Can BGP be used like EIGRP / OSPF?

[ranjipremandrewp]

Can BGP be used like EIGRP / OSPF? Can some one explain?

[Peter Paluch]

Hi,

Can BGP be used like EIGRP / OSPF? Can some one explain?

In general, it can not. BGP depends on EIGRP or OSPF (or any other IGP protocol) to provide full reachability within an autonomous system, for three primary purposes:

• IGP constructs a list of networks present inside an autonomous system that is subsequently imported into BGP and advertised to other autonomous systems. In other words, an IGP protocol is the source of routing information about an autonomous system that is fed into BGP.
• Inside an autonomous system, BGP peerings must either be configured mutually between all routers, or at least between a route reflector and all other routers. In both cases, these peerings are configured using neighbor's IP addresses and may (and usually will) span several hops. Without an IGP, a router would not know how to reach its neighbor's IP address to bring up the BGP peering. If you wanted to run only BGP in your autonomous system, this would create a chicken-and-egg problem.
• For routes learned via BGP from other autonomous systems, the next hop address is set to the boundary router's address that connects to the other autonomous system. In order to reach those external routes, your routers must first know how to reach the boundary router through which the external routes are reachable. This reachability is again provided by an IGP.

In common scenarios, BGP is deployed along with IGP.

Please feel welcome to ask further!

Best regards,

Peter

[Joseph W. Doherty]

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Posting

As Peter described, normally you wouldn't use BGP in place of an IGP like EIGRP or OSPF.

However, if you're a masochist, you could.

For example, if all your routers were connected by p2p links, you could configure each router in its own AS and run them as eBGP peers.

What Peter was alluding to, normally interior BGP peers run as iBGP peers, but then each requires a connection to every other peer (full mesh).  (There's also BGP route reflectors or BGP confederations, which limit the need for interior iBGP peering to all other iBGP routers, but neither deals with how the iBGP routers "know" how to route to their required peers.)  Normally an IGP protocol is used to provide reachability, but you could do the same with static routes.  Of course, whether you have an IGP or use static routes that maps out the whole interior topology they both beg the question of why also then have iBGP too (if being used just for the interior topology).

There's a limit to the number of AS numbers, especially if using just the private range, so you could combine the two approaches, such as having small clusters of iBGP peers (maybe 2 or 3) which eBGP peer to other clusters.

If you did go to all the trouble, you would then find that BGP will likley converge  slower than most IGPs and unless you do some manual policy configuration, BGP won't path select much better than RIP.

What BGP does well is handle massive route tables (like the Internet) and allow very sophiscated (manual) routing policies.  However, both are usually not as desirable for an IGP.