Border Gateway Protocol | External Gateway Protocol

Fig 1: Border Gateway Protocol

The Border Gateway Protocol connects an autonomous system to the Internet and other such networks. BGP is the Border Gateway Protocol. It is the mechanism used to exchange routing information between autonomous systems. BGP is used to exchange routes between autonomous systems. These routes are then used to exchange routing information between each other. BGP routes are usually exchanged between Autonomous Systems (AS). Each AS is a self-contained routing domain. BGP is the protocol which is use for the routing between the different autonomous systems. BGP is a external gateway protocol that we are using nowadays. Below is an image that show you the place of BGP where it configure. The exact place is between the different autonomous systems and autonomous system basically is the collection of the routers. Here below there are the two different autonomous system 100 and 200 respectively and BGP is configure between autonomous systems. This protocol is the largest protocol that holds the multiple autonomous system for the communication and this is known as the EBGP.

• Autonomous system numbers are 16 bits and 2^16=65535 means that are numbers of autonomous systems we can choose from this numbers.
• There are the public and private autonomous system numbers just like public and private addresses.
• The range 1-64511 are globally unique/public AS numbers that are for the commercial purposes.
• The range 64512-65535 are private autonomous system numbers.

When to use BGP ?

When autonomous system work as transit autonomous system and the other condition is when autonomous system connected to the multiple autonomous systems means when AS multi-homed.

When BGP is not appropriate ?

BGP is not appropriate if the autonomous system is single-home. When lack of resources there should no need to use BGP and the other main point is that when you are not familiar with the BGP how to use it and when you don't know about the BGP Route filtering and the processes of path selection. 

BGP Features

• BGP is also used internally means inside the autonomous system and is known as the IBGP.
• Border Gateway Protocol is the Open Standard Protocol.
• It is also known as the Path Vector Protocol because of when one route originated it specify all the AS Number where the route came from.
• It is designed for the inter domain routing .
• AS path is the feature of BGP that is used for loop avoidance routing.
• In this protocol we have forcefully establish the neighbor ship.
• BGP has no metric.
• BGP uses attributes for path selection.
• BGP is TCP based use the port no 179.
• BGP peering can be configured between directly connected and indirectly connected.
• BGP has two flavors 1)External Border Gateway Protocol 2)Internal Border Gateway Protocol. 
• Administrative distance of EBGP is 20.
• Administrative distance of IBGP is 200.