Dell FCX624-E Laptop User Manual


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926 PowerConnect B-Series FCX Configuration Guide
53-1002266-01
Overview of OSPF
29
NOTE
For details on how to configure the system to operate with the RFC 2178, refer to “Modifying the
OSPF standard compliance setting” on page 962.
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. Dell PowerConnect
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 141 shows an example of the AS External LSA reduction feature. In this example, Dell 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.