BGP Sessions

Unanswered Question
Oct 6th, 2009


we are running a BGP on the cisco 7206 VXR router

we have two BGP router BGP1 and BGP 2 with the

LAN - and running HSRP on the LAN side for hardware redunanacy

on WAN side we have 2 BGP session ( Session A and B ) with the same ISP

Currently all traffic from my LAN to WAN will go through the BGP1 with session A since we have configured

with the HSRP high priority

and also from WAN to LAN also the same since the ISP session A router is more expose to the Internet routes

Now my requirement is to load balance the traffic between the two BGP routers

how i can make the incoming traffic through the BGP 2 router on BGP SESSION B

Whether i depends on the ISP configuration or We can make some MED value ?

Thanks in advance

I have this problem too.
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vishwancc Wed, 10/07/2009 - 03:57

Hi Vinoth ,

If i understand correctly , your 2 BGP router are connected to 2 ISP router with 1 EBGP session on both the router.

To do load balancing for the incoming traffic you could use AS path prepenad and MED,but i will suggest to use AS path prepend since MED is a weak metrick since it come below AS is BGP path selection attributes.



Giuseppe Larosa Wed, 10/07/2009 - 10:27

Hello Vishwa,

in this case being the connections to same ISP/AS MED may be an acceptable tool in Vinoth's scenario.


With selective prepending is possible to divide IP subnets in two subsets one advertised without prepending by R1 and with prepending by R2; the opposite happens for the other subset of routes: R2 advertises them without prepending and R1 with prepending.

This may be a better solution if your border routers are in different sites to give preference to site specific routes.

Hope to help


vishwancc Thu, 10/08/2009 - 03:33

Hi Giuseppe ,

Are you saying that we should always use MED to infulence the incoming traffic if there are multiple connection with the same ISP.

Because my understanding is that both AS prepend and MED are used to infulence the incoming traffic and you should choose that path selection attribute that is above in the list like AS path.

What are the drawback of using AS prepending instead of MED ?



Giuseppe Larosa Thu, 10/08/2009 - 03:56

Hello Vishwa,

>> What are the drawback of using AS prepending instead of MED ?

its effect extends over the neighboring ISP.

MED is not propagated beyond it.

This may be a desired result when you want to be sure that traffic destined to subset1 of customer routes should come back on R1 for some reasons (symmetrical paths or site specific routes for example)

if both MED and AS Path prepending are set as you noted the second is considered first in BGP path selection hierarchy.

Hope to help


vishwancc Thu, 10/08/2009 - 04:16

Hi Giuseppe,

You are right when you say the AS prepend will propagate beyond the neighbor AS and i think now that make sence..HUUU something to think..




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