Sunday, May 20, 2012

Redistributing eBGP into anything!?

I was just witnessed a very surprising conversation through NANOG.  The person was asking if he can redistribute their full BGP table into OSFP.  Of course, the rest of a forum was able to assist this poor chap out.

This is a problem on several levels.   Frist of which, OSPF is designed as an IGP meant to find the best path to a prefix within a AS, not the entire internet, which BGP is designed for.  Therefore, it is not recommended to inject an EGP into an IGP such as OSPF.  On the other hand, BGP, or eBGP more specifically, is designed for route control, not best path discovery; or in other words, to provide the AS control of what routes get advertised out of the AS, what routes get advertised into the AS, and what path to take for egress and ingress traffic.

Second, a full internet BGP table is 400,000+ prefixes.  I cannot image a router that can calculate that many SPF algorithms, or any other IGP protocol, and make it through convergence, not to mention if there is a one area or several areas within OSPF.  I suppose it would depend on the amount of memory, and processing power of the router.  Maybe there is a router that can, but could you imagine maintaining a route table with 400,000 prefixes, it would be an administrative nightmare.

Unless you are an ISP, keep the EGP at the edge or within the core for specialty cases.  Also, unless you are multi-homed with 2 or more ISPs there is not really a reason to have a full BGP table.  I do acknowledge that there are reasons to redistibute individual routes into the IGP for path selection within the AS, but there are other ways of accomplishing this within the IGP.  I can imagine also redistributing a default route learned from BGP into the IGP, but OSPF does not support this feature in Cisco IOS, though it is supported in JUNOS.

If you have had experiences redistributing BGP into an IGP I would be very interested in hearing how you have utilized it.  I have myself redistributed a default route into EIGRP within a multi-homed environment, and also did some interesting things with MPLS from a practical perspective, and done other things in the lab.

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