• BGP stands for Border Gateway Protocol.

• It is an interdomain routing protocol based on path vector routing.

• Need of BGP: It was mainly developed for a need of efficient interdomain unicast routing protocol.

• For communication, BGP uses Classless Interdomain Routing (CIR) addresses.

• BGP Path: A BGP path is a list of attributed which gives some information about the path.

These attributes are of different types as shown ……………..

• Well-known Attribute: This attribute must be recognized by all BGP routers.

  (i) Mandatory Attribute: This attribute must appear in the description of the route.

  (ii) Discretionary Attribute: This attribute must be recognized but may not be included in an update message.

• Optional Attribute: This attribute may be recognized by BGP routers.

  (i) Transitive: If a router does not implement this attribute it must be passed to the next router.

  (ii) Nontransitive: If a router does not implement this attribute it must be discarded.

• BGP Session

BGP establishes a session to exchange information between routers.

When BGP is running inside an autonomous system, it is referred to as Internal BGP (IBGP). When it runs between autonomous systems, it is External BGP.

BGP Packets

• BGP has four different types of packets. They are :

(1) Open

A router running BGP uses open message to establish a TCP connection with its neighbours.

(2) Update

A router running BGP uses update message to either advertise a new route or withdraw a route that has been advertised previously.

(3) Keepalive

The routers running BGP use keepalive message to notify other routers that they are alive.

(4) Notification

A router running BGP uses notification message whenever it intends to close a connection or an error condition is detected.

• BGP packet format consists of the following fields:

  (1) Marker (16 byte): This field is used for authentication.

  (2) Length (2 byte): This field defines the total length of the message (including header).

  (3) Type (1 byte): This field indicates the type of packet. The various types of packets are Open, Update, Keepalive, and Notification.