Border Gateway Protocol

By | March 14, 2020

Border Gateway Protocol uses a wide metric of attributes that can be tune. If we are not tuning the attributes then BGP ( Border Gateway Protocol ) behaves like a distance vector routing protocol and uses ASN in the place of hop counts.

It is designed to work slowly because on internet or in a large network, every time many network went down and comes up. If it will work fast then router’s CPU and memory will be waste to calculate these routes again and again.

BGP is a reliable protocol and uses TCP port number 179. BGP advertises the routes in the place of networks and supports multiple network layer protocols. With BGP we need to manually configure the neighborship (peering).

It uses five types of control messages such as: open, keepalive , update , notification and route refresh.

BGP exchanges complete BGP table when first time neighborship is establishing after that it sends the trigger updates. Routes exchange is done using update messages and one update message can carry only one route and its path attributes.

One update message can also carry multiple routes but path attributes should be same for all the prefixes (routes).

BGP is available in two flavors: iBGP and eBGP

iBGP ( Interior Boarder Gateway Protocol)

iBGP ( Interior Boarder Gateway Protocol ) is used for routing under an autonomous system (AS). iBGP neighbor-ship can be establish indirectly. means neighbor can be more than one hop away.

It also forces a rule that all the iBGP routes should be fully meshed. But this restriction can be bypass using BGP route reflector or BGP confederations. iBGP uses the split horizon for loop avoidance.

So a iBGP don’t advertise the routes to its iBGP peer which were received from another iBGP peer. But iBGP peer can advertise the routes to its eBGP peer. Default iBGP administrative distance (AD) is 200.

Border Gateway Protocol iBGP

eBGP ( Exterior BGP)

eBGP ( Exterior Boarder Gateway Protocol ) is used for routing between autonomous systems. eBGP neighbors should be directly connected. But this restriction can be bypass using eBGP multihop command.

It uses AS-Path attribute for loop avoidance. Default eBGP administrative distance (AD) is 20.

Border Gateway Protocol eBGP

To configure the eBGP or make up an eBGP peer , we need only few commands to be configure. Below is the example:

Router# conf t
Router(config)# router bgp 100
Router(config-router)# neighbor remote-as 200

iBGP configuration example:

Router# conf t
Router(config)# router bgp 100
Router(config-router)# neighbor remote-as 100
Router(config-router)# neighbor update-source loopback 0 

I hope , now you are having a basic idea about the BGP. But still there is a question in your mind.

Why we need BGP ?  when we were already having multiple IGP protocols. I have listen many answers for this question but no one given 100% satisfactory answer. They were right but not 100 %. So I have tried to give you best answer for this question. You can read the answer on below link:

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One thought on “Border Gateway Protocol

  1. jong

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