BGP Multipath

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.

We have on the website, the most detailed , Design and Practial Hands-on BGP Training in the world.

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.


We have many other BGP posts on the website and we have 35-40 hours of Video-based, BGP Course on the website.

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Orhan Ergun

Orhan Ergun, CCIE/CCDE Trainer, Author of Many Networking Books, Network Design Advisor, and Cisco Champion 2019/2020/2021

He created OrhanErgun.Net 10 years ago and has been serving the IT industry with his renowned and awarded training.

Wrote many books, mostly on Network Design, joined many IETF RFCs, gave Public talks at many Forums, and mentored thousands of his students.  

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