ECMP Multicast Routing Cisco Nexus Switches

In multicast routing, data is transmitted from one sender to multiple receivers across a network. A critical part of this process is deciding which uplink to use. On Cisco Nexus switches, this decision is made using a hashing algorithm, specifically the s-g-hash (source-group hash). This algorithm is predictable and non-random, ensuring consistent and stable routing paths.

When you run the show ip multicast vrf default command on a Cisco Nexus switch, it displays multicast routing information for the default VRF (Virtual Routing and Forwarding). Here’s an example of the command output:

LEAF1# show ip multicast vrf default
Multicast Routing VRFs (1 VRFs)
VRF Name              VRF      Table       Route   Group   Source  (*,G)   State
                      ID       ID          Count   Count   Count   Count

default               1        0x00000001  3       1       1       1       Up
    Multipath configuration (1): s-g-hash
    Resilient configuration: Disabled

Multipath Configuration Shows that the s-g-hash algorithm is used for Equal-cost Multi-path routing multicast.

The s-g-hash algorithm used in this configuration is based on the source and group address. This means the hashing process will produce the same output for the same input every time. Unlike algorithms that use random values, the s-g-hash ensures predictable and repeatable routing decisions.