BGP: Border Gateway Protocol

It is an EGP Protocol (It forms neighborship between two different AS’s).
It is an Path Vector routing Protocol (It uses multiple attribute to select best path).
It is highly scalable than IGP (It can have lakhs of routes in its BGP table).
It is reliable protocol (It uses TCP).
It uses TCP port number 179 & before establishing neighborship, it form TCP connection with it’s neighbour using TCP 3-way handshake process.
Only BGP is fully reliable routing protocol.
It is more secure than any other routing protocol because all massages will travel on TCP protocol.
It is an classless routing protocol.
It support manual summarization.

A.S. size: 2bytes (0-65535) and 4bytes (0.0-65535.65535).
Public A.S= 1 to 64511, Private AS= 64512 to 65534.
BGP doesn’t support dynamic neighbor discovery.
It supports only md5 authentication
BGP convergence is slow as compaire to IGP.

Note: We can’t configure more than one AS on a single router.
Note: BGP send all type of packets as unicast.

Types of neighborship in BGP

IBGP : Internal border gateway protocol

EBGP Neighbor: external border gateway protocol

Administrative Distance:

IBGP: 200

EBGP: 20

Why we use BGP
IGP’s were developed for to handling upto 1000 of routes inside their routing table. Therefore can’t be implement in very large networks.
BGP can contain lakh’s of routes inside their BGP table or routing table.
We never use default route in ISP router to other ISP router.
In IGP’s if any link (prefix) goes down thn it effects to complete routing structure (topology). But in BGP, it does not affect routers bgp table.
When we are service provider or network size is very large.
When we have multiple connections for internet or multiple service provider.

Types of messages in BGP
There are four types of messages in BGP, each type of BGP packet will travell over TCP. It is not possible to transmit any single massage of BGP without forming TCP connection.

  1. OPEN message
    Open massage send only once when neighbourship comes up. After that BGP will not send any open massage.
    Open massage will be used by BGP to exchange initial configuration between two bgp peers which is require to form bgp neighorship.
    contents of Open massage
    version 4
    own AS
    Hold down timer (180sec)
  2. Keep-alive massage
    After exchanging open massage successfully between both router, router will start to send keep-alive massages periodically after every 60sec just to inform its neighbor that i am still alive.
    If any router doesn’t receive keep alive massage from its neighbor within 180sec (hold down time) than router will break down the neighborship
  3. Update
    update packets will be responsible to carry routes information. update packet having some components like Path Attributes, Prefix info/NLRI, Withdrawn routes.
    Note: if any router have multiple prefixes and all prefix attributes are same than router will generate a single update message.
  4. Notification
    if peering goes down than it will generate notification message and it will contain the cause of problem