What Is BGP?
BGP is the only widely implemented EGP (Exterior Gateway Protocol) and the only routing protocol linking networks to one another on the Internet. BGP was first specified in 1989 in RFC 1105. Version 4 was specified in 1994 in RFC 1654 and updated in RFC 1771. There also have been a number of documented extensions. Version 4's most significant contribution is the ability to aggregate advertisements from multiple contiguous routes in one routing-table entry, a.k.a. CIDR (Classless Inter-Domain Routing). BGP4 was implemented when big routing tables began overwhelming routers. CIDR protects you from many potential outages and instability on the Internet, and provides great relief for address depletion by more efficiently dividing addresses.

When enabled, BGP4 establishes relationships with adjacent routers, referred to as neighbors. Unlike OSPF (Open Shortest Path First) and EIGRP (Enhanced Interior Gateway Routing Protocol), which will automatically discover the routing neighbors, BGP won't exchange routing-table information until both routers have configured one another's IP addresses and ASNs (Autonomous System Numbers) on their corresponding interfaces. Once this is completed, the routers are considered peers.

Neighboring routers send small "keep-alive" messages to one another. If a neighbor stops receiving keep-alive messages for a predefined "hold time," it will update its routing table to reflect the loss in available routes. BGP also sends incremental updates when routes become unavailable. Otherwise, the full routing tables are exchanged only when two routers first establish or re-establish a peering relationship.

BGP is a Path Vector Protocol, which is similar to a Distance Vector Protocol, but with a key difference. A Distance Vector Protocol chooses routes based on the hop count (or routers traversed) and link speeds; BGP, in contrast, chooses a route that traverses the least number of Autonomous Systems (AS). As a routing advertisement passes through an AS, it prepends (adjusts the path length advertised) the ASN of the AS of origin to the path of other ASes it has traversed. By default, the path with the fewest ASNs is stored in the routing table as the optimal path to a destination network. One AS can contain multiple routers, so it's possible the actual hop count is higher than the AS path indicates.

However, with BGP's built-in flexibility, you can enhance this default behavior. For instance, you may want to control the path traffic takes leaving your network. When peering with multiple neighbors in an external AS, or in different external ASes, there will be multiple paths to the same destination network. By default, BGP determines the optimal path by picking the route that has traversed the fewest number of ASes. However, BGP does not take link speed or network load into consideration when computing paths, so the shortest path may not be the optimal one.

You can get around this by using BGP's Local-Pref attribute, which forces BGP to take a particular next-hop route in a scenario with multiple choices. Tell the router that all, or even some, of the routes advertised to one of your router interfaces should receive a higher Local-Pref weight than the same routes advertised to another interface. Because Local-Pref is always considered before the computed path-distance, the interface you designate with the highest Local-Pref will be chosen as the route.

Controlling traffic coming back into your network is more difficult. With geographically diverse networks, where one ISP connection is a lot closer to one part of the network than another, you may want to use the MED (multiexit discriminator) attribute, which specifies the path external traffic should use when destined for one of your internal networks. Although the MED attribute is a fairly simple way to control incoming traffic, it will work only if both Internet connections go to the same ISP because it won't be propagated outside that ISP's AS. Prepending is another way to control incoming traffic.

BGP routing can be controlled through the community attribute that puts a predefined code on a group or community of routes so the receiving router takes a predefined action based on the value of the code. This code can be user-defined, but the most common is a reserved or well-known community, called No-Export. When a BGP router sees a route come in with the No-Export community, it will not advertise the route outside its own AS. This can be handy for balancing incoming traffic.