About Broadcast And Multicast Routing; About Multicast Root Switch; Setting The Multicast Root Switch - Cisco AP776A - Nexus Converged Network Switch 5020 Configuration Manual

FSPF Routes
S e n d d o c u m e n t a t i o n c o m m e n t s t o m d s f e e d b a c k - d o c @ c i s c o . c o m

About Broadcast and Multicast Routing

Broadcast and multicast in a Fibre Channel fabric uses the concept of a distribution tree to reach all
switches in the fabric.
FSPF provides the topology information to compute the distribution tree. Fibre Channel defines 256
multicast groups and one broadcast address for each VSAN. Switches in the Cisco MDS 9000 Family
only use broadcast routing. By default, they use the principal switch as the root node to derive a loop-free
distribution tree for multicast and broadcast routing in a VSAN.
Caution All switches in the fabric should run the same multicast and broadcast distribution tree algorithm to
ensure the same distribution tree.
To interoperate with other vendor switches (following FC-SW3 guidelines), the SAN-OS and NX-OS
4.1(1b) and later software uses the lowest domain switch as the root to compute the multicast tree in
interop mode.

About Multicast Root Switch

By default, the native (non-interop) mode uses the principal switch as the root. If you change the default,
be sure to configure the same mode in all switches in the fabric. Otherwise, multicast traffic could face
potential loop and frame-drop problems.
Note The operational mode can be different from the configured interop mode. The interop mode always uses
the lowest domain switch as the root.
Use the mcast root lowest vsan command to change the multicast root from the principal switch to
lowest domain switch.

Setting the Multicast Root Switch

To use the lowest domain switch for the multicast tree computation, follow these steps:
Command
Step 1
switch# config t
switch(config)#
Step 2
switch(config)# mcast root lowest vsan 1
switch(config)# mcast root principal vsan 1
To display the configured and operational multicast mode and the selected root domain, use the show
mcast command.
switch# show mcast vsan 1
Multicast root for VSAN 1
Cisco MDS 9000 Family CLI Configuration Guide
26-12
Configured root mode : Principal switch
Operational root mode : Principal switch
Root Domain ID : 0xef(239)
Chapter 26 Configuring Fibre Channel Routing Services and Protocols
Purpose
Enters configuration mode.
Uses the lowest domain switch to compute
the multicast tree.
Defaults to using the principal switch to
compute the multicast tree.
OL-18084-01, Cisco MDS NX-OS Release 4.x