[c-nsp] iBGP v eBGP

[Vinny Abello]

At 10:09 AM 4/18/2006, Michael Robson wrote:
>For our shortly-to-be-upgraded network core, we are going to have each
>router
>iBGP peering to each other using loopbacks with OSPF as our IGP advertising
>these loopbacks; sites connecting to the core and our WAN links will connect
>using eBGP.
>
>The question: what are the benefits of using iBGP in the core instead of
>just using eBGP everywhere? Someone asked me this and I couldn't answer the
>question to my own satisfaction let alone his.

eBGP is a BGP connection to a different AS than your router's. Unless 
each router in your core is running a different AS on it (ewwww), you 
can't run eBGP in the core. I'm sure there are some convoluted ways 
someone could invalidate what I said, but in general iBGP is what you 
get when peering with routers in the same AS and what you want to do. 
iBGP by default will only update other iBGP peers with routes it is 
originating or has learned from an eBGP speaker. It won't 
redistribute other learned iBGP routes which is why you need a full 
mesh. You can use route-reflectors to overcome this if it makes sense 
to do so. iBGP will scale better than your IGP because it's designed 
to carry more routes. Putting only your loopback and maybe your links 
between core routers in your IGP will enable the IGP to converge very 
quickly. Think of iBGP as a layer on top of your IGP. iBGP will not 
converge as fast (although I think there are some enhancements that 
help convergence times on certain trains) but you can scale to a lot 
more prefixes. You shouldn't normally drop iBGP connections with your 
routers due to a failed link because the IGP is maintaining the 
reachability of all routers on your network.

Some of what I said I'm sure you already knew due to the upgrades you 
are doing.




Vinny Abello
Network Engineer
Server Management
vinny at tellurian.com
(973)300-9211 x 125
(973)940-6125 (Direct)
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