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What is BGP Load Sharing over Multiple Autonomous Systems

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TitleWhat is BGP Load Sharing over Multiple Autonomous Systems
Question
BGP Load Sharing is a common scenario; however, it is usually done in two or more links connected to a unique Autonomous System (ISP), if we have two or more ASs particular configuration is needed.
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Answer
BGP Load Sharing

BGP Load Sharing is a common scenario; however, it is usually done in two or more links connected to a unique Autonomous System (ISP), if we have two or more ASs particular configuration is needed.

One eBGP router to another eBGP router

Having two routing connections to a neighbor ASis acommon configuration used for redundancy in a single-homed AS environment, instead of having only onephysical link, we connect two of them, and theEBGP adjacency is establishedusing a Loopback interface, it is important to keep on mind additional configuration is needed such as ebgp-multihop for the OPEN message to have ahigher TTL value,and Update-source command for theadjacency to be sourced froma specific interface loopback.In this particular case BGP will perform load sharing by itself, since we only have one BGP adjacency against a neighbor AS over two routing links.

device(config)# router bgp
device(config-bgp-router)# neighbor 10.10.10.10 ebgp-multihop 2
device(config-bgp-router)# neighbor 10.10.10.10 Update-source loopback 1

One eBGP router to two eBGP routers in a unique AS

A different scenario is when we have two adjacencies for two routers part of the same neighborAS.In such a case, we will also need to inform instruct BGP to perform load sharing across those two linksusing the Maximum-paths feature.

device(config)# router bgp
device(config-bgp-router)# maximum-paths 4 use-load-sharing

One eBGP router to two different AS eBGP routers

Brocade alsosupports load-sharing across Autonomous systems, it is achieved by using the multipath feature.
To enable load sharing of IBGP paths only, enter the following command at the BGP4 configuration level of the CLI.

device(config-bgp)# multipath multi-as

multi-as - Multipath load sharing is enabled for paths from different autonomous systems.

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