BGP Terminology

Autonomous System
An AS is a group of routers that share similar routing policies and operate within a single AD. Unique AS number is managed by IANA.
- use 16 bit(1-65535)
- public AS number is 1-64511.
- private AS number is 64512-65535.

Internet Assigned Numbers Authority
- Divided into five RIRs(Regional Internet Registry). AFRINIC, APNIC, ARIN, LACNIC, RIPE NCC.
- assigned unique AS number and IP Address.

BGP
Internet is a collection of AS that are interconnected to allow communication among them. BGP provides the routing between these AS.
- is a path vector protocol.
- use TCP.
- latest version is BGPv4.

Later, I will discuss Comparison of BGP and IGPS.

EBGP - run between routers in different AS.
IBGP - run between routers in the same AS.

EBGP neighbors need to be directly connected.
- need TCP session (three way handshake)
- different AS numbers , AD - 20.
IBGP neighbors must be reachable usually by using an IGP. Loopback IP are used to identify IBGP neighbors.
- must be established TCP session.
- same AS number, AD - 200.

Transit AS
A transit AS is an AS that routes traffic from one external AS to another external AS.
IBGP in a transit AS - fully meshed BGP internetwork and BGP runs on all internal routers                                    and all routers establish IBGP sessions.
IBGP in a Non-transit AS - To avoid routing loop within an AS, BGP specifies that routes                                             learned through IBGP are never propagated to other IBGP                                                 peers.