BGP Multihoming

Unanswered Question
Jul 25th, 2003
User Badges:

Multihoming our internet connection to 2 different ISPs. Packets are coming in through both ISPs as I cannot control their routes.


Question is, in general and on average, how fast do the Internet BGP tables converge if one of my links to either ISP goes down?


For example, if incoming routes are coming through ISP1 and its link goes down, how long will it usually take for all incoming traffic to be redirected to ISP2?


Thanks!



James

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
Loading.
deilert Fri, 07/25/2003 - 06:57
User Badges:
  • Silver, 250 points or more

It will take at least 3 minutes for the neighbors to go down .


Say your linke to ISP one goes down it will take 3 minutes for BGP to drop, Now when the link goes down there is no gaurentee that routes you are learning from ISP 1 will be advertiesed by ISP 2 . What you can do is configure default routes on both rotuers with an admin distance of 250 , this way if one link drops and there is no route then traffic will always be fowarded out one of the links



ip route 0.0.0.0 0.0.0.0 nexthopIP 250



ruwhite Fri, 07/25/2003 - 08:35
User Badges:
  • Gold, 750 points or more

It's actually going to depend, as well, on how the BGP session is set up to the provider. If you're using bgp fast-external-fallover, and you are peering with the directly connected interfaces over a point-to-point of some type (not using eBGP multihop), then when the interface fails, the BGP session will fail.


Other than this, you could also set your BGP timers very low, if you're willing to incur the extra cost in TCP keep's, and the ISP's are willing to play along.


Russ.W

t.baranski Fri, 07/25/2003 - 18:24
User Badges:
  • Bronze, 100 points or more

In addition to the amount of time required for ISP1's router to detect that your circuit is down and stop advertising your routes (adjustable by BGP timers as mentioned), it also takes time for this change to propagate across the Internet. Routes to your network via ISP1 have to be withdrawn on Internet BGP routers, and the routes via ISP2 have to be added. This should normally take on the order of a minute or two, though it can take longer for certain networks to receive the updates due to busy routers, and other factors.

Actions

This Discussion