Brocade FastIron Ethernet Switch Layer 3 Routing Configuration Guide - Manual (Page 249)

OSPF RFC 1583 and 2328 compliance
Brocade devices are configured, by default, to be compliant with the RFC 1583 OSPF V2 specification.
Brocade devices can also be configured to operate with the latest OSPF standard, RFC 2328.
Reduction of equivalent AS external LSAs
An OSPF ASBR uses AS External link advertisements (AS External LSAs) to originate advertisements
of a route learned from 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. The device optimizes OSPF by
eliminating duplicate AS External LSAs in this case. The device 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 link state database on the
device. The AS External LSA reduction is described in RFC 2328
In this example, Routers 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.
OSPF RFC 1583 and 2328 compliance
FastIron Ethernet Switch Layer 3 Routing Configuration Guide 249
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