Advertising 2 different AS BGP via same ISP

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First of all thank you, this is the situation: I have 2 different ISP's, ISP 1 with IP block 64.x.x.x and ISP 2 with 65.x.x.x, I got a 3rd ISP that I would like to BGP peer and start advertising these 2 IP blocks via the 3rd ISP pipe, please see visio, is this type of scenario doable? thanks again for your help.

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sundar.palaniappan Wed, 02/21/2007 - 12:37

There should be no problem with that provided, the 3rd ISP agrees to route traffic for the two networks coming from the other two ISP's IP scope. The 3rd ISP needs to accept the two prefixes from you and has to advertise them out to the Internet.

HTH

Sundar

Richard Burts Wed, 02/21/2007 - 13:14

I am not so sure that I would go as far as Sundar and say no problem. I believe that there are a couple of things to consider which may turn out to be problems.

First - many service providers enforce limits on the length of prefix that they will accept and advertise. Your drawing does not show the mask or prefix length so we do not know how likely it is to be impacted by these policies. But it is something to consider.

Second - it is likely that the original providers are advertising your address space aggregated into their address space (they probably just announce their big address block and not your individual part of it). If you successfully advertise your part of their address space to ISP 3 then you are de-aggregating the address space and if routers in the Internet have your specific block and the original provider bigger block then they will choose to route all your traffic to you via ISP 3 (longest match wins in the routing table). Is that what you intend?

I agree with Sundar that just from the operation of the routing protocol that you can do this. But from an operational perspective I believe that there are some problems.

HTH

Rick

sundar.palaniappan Wed, 02/21/2007 - 13:27

While I agree with Rick the issues that might arise with this setup there's workaround available to address these issues.

First - We don't know what the mask of those prefixes are. The original posting shows the last 3 octets were grayed out and even if the network is not using /8 bit mask and there's a good chance it could be a /24 bit mask. AFAIK, most providers do accept prefixes with /24 bit mask.

Second - To address the problem of more specific routes being advertised by the 3rd ISP, which would cause them to be the preferred path for that network, in advertisements to the 3rd ISP the as-path can be prepended to make the route less desirable via that provider from the Internet.

Either way, you need to discuss this with the 3rd ISP to work out the details to accomplish what you intend to do.

HTH

Sundar

Jon Marshall Wed, 02/21/2007 - 13:27

Hi

Yes, to follow on from what Rick says in his second point. If you don't want all your routing to go via ISP3 then you will need to convince your two other ISP's to punch holes in their aggregate addressing and advertise your 64.x.x.x and 65.x.x.x networks out.

Most ISP's are reluctant to do this as increases the number of routes that need to be held in internet router tables.

Jon

sundar.palaniappan Wed, 02/21/2007 - 13:34

Most providers are reluctant to do this as they normally do not want to make exceptions to individual customers from what their preset policy dictates. However, many providers do accept the prepended as_path, as I had mentioned in my previous posting, and this would cause that path less desirable.

HTH

Sundar

Richard Burts Wed, 02/21/2007 - 13:43

Sundar

With all due respect: no matter how many times you prepend you will never overcome the fact that a /24 will always be preferable to a /16 (or whatever the masking turns out to be).

Prepend is only useful when you are advertising the same prefix through multiple providers.

The only real solution to this is to convince ISP 1 and ISP 2 to de-aggregate their address space and to advertise the more specific route.

HTH

Rick

sundar.palaniappan Wed, 02/21/2007 - 13:53

True, that was an oversight on my part and yes, the longer/shorter prefix would come into play before anything. I was quickly responding to this post and moved on to something that I was doing at that time. Yes, indeed, this would require the other two ISP's to advertise the prefix with the same mask at which point as_path prepend is applicable.

HTH

Sundar

Jon Marshall Wed, 02/21/2007 - 13:45

Hi Sundar

Out of interest, in the scenario we are discussing is there a way that you could load balance incoming traffic so that 64.x.x.x came in via ISP1 and ISP3 and 65.x.x.x came in via ISP2 & ISP3 ?

This assumes that ISP1 & ISP2 will not punch holes in their aggregate addressing.

Thanks

Jon

sundar.palaniappan Wed, 02/21/2007 - 14:01

Jon,

Assuming all three providers advertise the prefix(s) with the same mask what you are saying is possible. There's no workaround to the basic rule of routing, longer prefixes are preferred over shorter ones. This setup would require the explicit cooperation/coordination of all three providers to route as intended.

HTH

Sundar

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