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.
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 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.
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:
switch# config t
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
switch# show mcast vsan 1
Multicast root for VSAN 1
Cisco MDS 9000 Family CLI Configuration Guide
Configured root mode : Principal switch
Operational root mode : Principal switch
Root Domain ID : 0xef(239)
Configuring Fibre Channel Routing Services and Protocols
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-16184-01, Cisco MDS SAN-OS Release 3.x