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Three ISP - Three Edge Routers - iBGP Mesh


Currently I have two service providers and two edge routers. RTR1 is iBGP peered to RTR2 and RTR2 is iBGP peered to RTR1. We are planning to install a third service provider and install RTR3. Making sure I have my iBGP mesh setup properly, and also considering "Three Different Service Providers" configurations aren't typically found with Google search can I get some feedback on the topo above? Seems simply but didn't want to run into any gotcha moments on deployment day!

RTR1 -> RTR2 | RTR1 -> RTR3

RTR2 -> RTR1 | RTR2 -> RTR3

RTR3 -> RTR1 | RTR3 -> RTR2

New Member

Re: Three ISP - Three Edge Routers - iBGP Mesh

Yep ibgp peers as full mesh as they are all ebgp peers as well. Any more and you would be better with one as a route reflector

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Three ISP - Three Edge Routers - iBGP Mesh


Yes, you got it perfect for iBGP mesh.

However, if your are using any other IGP between your CE, then I would suggest disable bgp syncronization.


Are all 3 CE connected to same LAN segment or are they catering different.

If they are connected to same LAN segment, then you can use GLBP to load balance across 3 links.



Three ISP - Three Edge Routers - iBGP Mesh


I'd be careful here.

GLBP might be nice for outgoing traffic load sharing if a LAN with PCs is connected.

But in this case it's not so easy:

a) even when a PC is using one router as it's default gw, the router can still forward the traffic to another iBGP neighbor here if receiving a better route via iBGP,

b) traffic returning to the PC can take a different router (=link) on the returning path.

So an asymmetric routing can occur here easily - this is the true challenge in topologies like this.

I even don't see in the topology description:

Is this connecting the LAN to the Internet or some entreprise backbone?

If enterprise, are there some sites connnected to multiple providers or any kind of gateways between them?



Three ISP - Three Edge Routers - iBGP Mesh

Hi ,

I can see that you are using HSRP between RTR1 and RTR2 and the RTR3 is the third in the group so at any given time you have a primary /back up situation. so thats good in terms of redundancy.

now coming to BGP.

make sure to have local pref and AS-path prepend configured correctly so tht you dont end up with assymetic routing.

other than that it should be fine.

for eg: RTR 1- local pref 140

         RTR2  - local pref 120 , as-path prepend

          RTR3 - local pref 110 , as parth prepend

so routing is consistent