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:
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.
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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