance-vector approach and a strict area hierarchy permits
avoidance of the "counting to infinity" problem. OSPF prevents
inter-area routing loops by implementing a split-horizon mechanism,
allowing ABRs to inject into the backbone only Summary-LSAs derived
intra-area routes, and limiting ABRs' SPF calculation to consider
only Summary-LSAs in the backbone area's link-state database.
The last restriction leads to a problem when an ABR has no backbone
connection (in OSPF, an ABR does not need to be attached to the
backbone). Consider a sample OSPF domain depicted in the Figure 1.
. Area 0 .
. +--+ .. +--+ .
. .. .
. +--+ .
. Area1 |R3| Area2 .
. +--+ +--+ .
. .. |R4| .
. . . +--+ .
Figure 1. ABR dropping transit traffic
In this example R1, R2, and R3 are ABRs. R1 and R2 have backbone
connections, while R3 doesn't.
Following the section 12.4.1 of [Ref1], R3 will identify itself as an
ABR by setting the bit B in its router-LSA. Being an ABR, R3 can
only consider summary-LSAs from the backbone when building the
routing table (according to section 16.2 of [Ref1]), so it will not
have any inter-area routes in its routing table, but only intra-area
routes from both Area 1 and Area 2. Consequently, according to
section 12.4.3 of [Ref1], R3 will originate into Areas 1 and 2 only
summary-LSAs covering destinations in the directly attached areas,
i.e., in Area 2---the summary-LSAs for Area 1, and in Area 1---the
summary-LSAs for Area 2.
Zinin, et al. Informational [Page 2]RFC 3509 OSPF ABR Behavior April 2003
At the same time, router R2, as an ABR connected to the backbone,
will inject into Area 2 summary-LSAs describing the destinations in
Area 0 (the backbone), Area 1 and other areas reachable through the