Quick Reference Guide

PowerConnect B-Series TI24X Configuration Guide 661
53-1002269-02
Overview of OSPF
23
NOTE
For details on how to configure the system to operate with the RFC 2178, refer to “Modify OSPF
standard compliance setting” on page 694.
Reduction of equivalent AS External LSAs
An OSPF ASBR uses AS External link advertisements (AS External LSAs) to originate advertisements
of a route to another routing domain, such as a BGP4 or RIP domain. The ASBR advertises the
route to the external domain by flooding AS External LSAs to all the other OSPF routers (except
those inside stub networks) within the local OSPF Autonomous System (AS).
In some cases, multiple ASBRs in an AS can originate equivalent LSAs. The LSAs are equivalent
when they have the same cost, the same next hop, and the same destination. devices optimize
OSPF by eliminating duplicate AS External LSAs in this case. The Layer 3 Switch with the lower
router ID flushes the duplicate External LSAs from its database and thus does not flood the
duplicate External LSAs into the OSPF AS. AS External LSA reduction therefore reduces the size of
the Layer 3 Switch link state database.
This enhancement implements the portion of RFC 2328 that describes AS External LSA reduction.
This enhancement is enabled by default, requires no configuration, and cannot be disabled.