December2017

Unequal Cost load-balancing with BGP

Did you know that you can loadbalance over unequal cost paths with BGP, just as we can do with EIGRP?

It is true. As long as bestpath selection is equal for all attributes up to MED – weight, local pref, as_path, origin, metric. And the neighbor type have to be external. To enable this feature in EIGRP we have to rely on the feasibility check to be sure that we avoid routing loops, in BGP we rely on the AS-path check. So for external routes with equal attributes including MED we can import multiple paths and loadbalance between them.

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OSPF NSSA P-bit

Which path does the router use to reach a redistributed?

In the topology we see an ASBR doing mutual redistribution between BGP and OSPF. It is connected to two areas, one normal area and one no-so-stubby-area. The question in the task is which path does the bottommost router use to reach a route redistributed into OSPF from BGP. That this is some kind of trap is obvoius. What happens to a redistributed route when traversing those areas in this topology?

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