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
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
- If a loopback interface is configured, the router ID is set to the IP address of the loopback interface. If multiple loopback interfaces are configured, the router ID is set to the IP address of the loopback interface with the highest IP address.
- If no loopback interface is configured, the router ID is set to the highest IP address on a physical interface.