Table of Contents

Configuration in SONiC

Enabling/disabling WCMP

FRR WCMP configuration

Configuration in SONiC

In normal ECMP, the route to a destination has multiple next hops and traffic is expected to be equally distributed across these next hops. In practice, flow-based hashing is used so that all traffic associated with a particular flow uses the same next hop, and by extension, the same path across the network.

Weighted ECMP using BGP link bandwidth introduces support for network-wide UCMP to an IP destination. The unequal cost load balancing is implemented by the forwarding plane based on the weights associated with the next hops of the IP prefix. These weights are computed based on the bandwidths of the corresponding multipaths which are encoded in the BGP link bandwidth extended community. Exchange of an appropriate BGP link bandwidth value for a prefix across the network results in network-wide unequal cost multipathing.

WCMP is applicable in a pure L3 network as well as in a EVPN network.

Example model & SONiC version:

• Aurora 830, Aurora 721, Aurora 621, Aurora 221
• Netberg SONiC: sonic-broadcom-202411.n0

Enabling/disabling WCMP

admin@sonic:~$ sudo config bgp device-global wcmp enabled|disabled

Check the status:

admin@sonic:~$ show bgp device-global
TSA       W-ECMP
--------  --------
disabled  enabled
admin@sonic:~$ show bgp device-global -j
{
    "tsa": "disabled",
    "w-ecmp": "enabled"
}

FRR WCMP configuration

The configuration for weighted ECMP using BGP link bandwidth requires using a route-map to inject the link bandwidth extended community.

There is no configuration necessary to process received link bandwidth and translate it into the weight associated with the corresponding next hop; that happens by default. If some of the multipaths do not have the link bandwidth extended community, the default behavior is to revert to normal ECMP.

At the entry point router that is injecting the prefix to which weighted load balancing must be performed, a route-map must be configured to attach the link bandwidth extended community.

For the use case of providing weighted load balancing for an anycast service, this configuration will typically need to be applied at the ToR or Leaf router that is connected to servers which provide the anycast service and the bandwidth would be based on the number of multipaths for the destination.

L3 network

Pre-requisite – Border Gateway Protocol.

Example configuration:

!
route-map wcmp-map permit 100
  set extcommunity bandwidth num-multipaths
exit
!
router bgp 65100
  neighbor SPINE peer-group
  neighbor 1.1.1.1 peer-group SPINE
  neighbor 1.1.1.1 remote-as 65200
  neighbor 2.2.2.2 peer-group SPINE
  neighbor 2.2.2.2 remote-as 65200
  !
  address-family ipv4 unicast
    neighbor SPINE route-map wcmp-map out
    neighbor SPINE activate
  exit-address-family
  !
end

EVPN network

Pre-requisite – EVPN L2 VxLAN Configuration.

Example configuration:

!
route-map wcmp-map permit 100
  set extcommunity bandwidth num-multipaths
exit
!
router bgp 65100 vrf vrf1
  neighbor SPINE peer-group
  neighbor 1.1.1.1 peer-group SPINE
  neighbor 1.1.1.1 remote-as 65200
  neighbor 2.2.2.2 peer-group SPINE
  neighbor 2.2.2.2 remote-as 65200
  !
  address-family l2vpn evpn
    advertise ipv4 unicast route-map wcmp-map
    neighbor SPINE activate
  exit-address-family
  !
end