North American Network Operators Group|
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Re: weird BGP cisco-ism?
if the primary route becomes unavailable and routing falls over to the "nailed up" route, a bgp update is still sent. (if dampening is enabled) this update is recorded by ebgp peers as a "flap". i'd guess from your message below that when this occurs your router(s) are also changing the med attached to the prefix, which seems normal to me... i'd suggest you have a look at the stability of the interface to which the primary route is attached (?carrier transitions, interface resets, etc...?). you might also consider breaking the primary route (/20) into 2 /21 blocks internally and allowing the longer "nailed up" route to be the permanent source of the /20 advertisement. for example: rather than: ip route 184.108.40.206 255.255.240.0 <interface> !"primary" route ip route 220.127.116.11 255.255.240.0 null0 254 !"nailed up" route ! router bgp <as> network 18.104.22.168 mask 255.255.240 try this: ip route 22.214.171.124 255.255.248.0 <interface> !"primary" route ip route 126.96.36.199 255.255.248.0 <interface> !"primary" route ip route 188.8.131.52 255.255.240.0 null0 <admin. distance> !"nailed up" route ! router bgp <as> network 184.108.40.206 mask 255.255.240 although correcting the stability problem is the correct solution. -danny > > I have a Cisco 7505 which is advertising about 50 routes to about 40 > peers at mae-west, and a few others. One set of customers has been complaining > that their connectivity is going away right at that router, and then coming > back. Narrowed the set of customers down to a single CIDR block, at > 220.127.116.11/20. > > So, some of our peers are claiming that the route is flapping... that's > weird, we have them all nailed up to static routes... especially the CIDR > blocks. So I wrote a tool which you can peer a router with, and it watches > the BGP traffic and prints anything it gets, formatted, to standard out. > > My Cisco is sending fresh advertisements every 10-30 minutes for that route, > and not for any other of the routes it has, and it appeared to be all the same, > but on careful examination, it appears that each advertisement reflects a > change in the MULTI_EXIT_DISC from 0x00000000 to 0x00000014 and then back > again in the next advertisement. > > What the heck am I seeing here? Is someone's flap damping code seeing the > repeated advertisements and suppressing me? Is my Cisco going crazy? > > -matthew kaufman > firstname.lastname@example.org