Brocade TurboIron 24X Series Configuration Guide - Manual (Page 644)

610 Brocade TurboIron 24X Series Configuration Guide
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Overview of OSPF
NOTE
For details on how to configure the system to operate with the RFC 2178, refer to “Modify OSPF
standard compliance setting” on page 643.
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.
Figure 97 shows an example of the AS External LSA reduction feature. In this example, Layer 3
Switches D and E are OSPF ASBRs, and thus communicate route information between the OSPF
AS, which contains Routers A, B, and C, and another routing domain, which contains Router F. The
other routing domain is running another routing protocol, such as BGP4 or RIP. Routers D, E, and F,
therefore, are each running both OSPF and either BGP4 or RIP.