BGP Multipath

BGP on its own does not perform ECMP¹.

To make BGP behave more like an IGP, and especially at scale with multiple ECMP links, this feature is needed.

Topology

R1 is connected to the ISP via two eBGP links.

For any given route, eBGP on its own will install one best path.

          ┌───── eBGP ───────┐           
          ▼                  ▼           
             192.168.0.0/31              
┌────────┐.0                .1┌────────┐ 
│   R1   ├────────────────────┤  ISP   │ 
│AS 64496├────────────────────┤AS 64511│ 
└────────┘.2                .3└────────┘ 
             192.168.0.2/31              
          ▲                  ▲           
          └───── eBGP ───────┘           

Config

IOS-XE, no VRF

Courtesy of the config guide.

router bgp 64496
  neighbor 192.168.0.1 remote-as 64511
  neighbor 192.168.0.3 remote-as 64511
  address-family ipv4 unicast
    maximum-paths eibgp 2
  !
  ! Not required for this diagram, but you know the command now
  !
  address-family ipv6 unicast
    maximum-paths eibgp 2

References

Cisco Live - A Deep Dive into Basic and Design Best Practices for BGP and L3VPN

Cisco - eiBGP Multipath for Non-VRF Interfaces (IPv4/IPv6) - IP Routing Configuration Guide, IOS-XE 17.x

1. Equal Cost Multi Path. ↩