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680 BigIron RX Series Configuration Guide
53-1001810-01
Overview of OSPF (Open Shortest Path First)
25
Algorithm for AS external LSA reduction
Figure 104 shows an example in which the normal AS External LSA reduction feature is in effect.
The behavior changes under the following conditions:
•
There is one ASBR advertising (originating) a route to the external destination, but one of the
following happens:
•
A second ASBR comes on-line
•
A second ASBR that is already on-line begins advertising an equivalent route to the same
destination.
In either case above, the router with the higher router ID floods the AS External LSAs and the
other router flushes its equivalent AS External LSAs. For example, if Router D is offline, Router
E is the only source for a route to the external routing domain. When Router D comes on-line, it
takes over flooding of the AS External LSAs to Router F, while Router E flushes its equivalent AS
External LSAs to Router F.
•
One of the ASBRs starts advertising a route that is no longer equivalent to the route the other
ASBR is advertising. In this case, the ASBRs each flood AS External LSAs. Since the LSAs
either no longer have the same cost or no longer have the same next-hop router, the LSAs are
no longer equivalent, and the LSA reduction feature no longer applies.
•
The ASBR with the higher router ID becomes unavailable or is reconfigured so that it is no
longer an ASBR. In this case, the other ASBR floods the AS External LSAs. For example, if
Router D goes off-line, then Router E starts flooding the AS with AS External LSAs for the route
to Router F.
Support for OSPF RFC 2328 appendix E
BigIron RX provides support for Appendix E in OSPF RFC 2328. Appendix E describes a method to
ensure that an OSPF router generates unique link state IDs for type-5 (External) link state
advertisements (LSAs) in cases where two networks have the same network address but different
network masks.
NOTE
Support for Appendix E of RFC 2328 is enabled automatically and cannot be disabled. No user
configuration is required.
Normally, an OSPF router uses the network address alone for the link state ID of the link state
advertisement (LSA) for the network. For example, if the router needs to generate an LSA for
network 10.1.2.3 255.0.0.0, the router generates ID 10.1.2.3 for the LSA.
However, suppose that an OSPF router needs to generate LSAs for all the following networks:
•
10.0.0.0 255.0.0.0
•
10.0.0.0 255.255.0.0
•
10.0.0.0 255.255.255.0
All three networks have the same network address, 10.0.0.0. Without support for RFC 2328
Appendix E, an OSPF router uses the same link state ID, 10.0.0.0, for the LSAs for all three
networks. For example, if the router generates an LSA with ID 10.0.0.0 for network 10.0.0.0
255.0.0.0, this LSA conflicts with the LSA generated for network 10.0.0.0 255.255.0.0 or 10.0.0.0
255.255.255.0. The result is multiple LSAs that have the same ID but that contain different route
information.