BGP - [Part 5] - LOOP PREVENTION MECHANISM

BGP uses two mechanism to prevent loops:-

Split Horizon Rule

• As per BGP loop avoidance mechanism-Split Horizon Rule(Within IBGP), when a router learn routes from an IBGP peer, that router does not advertise the same routes to another IBGP peer.

• Check the below diagram, there are three Routers within the same Autonomous system, and all of these are IBGP peer of each other, Router R1 will send the routes to R2, and Router R2 will learn these routes, but Router R2 cannot advertise the same routes to Router R3.

• So according to Split Horizon rule, all iBGP peers within an AS must be fully meshed. But we can break this rule via Route Reflector.

• So we used the Route Reflector to break the split horizon rule and to reduced number of IBGP peering.

AS_Path Rule

• When an EBGP peer advertises to an EBGP peer, then BGP router adds its own ASN to the AS_PATH. If a BGP router receives an update then it will check AS_PATH list, if it found its own ASN in that list , the router will ignores that route.

• Check the below diagram there are four different Router in four different Autonomous System, all of these are EBGP peer for each other, when Router R1 is advertising to EBGP peer R2, then it will add its own Autonomous System number 500 to AS_Path, and the same thing will do all other router, and finally when Router R1 receives an update, and it found its own ASN in the AS Path list, so it will ignore this routes

Note - A BGP router does not add its ASN when advertising to an IBGP peer.

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Anubhav Upadhyay

Hello and welcome to networktopic Blog. My name is Anubhav. I am a Senior Network Egineer. I have created this blog specially to serve interview questions and answer on Network Routing and Switching, I will try my best to serve correct and updated networking knowledge for you as per my corporate experience.

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