iBGP or IGP in this design?

[Saman Shamim]

Hi,

According to this example from official CCNP book, the BGP design calls for making the packets flow over the route as shown.

And according to that book, two actions must take place for this design to work:

1. Make BGP specify a prefix as being best reached through a particular link (BGP PA settings). I'm fine with that, no questions here.
   
2. Need to run BGP between multiple routers. E1 and E2 must communicate BGP routes using iBGP connection and because packet forwarding between E1 and E2 goes through other routers (such as Core1 and Core2), those routers also need to run BGP.
   

Here is my question:

Instead of running iBGP on non-enterprise routers, can't we just add IGP to Enterprise routers and have them to redistribute their eBGP learned routes into IGP, forward them to enterprise network and leave the non-enterprise routers running IGP?

Thanks,

Saman

[Joseph W. Doherty]

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Posting

Here is my question:
Instead of running iBGP on non-enterprise routers, can't we just add IGP to Enterprise routers and have them to redistribute their eBGP learned routes into IGP, forward them to enterprise network and leave the non-enterprise routers running IGP?

Perhaps, but often the two principle reasons for using BGP is number of routes being distributed, e.g. IGPs generally cannot deal with the number of routes in the Internet, and/or we want to engineer a path for specific traffic.

[Alessio Andreoli · Accepted solution]

Hi Saman,

i totally agree with you. An IGP (OSPF,EIGRP or IS-IS) would be even more flexible in this scenario that otherwise in the near future can represent some scalability problems should you add some routers more due to the fact that no BGP route reflectors are in place. If the Joseph guessing is right (you have a lot of prefixes) just use summarization and you will definitely see benefits in your design. Except for SP environment, iBGP should never be extensively deployed in an enterprise. You you want a more valuable opinion about this topic, Jeff Doyle (TCP/IP Vol 2) makes clear why we use BGP and ...it is very interesting

Alessio

[Saman Shamim]

Thanks you Joseph and Alessio.

In our real world project, the enterprise routers will communicate with a Data Center in a Virtual Private Cloud over the MPLS. So they will just recevie one single prefix advertised by Data Center routers. No outbound routing toward the internet will happen here. The motivation to use BGP is to provide failover at the both side of the network. I think in this case, it's safe and better to use IGP within the enterprise.

Alessio, I'll check out that book.

Thanks again,

Saman