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New Member

BGP with 2 ISP, but not load balancing

Hi

I have connection to 2 ISP's: one - default, one - backup. Let say: ISP1 and ISP2.

I had confgured BGP with AS-PATH prepend to use backup channel in case of default going down.

Now, to the ISP2 connects some other ISP's (ISP3, ISP4).

I got networks for these ISP's with AS prepend. For me, this is not a problem.

The problem is that other ISP's (ISP3 and ISP4) got my networks with AS prepend. They have full BGP table, and also got my networks from Internet without prepend.

How I must better implement this configuration?

Thanks

Vladimir Girnet,

MEGADAT.COM Network Administrator

http://www.megadat.com

4 REPLIES
New Member

Re: BGP with 2 ISP, but not load balancing

Hi.

How many times did you prepend your AS?

If ISP3 and ISP4 get the networks from your AS through the Internet with a short AS_PATH, there's nothing you can do... Ok, you can always shorten your prepends, but this doesn't guarantee they will prefer a certain path to your AS.

Hope it helps.

Nuno Morais

New Member

Re: BGP with 2 ISP, but not load balancing

My AS is prepended 6 times.

I think, the only good configuration will be, if the ISP2 (my backup) will announce my networks to Internet with AS prepend, but to other ISP's, connected to him - without prepend.

In this case, I must not prepend my AS on my router.

Am I right?

Thanks

New Member

Re: BGP with 2 ISP, but not load balancing

Hi,

Considering that ISP3/4 got your AS with the prepends and without it is quite correct. That is because they got one path you you AS for your ISP1 (without the prepend) and the other path from ISP2 with prepend. Now the BGP best path selection is steping in. They may have multiple paths to your AS but only one is best (as you probably already know). Now while your ISP 1 is up and runing you get your traffic from that side and I bet that in the BGP tables of ISP3/4 they have your network as best path through ISP1. Let say ISP1 goes down. After about 180 seconds (default value) the entry in ISP3/4 tables about you network that was pointing the best path through ISP1 will dissapear and the best path will be through ISP2. And that is what you wanted to achieve right :-))

Later

Velimir

New Member

Re: BGP with 2 ISP, but not load balancing

>>And that is what you wanted to achieve right :-))

Not so. I want that ISP3/4 always got my networks from ISP2 as "the best". And only if ISP2 goes down, then, ISP3/4 got routes from ISP1.

OK, are there any other ways of announcing routes to internet via a backup link without AS prepend, if connected to 2 different ISP's?

thanks,

Vladimir

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