Configuration of BGP

Before moving to BGP configuration first you have to understand its working, terminology, attributes how does it works...
So in this chapter I am going to write BGP Terminology.

1. BGP Terminology-

Main Terms which used in BGP are-
a) BGP Peers
b)Autonomous System (AS)
c)IANA & RIRs
d)NLRI
e)BGP Types
f)BGP Attributes

a) BGP Peers and Peering -

     * BGP Neighbors are called BGP Peers and when BGP Routers exchange routes with another BGP speaking device called BGP Peering.
    * Neighbours established by manual configuration.

b) Autonomous Syastem (AS)-

     * Group of routers that share similar routing policies & operate within a single administrative  domain, typically belongs to one organization can be between 1 to 65,535.

c) IANA and RIRs - 

    * IANA- Internet Assigned Number Authority -( work to manage IP address and AS no.)
    * RIR - Regional Internet Registries -(IANA use RIRs to distribute IP and AS no, there are 5 RIRs across the world)

d) NLRI-

   * Network Layer Reachability Information - By using this we can reach to other network.

e) BGP Types- 

    1.iBGP (Internal BGP)--- Neighbors that belongs to same AS and need not to connected directly.
    2.eBGP(External BGP)--- Neighbors that belongs to different AS and must connected directly.

f) BGP Attributes-

The metrics used by BGP are called path attributes.

    1.AS Path
    2.Next Hop
    3.Local Preference etc... 




2.BGP Message Type-- 


BGP use 4 message type for its functioning. ie whenever a BGP Speaking Router wants to communicate with other BGP speaking Router then Routers use these 4 messages...

a)Open
b)Update
c)Keepalive
d)Notification


a)open-  

     After TCp connection has been established (Note- BGP use port no-179 of TCP for working) open message is sent. This OPEN message contains set of parameters (eg- MD5 password). And before establishing full BGP adjacency neighbor router have to be agreed on these parameters.

b)Update-

     If open message is sent and both routers are agreed on parameters then update message is sent.
     It contains the actual route updates.
     Update Message is composed of
        (i)NLRI
        (ii) AS-Path
        (iii) AS-Path Attributes.

c)Keep-alive Message-

     If routers are not sending information then they can have information about their neighbor with help of Keepalive Message.Keepalive message keep the session running when there is no updates. If a router don't get keepalive message then router remove all paths header from that router which not sending keepalive message.


d)Notification Message-   

     Notification message is used to send error message. For eg update recived is corrupt or router needs to turn down the session unexpectedly. 




3.BGP States-

We can understand BGP state by this FSM (Finite State Machine)--

i)Idle -
       - Not accepting incoming BGP connection.
       - Start a TCP connection with BGP peer.
       - Listens for TCP connection from peer.

ii)Connect -
      - Listens for a TCP connection from peer.
      - If connection is successful then send open message and goto "OPEN SENT".
      - If unsuccessful then go to "ACTIVE".

iii)Active -
      - Unable to establish TCP session.
      - Restart another TCP session with the peer "IDLE".

iv)Open Sent -
      -Listen for an open message from it's peer.
      -Router check the validity of the open message like "BGP Version, MD5 password".
      - If any error occur then sends a notification message to the peer and Indicate why error occured.
      -If there is no error then send Keep alive  message, various timers are set and state is changed to                      "Open Confirmed"

v)Open Confirmed -
     - Peer is listening for a keep alive message.

vi)Established -
    - Peers are send update message.
    - Exchange information about each route being advertised to BGP Peer.