BGP

BGP (Border Gateway Protocol)

BGP is an Exterior Gateway Protocol (EGP), which means that it exchanges routing information between Autonomous Systems (ASs). This is different from purely IGPs(interior gateway Protocols), such as RIP, EIGRP, and OSPF, which we discussed earlier.

It uses a different basic algorithm for building a loop-free topology than any of those protocols. RIP is a distance vector protocol, OSPF is a link state protocol, and EIGRP is a distance vector protocol that incorporates many of the advantages of a link state protocol. BGP, on the other hand, uses a path vector algorithm. This means that instead of reducing each route's relative importance in the routing table to a single metric or cost value, BGP keeps a list of every AS that the path passes through. It uses this list to eliminate loops, because a router can check whether a route has already passed through a particular AS by simply looking at the path.

BGP is especially used for exchanging routing information between all of the major Internet Service Providers (ISPs), as well between larger client sites and their respective ISPs. And, in some large enterprise networks, BGP is used to interconnect different geographical or administrative regions.

Primarily to support the complexity of the public Internet, Cisco has added several clever and useful features to its BGP implementation. Some of the primary attributes of BGP is the use of pieces of information about a known route, where it came from, and how to reach it, A BGP router will also generate an error message if it receives a route that is missing these are mandatory attributes.

Whenever a router passes along a route that it has learned via BGP to another BGP router, it must include all of the well-known attributes that came with this route, including any discretionary attributes learned, Of course, the router may need to update some of these attributes before passing them along, to include itself in the path.

There are some optional attributes too; these are not necessarily supported by all BGP implementations. Optional attributes can be either transitive or non-transitive, which is specified by a special flag in the attribute type field. If a router receives a route with a transitive optional attribute, it will pass this information along intact to other BGP routers, even if it doesn't understand the option. The router will mark the Partial bit in the attribute flags to indicate that it was unable to handle this attribute, however.