Russ

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BGP Persistent Oscillation

After Daniel Walton visited the History of Networking at the Network Collective, I went back and poked at BGP permanent route oscillations just to refresh my memory. Since I spent the time, I thought it was worth a post, with some observations. When working with networking problems, it is always wise to begin with a network, so…

For those who are interested, I’m pretty much following RFC3345 in this explanation.

There are two BGP route reflectors here, in two different clusters, labeled A and D. The metric for each link is listed on the links between the RR clients, B, C, and E, and the RRs; the cost of the link between the RRs is 1. A single route, 2001:db8:3e8:100::/64 is being advertised in with an AS path of the same length from three different eBGP peering points, each with a different MED. E is receiving the route with a MED of 0, C with a MED of 1, and B with a MED of 10.

Starting with A, walk through one cycle of the persistent oscillation. At A there are two routes—

edge MED IGP Cost
C    1   4
B    10  5 (BEST)

When A runs the bestpath calculation, Continue reading

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