Brocade Communications Systems ICX 7250 series, Configuration Manual

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The DR and BDR election process is performed when one of the following events occurs:
• An interface is in a waiting state and the wait time expires.
• An interface is in a waiting state and receives a hello packet that addresses the BDR.
• A change in the neighbor state occurs, such as the following:
–
A neighbor state transitions from ATTEMPT state to a higher state.
– Communication to a neighbor is lost.
– A neighbor declares itself to be the DR or BDR for the first time.
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.
Reduction of equivalent AS external LSAs
Brocade FastIron Layer 3 Routing Configuration Guide
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