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Practical BGP by Sangli Srihari, Russ White, Danny McPherson

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Inconsistent Routing

There are two common reasons for inconsistent routing within BGP: holes in the BGP specification, and vendors modifying the best path algorithm (to reduce the rate of flapping routes in the Internet, for instance). We'll examine two of these issues here, the indeterminism of the multiple exit discriminator and a simple change to the best path algorithm that places the older route over the route learned from the peer with the highest router ID.

Multiple Exit Discriminator Indeterminism

The MULTI_EXIT_DISC attribute may be used on external (inter-AS) links to discriminate among multiple exit or entry points to the same neighboring AS. [RFC1771].

According to RFC 1771, the multiple exist discriminator is not always compared ...

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