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as-path prepending/appending for in/out routing

Hi, I am configuring Internet border routers for a company doing ecommerce business. I want to prefer/deprefer certain paths in a mulit-provider uplink scenario (2 routers / 2 ISP's). To do that, I have chosen to prepend local AS 1-4 times based on desired result. This is a pretty common configuration:

route-map RM_eB_ISP_1xAS_OUT permit 10

description Deprefer local route through ISP by 1x
match ip address prefix-list PL_ASxyz_ONLY
set as-path prepend ASxyz

neighbor w.x.y.z route-map RM_eB_ISP_1xAS_OUT out

Since this affects inbound traffic to the AS, and I would like to maintain routing symmetry as much as possible, so I want to adjust the outbound paths as well, in the same fashion as the internet will see the local AS.

In researching this, I came across:

set as-path prepend last-as X

I am configrung this as follows:

route-map RM_eB_ISP_1xAS_IN permit 10

description Deprefer foreign routes though ISP by 1x
set as-path prepend last-as 1

neighbor w.x.y.z route-map RM_eB_ISP_1xAS_IN IN

The idea here would be to add and extra ISP AS hop to all paths coming in so the symmetry matches the outbound prepend:

Questions:

1) I am accepting full internet route tables. Is this a bad idea due to CPU constraints? In other words, every route from the ISP will have to be adjusted with the last-as append. This is a lot of processing power.

2) Will the last-as function be smart enough to control some sort of logic as follows: I see the ISP AS is the last path. since this is my BGP partner, i wont past-as append for that AS since I am directly connected to it?

3) Has anyone gotten this to work? in the lab, it doesn't appear to work for me running 12.4(25d) IOS. When I remove the inbound route-map, i get some AS paths from ISP, but when I put it back in there, i don’t get those AS's.

4) Is there a better way to accomplish this?

many thx in advance,

Will

2 REPLIES

Re: as-path prepending/appending for in/out routing

Using route map with AS prepend to effect inbound traffic and using local preference to effect outbound traffic is most common method to be used in a case like yours

Assuming you are runing ibgp between your edge routers

Also with route map make sure u use another sequence with permit to let other route to pass without action

Otherwise route not match by the first route map prefix list will not be allowed

Hope this help

If helpful rate

Sent from Cisco Technical Support iPhone App

New Member

Re: as-path prepending/appending for in/out routing

Hi Marwan, Using local preference for outbound policy seems to be just a binary process - which breaks the routing symmetry. For example, if I say local Pref = 110 for all routes through my local R1, this would then favor all internet routes to go through this R1 router from my AS. I really don’t want to do that. I want to consider the number of "hops" in the routing decision, just like prepend outbound is doing on inbound traffic. So to affect this in a granular way, and prefer some routes through R1 and others through R2 (to maintain symmetry), seems like I want to do another prepend/append on all inbound paths received by my ISP BGP peer.

Hope that clarifies what I want. Perhaps this is not an effective or useful thing to do with BGP, but it seem like it would be? Thank you for your response!

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