BGP route-map question!

Unanswered Question
Aug 10th, 2007


I?m running BGP using local-preference to determine the best route for each package (no weight is used). If I create a new route-map in one of my routers with the highest local preference pointing to one of my ISP (no other statements configured inside the route-map), Does this configuration will re-routed all the traffic from my BGP neighbors under same AS# to the ISP in question?.

I really appreciate a feedback on this question.


I have this problem too.
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deveshkumar Fri, 08/10/2007 - 06:13

Here's the idea of local preference:

consider router A and Router B connected to external AS.

consider Router C connected to Router A and Router B is Same AS (IBGP)

Router B has high local preference than Router A has.

Router C will choose path through Router B bcos of High local pref.

So, local prefrence is just to influence your path selection wihtin an AS, for outgoing traffic.

while for weight here's the catch:-

consider the Same Router B has two links connected to diferent ISP. Weighh will influenc only Router B's behaviour for path selection . higher the weigth better the router.

use following link for to explore more...

Hope this helps

erodrigue Fri, 08/10/2007 - 07:13


Thank you for your quick response and for your comments. I will follow up the link in detail.

Using IBGP, Can you also confirm this for me:

If no "match ip add prefix-list" is configured into the route-map, Do the router (Ex: router B) will sent all the traffic from other routers using same AS to the interface configured for the ISP in question causing network congestion or down?


deveshkumar Fri, 08/10/2007 - 07:24

i'm not able to get ur question completely..

is that qestion is : wihout prefix list, router b will send all traffice from other routers or

question is without prefix list will router b will send all traffice using same AS??

While running bgp u need to take care of two things :-

address u avertise ?


AS nos u advertise.

you shld use prefix list inorder to restict the address space you advertsie.

and u shd use as-path list to restrict the routes learned from different AS (ISP) being advertised to other AS (ISP) ..thus making you a Transit AS for the ISP..that too after you paying them......

erodrigue Fri, 08/10/2007 - 07:44

Basically, my question is related to your second question using same AS on different routers. Now, I believe from your comments that if I?m not restricting the address space thru the use of prefix list, all the traffic that I?m advertising will be route to the ISP configured on router b with the highest local preference. Correct?

Thanks again!!

deveshkumar Sun, 08/12/2007 - 19:40

Partly correct,

What ever address space u advertise on Router A and Router B and Router C, will be advertised to ISP , if synchronisation rule satisfies, or u have disable synchronization.

But both the routers will advertise the routes, howerver you outgoin traffic will be thru router having highest Local preference. remember Local preference is to influence outgoin traffic from ur AS.

BGP syncronization rule say, it will advertise the routes/or install the routes in BGP table only if it gets matching route via IGP (OSPF,RIP,EIGRP etc..).

Hope now u have got ur point.

If better u let me know what u want to achieve i'll send to sample config mate...

erodrigue Mon, 08/13/2007 - 08:07


I really appreciate all your help / comments. I need to have a better understanding of BGP algorithm before submit another question to you. Thank you for letting me knows that I can count on you.

Have a great day?


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