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Start BGP Advertisements

Forgive my ignorance of BGP. I'm a relatively new Network Admin, and I inherited the network I now manage. We are testing our BGP failover scheme this weekend which uses LP to advertise the routes for our /20 block. We have a /24 at the DR site that is cut out of our /20, that we want to fail over to.

For our HQ campus we advertise the whole /20 with a local pref of 100, and the /24 at our DR site with a local pref of 90 separately.

At the DR site, we advertise the /24 with a LP of 90, and the whole /20 with a LP of 100. All of this is based on the fact that more specific routing always wins (as I've read anyway).

My question is just this - when I pull the plug, test the failover (accessing a replicated web server), and then plug back in - is there a way to force the BGP advertisements to start immediately? Or does this happen as soon as the link is up? If we are having problems, what would be some good troubleshooting commands?

The router the advertisements are coming from is the 7206vxr running ver 12.3.

Thanks in advance

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1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

Re: Start BGP Advertisements

Hi,

BGP needs to establish a TCP session to the configured neighbor first, before exchanging updates. Therefore, you need IP connectivity (interface up/up, show ip route connected) to establish the TCP session. Once the session is established, some checks are performed (neighbor in correct AS, etc.) and then the updates are exchanged.

Be aware that this by no means results in immediate connectivity to the whole internet. To maintain stability BGP has very conservative timers. F.e. between eBGP updates - from one AS to another AS - the update timer is 30 seconds. This means, if between your AS and the destination AS three other ASes are in the path, it could take 2 minutes to receive the update.

Bottom line: between your router and the provider router it will take a couple of seconds at least to establish the BGP session and announce a prefix. To get this update out to the rest of the internet might take several minutes.

Hope this helps! Please use the rating system.

Regards, Martin

P.S.: BGP is a truck, not a race car.

2 REPLIES
Cisco Employee

Re: Start BGP Advertisements

Hi,

BGP needs to establish a TCP session to the configured neighbor first, before exchanging updates. Therefore, you need IP connectivity (interface up/up, show ip route connected) to establish the TCP session. Once the session is established, some checks are performed (neighbor in correct AS, etc.) and then the updates are exchanged.

Be aware that this by no means results in immediate connectivity to the whole internet. To maintain stability BGP has very conservative timers. F.e. between eBGP updates - from one AS to another AS - the update timer is 30 seconds. This means, if between your AS and the destination AS three other ASes are in the path, it could take 2 minutes to receive the update.

Bottom line: between your router and the provider router it will take a couple of seconds at least to establish the BGP session and announce a prefix. To get this update out to the rest of the internet might take several minutes.

Hope this helps! Please use the rating system.

Regards, Martin

P.S.: BGP is a truck, not a race car.

Re: Start BGP Advertisements

Thanks mheusing - that answers my question perfectly!

If this posts answers your question or is helpful, please consider rating it and/or marking as answered.
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