BGP Load Balancing Overview

The Border Gateway Protocol (BGP) is the core routing protocol of the Internet. It maintains a table of IP networks or ‘prefixes’ which designate network reachability among autonomous systems (AS). It is described as a path vector protocol. BGP does not use traditional IGP metrics, but makes routing decisions based on path, network policies and/or rulesets.

This short video provides a good introduction to BGP load balancing.

BGP was created to replace the EGP routing protocol to allow fully decentralized routing in order to allow the removal of the NSFNet Internet backbone network. This allowed the Internet to become a truly decentralized system. Since 1994, version four of the protocol has been in use on the Internet. All previous versions are now obsolete. The major enhancement in version 4 was support of Classless Inter-Domain Routing and use of route aggregation to decrease the size of routing tables. Since January 2006, version 4 is codified in RFC 4271, which went through well over 20 drafts based on the earlier version 4. The version corrected a number of errors, clarified ambiguities, and also brought the RFC much closer to industry practices.

Load-balancing problem with BGP

A factor causing growth of a routing table is the need for load balancing of multi-homed networks. It is not a trivial task to balance the inbound traffic to a multi-homed network across its multiple inbound paths, due to limitation of the BGP route selection process. For a multi-homed network, if it announces the same network blocks across all of its BGP peers, the result may be that one or several of its inbound links become congested while the other links remain under-utilized, because external networks all picked that set of
congested paths as optimal. Like most other routing protocols, the BGP protocol does not detect congestion.

To workaround this problem, BGP administrators of that multihomed network may divide a large continuous IP address block into smaller blocks, and tweak the route announcement to make different blocks look
optimal on different paths, so that external networks will choose a different path to reach different blocks of that multi-homed network. Such cases will increase the number of routes as seen on the global BGP
table.

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