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AS-Path Manipulation

Hi,

We are an ISP that has two transit feeds to Level3. We wanted to "load-balance" ingress traffic on each 1GB link. We decided to use AS-Path Prepending which works very well indeed. However, when I look at any route-server on the Internet and do a show ip bgp regexp _ourASN_ I don't see our prepends (I only see our one AS number so it reads like 3356 OurASN) and yet L3 are deffo getting them (as theyve given us the various show commands on their directly connected routers to us ANd traffic IS load balance quite nicely). I didn't think there was a way of manipulating the AS-Path and removing prepends from downstream ISPs?? How is L3 doing this?

Thanks

Ryan

Reply to
ryanfinnerty
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L3 will only advertise best to their peers

Reply to
Merv

I understand the reason for removing the prepended paths but I am intrested in how they do it since I didn't realise an AS-Path could be manipulated in such a way. Do you know how they do this?

Reply to
ryanfinnerty

I am assuming that you are advertising the same prefix over each of the links to L3, on one link you prepend and on the other link for the same prefix you don't - is that correct ?

Do both links terminate on the same L3 router ?

Reply to
Merv

Well we have the address space split in half and half prepended on one link - half on the other. Two links to L3. But like i say I dont see the prepends on a route server outside of L3. Just wondered why.

Reply to
ryanfinnerty

The reason is easy - BGP routers slects what is known as the "best" route from all the BGP routes its receives and stores in its BGP table (not its routing table). This "best" route is the one install in the main routing table. It is also the ONLY route that the BGP router then advertises to its BGP peers.

So the routes with the AS prepends will only exist within the L3 AS and never outside.

The L3 is not stripping any AS prepend off; it is just not advertising them outside due to the BGP best route slection rules.

Reply to
Merv

Ahh - Yes - its always the simple things that get you isn't it :-)

Thanks for the post!

Reply to
ryanfinnerty

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