About Multicast Root Switch; Setting The Multicast Root Switch - Cisco MDS 9000 Series Configuration Manual

Configuring Fibre Channel Routing Services and Protocols
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 encounter
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:
Step 1
switch# config terminal
switch(config)#
Enters configuration mode.
Step 2 switch(config)# mcast root lowest vsan 1
Uses the lowest domain switch to compute the multicast tree.
Step 3
switch(config)# mcast root principal vsan 1
Defaults to using the principal switch to compute the multicast tree.
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
Configured root mode : Principal switch
Cisco MDS 9000 Series Fabric Configuration Guide, Release 8.x
About Multicast Root Switch
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