I’ve found one of the best answers to the question “What is BGP for?” in one of Philip Smith’s presentations (google for “BGP Best Current Practices”): “What is an IGP not for?”

Internal Routing Protocols (IGPs), like IS-IS and OSPF, are used to maintain infrastructure connectivity. They carry infrastructure addresses and are designed for rapid convergence within reasonably sized and manageable routing domains. On the other hand, BGP is designed primarily as a routing policy tool. External BGP carries Internet prefixes and exchanges them between autonomous systems according to some policy and rules of good behaviour. BGP is also used internally to carry customer prefixes and adds full flexibility to the routing decisions. BGP can handle huge routing tables, but in a somehow stable environment. It is designed to scale with expanding networks but not to respond quickly to topology changes.

It is very common in small networks that some sort of IGP is “home alone routing protocol” and it is used also to carry customer routes. Redistribution is being used to insert customer prefixes in IGP, backup connectivity is provided by a combination of static routing and some flavour of dynamic IGP. Routing policy? If this is really needed in some obscure case, we can always configure distribute-lists, prefix-lists, route-maps, you name it. It works fine in a small network.

Just a second! I’ve just heard somebody shouting: “Please, define ‘reasonably sized’ and ‘small’! What is the magic size of the network to go for BGP, spend more money for routing software licensing and buy more expensive hardware?” Well, I can’t argue that – poor men can always do all the work with static routing ;-).
It is not the question about the size of the network – it is all about a proper design that aims toward:

  • Minimising the network resources (lower cost)
  • Speeding up routing convergence (better user experience)
  • Efficiently implementing flexible routing policies (good strategy)

A combination of an IGP and iBGP brings best from both worlds to our networks: rapid convergence, scalability and flexible routing policy decisions. It is the winning combination.

And, hey guys – BGP is not a luxury anymore! It often comes along with all standard IGPs in affordable IP-routing packages like Cisco’s IP Services IOS. No big deal in the year 2013.