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Community Member

BGP AS Number question.

I have a customer that owns 4 class C subnets. We are beginning a project to multihome them between two providers. The design we want to accomplish is to have 3 of the subnet to utilize provider A for inbound and outbound service while the fourth remaining subnet uses provider B for both inbound and outbound service. In the event of a provider failure all subnets need to switch to the remaining provider.

Question is: In thinking this through, I think I need two AS numbers if I want to insure both inbound and outbound traffic are isolated to a single provider under normal operation. Can some enlighten me if I can get it done using just a single AS number.


Community Member

Re: BGP AS Number question.

You can accomplish this with one AS. The following assumes you have 2 routers.

You can put three subnets on the router from provider A and the other subnet on the router from provider B. By using the weight attribute or the local preference attribute, you can select eBGP routes over iBGP routes. If your eBGP connection goes down, iBGP will take over. Depending on your network design, you might be able to use policy routing or HSRP to guarantee failover between the routers.

That was for outgoing. For incoming, you will probably have to do some kind of prepending to make sure subnets 1-3 prefer provider A while subnet 4 prefers provider B.



Re: BGP AS Number question.

One ASN is all you need. Use the ideas above for outbound traffic. For inbound traffic, talk to your ISPs. Most support community strings which allow treating an advertisement as "for backup use only." That way, _ALL_ inbound traffic will use the desired access path, even if originating local to the ISP and the preferred path is via another ISP. Alternatively, you can use AS Path prepending to influence inbound routing. Or you can ask the ISP to adjust their configuration (using any of the techniques available to you for outbound routing) to send traffic the way you want (not recommended unless you have no choice because then you need to get the ISP involved whenever you want to make an adjustment).

If your two ISPs are not very cooperative, get yourself a copy of Halabi's "Internet Routing Architectures" and learn more than you want to know about BGP. Bottom line, you can do just about anything you want, but you may not want to... You may also find chapter 8 of my book useful, but I focus on the availability side only, and generally ignore load sharing considerations.

Good luck and have fun!

Vincent C Jones

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