iBGP and eBGP use case scenarios

[dtran]

Hello everyone,

I am trying to figure out the use cases for iBGP vs eBGP. I understand the differences between the two but I am having a hard time figuring out in what scenario would you use iBGP vs eBGP.

Thanks everyone in advance !!! I appreciate any inputs / suggestions !!!

Danny

[paul driver · Accepted solution]

Hello

One scenario would be for bgp resiliency.

When you have a site with dual EBGP rtrs that are peering with say dual ISP rtrs, These site rtrs will always relate internally to a single site bgp ASN that each isp wan rtr needs to peer towards.

Now to provide resiliency for your site between these ISP's both your site bgp rtrs will be required to peer internally between each other forming a internal bgp peering (IBGP).

Now any ebgp routes learned from either ISP will be advertised internally between the ibgp peers/rtrs.

[dtran]

Hello Paul,

Thanks Paul !!! I very much appreciate your inputs !!!

Below diagram is one of the scenarios that I am trying to solve. As you can see my Nexus 7010 switches are connected to two WAN devices. The Cisco 4431 router is connected to WAN 2 talking EIGRP to the Nexus 7010's on the LAN side, where as my other third-party device is connected to WAN 1 talking BGP to the Nexus 7010's on the LAN side. My end goal is I would like the Nexus 7010's to always prefer WAN 1 first, so in this scenario I would want to use EBGP between the Nexus 7010's and the third-party device ?

Thanks Paul !!!

Danny

[Jon Marshall · Accepted solution]

EBGP will have a lower AD than EIGRP so yes you can use that with no modifications or you could run IBGP but you would then need to modify the AD of either EIGRP or IBGP routes as EIGRP now has the lower AD. 

Jon

[dtran]

Hello Jon,

Thanks Jon !!! I appreciate your response !!!

Danny