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These are all posts about BGP. Archive for 2016.

Mixing IPv4 and IPv6 in BGP

When I first started running IPv6 over BGP, I ran into an interesting problem. The Cisco router I was working on had four full BGP feeds over IPv4. But when did show ip bgp, I noticed the router had five copies of each prefix. One of these paths had a really weird next hop address and was marked as unreachable.

Turned out those were IPv4 prefixes learned over an IPv6 BGP session.

Read the article - posted 2016-03-18

Cisco BGP identifiers

One of the tiebreakers in the BGP best path selection algorithm is to prefer the path learned from a BGP speaker with the lowest BGP identifier. So how are BGP identifier selected when they're not configured explicitly?

I always forget whether it's the highest or the lowest IP address configured on a Cisco router. Turns out this is remarkably hard to find in Google, but if you know where to look it's in Cisco's IOS command reference:

Permalink - posted 2016-04-22

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