BGP Key Concepts Part I

Border Gateway Protocol (BGP) is an Exterior Gateway Protocol (EGP) used for routing and reachability between inter-autonomous systems.  Autonomous system can be defined as an administrative unit with common policies for routing.  IGPs are used for routing and reachability within an autonomous system (as earlier defined; note different from concept of Areas in OSPF); typically within a customer network.  BGP is used on the internet and between ISPS.

BGP comes in two forms IBGP and EBGP.  Internal BGP (IBGP) is BGP used to exchange routes within an Autonomous System (AS).  External BGP (EBGP) is BGP used to exchange routes between Autonomous System.  BGP uses TCP port 179 to exchange routes between BGP peers (BGP neighbours manually configured).  External BGP’s default administrative distance is 20 and internal BGP’s is 200.

BGP attributes are used to determine the best routes to a destination when multiple paths are available to a particular destination.  The following lists the attributes BGP uses in the route selection process.

1. Weight
2. Local preference (LP)
3. Multi-exit discriminator (MED)
4. Origin
5. AS path
6. Next hop
7. Community