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BGP Session



If you are multihomed with 2 different providers and one is performing maintenance what is the best was to gratefully steer ingress traffic away and the quickest for the least amount of downtime.

If you shutdown the neighbor facing the transit provider will this withdraw routes straight away or will we still have to wait for BGP timeout? or the other option is AS-Prepend how long will it take for ingress traffic to come in via the other ISP?

looking at both options just looking from an ingress point of view the best way?


5 Replies 5


It's very hard to do without at minimum a small glitch. Usual way I do this if I am not too worried about any traffic glitches occurring is to prepend (3-4 at most, anymore rarely makes a difference) out the transit provider i am routing away from. Give the traffic 10-20 minutes to drop then withdraw routes or shutdown the peer. You probably won't get rid of all traffic this way and will see some loss.

If you want to be super clean you could advertise just supernets out the provider doing maintenance meaning more direct routes are available via your other provider. As long as you have more specific routes for all the subnets in your supernets via that provider then all traffic should move cleanly over. Again give it 10-20 minutes and you should see almost all traffic move.

But yes if you just shutdown the neighbor to your transit you will definitely see loss for potentially a few minutes as BGP converges across the internet to your other provider. Probably easiest/quickest way however.

Hope that helps.



So your saying even if you prepend there will be a glitch..what would this be as traffic should streer via the provider?

we advertise the same supernets out of our border routers anyway so don't think this would be an option. 

Just shutting down the session wouldn't be an option as you say doing this will cause downtime, so like you say option 1 prepend would be the best and LP for the egress traffic.

Thanks for you help.

There probably will be a glitch still with prepends as some ISPs, or the transit provider themselves, will be local preferencing via that route so won't move over to your other provider however many prepends you put onto the advertisement. Only way to force it is offer more direct routes to make them prefer one link over the other.

No worries, as long as you withdraw the routes or hard shutdown the neighbor along with some earlier prepends you shouldn't see too many problems.

Good luck.

By hard shutdown the neighbor, i of course mean neighbor x.x.x.x shut, not shut the interface! Last thing you want is to shut the interface and have to wait 180 seconds for the ISP to realise you're down (as there may be l2 devices inbetween you.) :)

Thanks - we actually have around 5 transit providers. The one we are having maintenance on we have 2 sessions to them. So by just shutting down bgp session will we still have the issue of convergence across the rest of the internet as traffic will be still drawn to the ISP's upstream router ( the session we shutdown ) then it will run over there core network via i presume IBGP then over our second session to them.

Not saying i will use this method - just wondering if this will happen?


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