BGP Multipath - By default installs only a single path in IBGP and EBGP deployment.If prefixes are learned via multiple paths, BGP supports multipath for IBGP, EBGP, or across both IBGP and EBGP via EIBGP Multipath feature. In this blog post, we will explore each of these options: IBGP Multipath, EBGP Multipath, and EIBGP Multipath.
First of all, we should know that BGP Multipath feature should be enabled manually on the network device, unlike IGP protocols.
In IGP Protocols, when there is Multipath, commonly referred to as ECMP (Equal Cost Multipath), Routers install all next-hops for the destination prefixes, to the routing and forwarding table automatically, without manual human operation.
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Let's start with the first EBGP Multipath:
EBGP Multipath
Figure - EBGP Multipath
We have two rules for EBGP Multipath.
1. For the destination prefix, if there is more than one path, total number of AS - Autonomous System should be same across different
paths
2. For the destination prefix, if there is more than one path, same AS numbers should be in the AS Path.
On the above figure, since from AS 100 to the destination prefix, 192.168.0.0/24 there are two paths and both have 2 AS-Path lengths,
Rule 1 is satisfied.
Also, AS numbers are the same towards the destination prefix, which is AS100 - AS200, and finally the destination.
Let's have a look at now IBGP Multipath:
IBGP Multipath
Figure - IBGP Multipath
For IBGP Multipath, all the other best path selection steps until the Multipath tie-breaker should be the same.
Local Preference, AS-Path Length, Origin. Med, all should be the same. In fact, the IGP cost to the BGP next-hop has to be the same as
well.
Then, IBGP Multipath manually can be enabled and for the destination prefix, both BGP Next-hops can be used.
Lastly, let's have a look at EIBGP Multipath:
EIBGP Multipath
Figure - EIBGP Multipath
BGP Best path selection algorithm prefers EBGP paths over IBGP paths.
This prevents having both IBGP and EBGP prefixes be installed in the routing table at the same time.
EIBGP multipath feature allows the same prefix to be installed both with IBGP and EBGP next hops.
Configuring BGP Multipath on Cisco Routers:
After explaining IBGP Multipath, EBGP Multipath, and EIBGP Multipath, let's have a look at BGP Multipath configuration examples on Cisco devices:
Router(config-router)# maximum-paths ibgp maximum-number
Controls the maximum number of parallel IBGP routes that can be installed in a routing table.
BGP Load Balancing and BGP ECMP
In this post so far, I have been explaining BGP Multipath for IBGP Multipath, EBGP Multipath, and EIBGP Multipath features and their usage. Sometimes, Network Engineers use instead of BGP Multipathing, BGP Load Balancing, or BGP ECMP terms. They all are the same thing.
In BGP usually, we don't use BGP ECMP or BGP Load Balancing, or BGP Load Sharing but we use BGP Multipath keywords mostly. By highlighting this, I wanted to avoid any future confusion for you.
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