BGP

BGP is a path vector routing protocol. He care about how many AS path numbers to pass not routers that why path vector protocol. BGP uses TCP port 179. We need to advertise exact network at BGP if not we don't see any network. We shouldn't forget about TTL of BGP. eBGP TTL is 1 and iBGP TTL is 255.

BGP have 6 states:
1. Idle
    Process shutdown or waiting for next retry.

2. Connect
    Waiting for TCP connection to complete.

3. Active
    TCP conection ready, No BGP messages sent.

4. Opensent.
    Open message send.

5. Openconfirm
    Receive open message from neighbor.

6. Established
    Finish peering and start routing.

eBGP and iBGP is difference only two word, between and within.

eBGP AD 20.
iBGP AD 200.

Difference AS but want to advertise as a same AS use confederation.

Ref : Wikipedia, Cisco