Mumbai University > Information Technology > Sem 4 > CN

Marks: 5M

Year: May 2016 > Dec 2016

• 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.