BGP’s Loop Prevention Mechanism
Border Gateway Protocol (BGP) is mainly used for communication between different AS numbers. There are two types of BGP, IBGP and EBGP. IBGP, Interior Border Gateway Protocol, is used to communicate in the same autonomous system and EBGP, Exterior Border Gateway Protocol, is used to communicate between different autonomous systems. First of all what is a loop and what are the consequences of the formation of loops? Whenever one networking device receives its own sent packets it is called a loop. Due to loop formation number of packages in the system increases and more duplicate packets are introduced which are not required and increases the load on the networking device and uses unnecessary memory of that device and slows them down. BGP has its own default loop prevention mechanism which works depending on whether the system is IBGP or EBGP. In EBGP there is communication between more than one autonomous system. Whenever a packet is received at the destination, AS path is attached to that packet which contains information about its journey that from how many AS that packet has passed to reach the destination and also its origin AS. So when any AS receives a packet with his own AS written on a packet it will discard that packet due to which there is no formation of a loop. This is a loop prevention mechanism in EBGP. In IBGP There is communication between only one AS. Information send by one IBGP neighbor will not be sent back to other IBGP neighbors instead it will send his own information. Let us consider it with example. Suppose there are 4 routers connected into a ring topology. Give numbers to routers in a clockwise direction starting from the left side router. R2 and R4 are connected to R1 so they will get information about networks connected to R1 as they are neighbors but R2 and R4 will not send that same information, about R1, to R3 and instead, they will send information about their own networks. This is called the Split Horizon rule which acts as a loop prevention mechanism in IBGP. Hence R3 has no information about R1. The same thing can be applied to all routers that R1 has no information about R3, R2 has no information about R4 and R4 have no information about R2. Full Mesh topology, Route Reflectors, and BGP Confederation are possible solutions to overcome the split-horizon rule.
