Easy question on BGP approach - production

Unanswered Question
Feb 15th, 2010


Situation:imagine that all traffic from my network currently flows thru Site1 - 4507 then go to the C7206VXR -> GW.

As a backup, I could order a physical link between the Site2 going to a GW-2 (corporate). That way in case Site1 4507 or site1 physical connections are unavailable, I could have traffic flowing thru Site2 instead and reaching the Corporate network OK.

That said, my questions regarding BGP:

a) if I set site1-4507 on BGP AS#100, and then I make site2-4507 on AS#100;

b) then I install a new C7206VXR-2 connected to Site2 - 4507, similarly to what I have on Site1. Then this new C7206VXR-2 will be the Corporate BGP peer on AS#200. I also make C7206VXR on site1 on AS#200.

Does that make sense?

At that point I could use local-preference to control outbound traffic, correct?

Please let me know whether that approach makes sense.

I have this problem too.
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Giuseppe Larosa Mon, 02/15/2010 - 14:29

Hello Marlon,

>> At that point I could use local-preference to control outbound traffic, correct?

local preference can be used only within a single AS domain.

The typical design for a scenario like yours would be to put all your devices in same AS number with iBGP sessions between all devices (a full mesh is required fo the iBGP split-horizon rule).

eBGP sessions with providers gateways GW1 and GW2 are propagated on the iBGP full mesh.

local preference in a single AS can be used to choice a preferred exit path per destination route basis.

Hope to help


news2010a Wed, 02/17/2010 - 08:44


Just a quick question, from reading some documentation on BGP design, it seems in practical terms I should not worry with load-balancing. I mean, if my current connection from existing Site1 to Corporate network is 100Mbps, then if I add the Site2 to Corporate at 100Mbps I would just keep Site1 -> Corporate as my preferred route. Then I would use Site2 to Corporate only in case of failure on Site1->Corporate and I should forget about trying to load-balance this.

Do you agree that is a good approach? Do not worry with load-balancing complexities in this scenario even though I will have an idle link on Site2->Corporate? You can assume that in this case, Site1->Corporate link is underutilized so no imminent congestion situation or performance is driving my question.

Giuseppe Larosa Wed, 02/17/2010 - 12:55

Hello Marlon,

yes in an iBGP context in classical IP ( no MPLS L3 VPN) outbound load balancing is difficult to achieve.

So now I understand why you had thought to use two different AS numbers.

However, if the two C4500 take part in the iBGP full mesh, one can prefer the routes coming from first C7206VXR and the second can prefer paths coming from second C7206 VXR.

if so then you can get that Site1 goes to corporate via link1, C7206VXR1 and site2 via link2 C7206VXR2

Hope to help



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