Adjusting BGP Administrative Distance

[scott.moyle]

I am attempting to adjust the default BGP Administrative distance of 20 to 190. This is to make it less preferred that an EIGRP route with administrative distance of 170.

My issue is that I want this to only occur for routes with a specific AS path. This is due to my provider running the AS over ride feautre for me allowing routes to be shared over the same AS. This is setup for DR purposes. Under normal circumstances the EIGRP routes are redistibuted into BGP with a higher weight and therefore preferred.

Once a disaster scenario occurs the router only knows about some routes via BGP and therefore they are preferred. Once the scenario has completed the EIGRP routes come back but are ignored. Obviously if I remove the BGP version of the routes the EIGRP routes get advertised with a higher weight and therefore when the BGP routes are returned they are ignored.

I have tried to do this using an AS path access-list but from looking this can only be used in an route-map where there is no set option for administrative distance that I can see.

I have tried to set the administrative distance under the router setup. Whist this can be done via an access-list it does not seem possible to set it against an as path access list.

I hope this makes sense any suggestions would be great.

[scott.moyle]

Also I have looked into using the BGP backdoor command but it seems this can only be used for defining individual networks.

[Peter Paluch]

Hello Scott,

Modifying the administrative distance for paths selected by AS_PATH seems indeed to be unavailable in BGP.

My first question is if you believe it would be possible to modify the EIGRP administrative distance instead, based on the IP address of the EIGRP neighbor and optionally an ACL selecting the networks for which the AD is to be modified (in this case, lowered to at least 19). From what I have understood, you are receiving a set of destinations via BGP and there are some destinations with a characteristic AS_PATH you would like to handle specially. Also I assume that exactly those destinations are also advertised to you via EIGRP peering, and you want to prefer this EIGRP peering over BGP. If the other location is reachable through a single EIGRP neighbor or a small number of EIGRP neighbors, it might be easier to modify the EIGRP distances - assuming that the only EIGRP-learned destinations are exactly those which you would like to "un-prefer" in BGP.

Would this work for you?

Best regards,

Peter

[Jon Marshall]

Scott, apologies for using this thread for quick message

Peter, just in case you missed it there is a new ask the Experts session in LAN Switching/Routing on all switches so it may be a good place to as those questions about the 2960 that you didn't get an answer to. Just a thought.

Jon

[Marwan ALshawi]

just to Add to Peter's suggestion,

you might try this simple option which is send the BGP from the main site to you SP as summarized for those prefixes

and eigrp supposed to send more specific route in this case regardless what's this AD is, the more specific will be selected,

once the DR stop receiving the more specific route through EIgrp it will use the summary over BGP ---SP

HTH

[Peter Paluch]

Marwan,

Good point! Rated as deserved. The KISS principle wins, as usual

Best regards,

Peter

[scott.moyle]

Hi Guys,

Thanks for your responses they definitely got me thinking.

I should point out that this site is also used to advertise routes which are received via 3G backup connections from remote sites when there is issues with there primary link. These routes are learnt via the same EIGRP process and are always populated just with poor metrics so they are not preffered internally.

Once these sites come back online they re-advertise there networks via BGP which then need to be preffered over those routes advertised via the EIGRP process. This is controlled by setting the weight on these routes to 0 matching the routes which come from the provider but they have a lower local preference.

Hence I have an issue if I try to adjust the AD of the EIGRP routes unless I can do this via Tags which having a quick look does not look possible?

These routes would be summarised by BGP as they are similar to the others unfortunately and therefore they would not be preffered once a remote site returns to its primary service.

I hope this all makes sense.

[Peter Paluch]

Scott,

Do you believe you could post a schematic exhibit of your network, explaining the flows of routing information and the location of the BGP and EIGRP cloud? This is starting to be somewhat confusing, and seeing your network topology would definitely help.

Best regards,

Peter

[Marwan ALshawi]

agree with Peter it looks more complicated than to be addressed using txt only

by the way thanks peter for the Rating