O men, when you start to write about BGP it is probably the time then you seriously start questioning yourself where did I go with myself. That is probably the moment in which you realize that there is a network geek sitting somewhere inside you. At least that is what happened to me when I finished to write this huge post. Don’t be scared, it’s fun to know about this thing below.
Every local network is managed by his own network administrator. If the network become big enough and there are more than few sub-segments inside that network there will probably be some kind of routing protocol running inside. That routing protocol will be IGP or interior gateway protocol more probably OSPF as it’s vendor independent.
When we want to connect our network to other networks across the world, we are trying to connect it to the internet. The Internet is the network connecting most of the networks today and in that way it became the biggest inter-networking system in the world. To be able to get that huge network to function and get our LANs to act jointly there must be a routing protocol that enables it.
BGP – Border Gateway Protocol is that one.
Every individual network has his own policies that are enabling that network to behave as the administrator want. When connection networks to the internet network all those policies need to be tied together with BGP protocol in order to influence outside communication entering the local network and communications initiated from the local network going outside somewhere on the internet. This is done using more that few different BGP attributes. Those attributes are forwarded across specific prefixes. Sometimes those attributes are not only forwarded but also modified on the way, one of which is the community attribute.