multicast groups to these devices. This periodic flooding is not necessary if all downstream multicast routers are
switches covered in this guide.
Flow capacity
The routing switch provides an ample multicast environment, supporting 2046 multicast flows in hardware across
a maximum of 64 VLANs. (A flow comprises a unicast source address and a multicast group address, regardless
of the number of active hosts belonging to the multicast group at any given time.)
IGMP traffic high-priority disabled
Enabling IP multicast routing to support PIM-DM operation has the effect of disabling IGMP traffic high-priority, if
configured.
ACLs and PIM
With V2 modules, the outbound routed ACLs only match IP unicast addresses and not IP multicast addresses.
When the chassis is in V3 only mode (no-allow-v2), outbound routed ACLs can also match IP multicast
addresses.
When to enable IGMP on a VLAN
When PIM is enabled on a VLAN, it is not necessary to also enable IGMP unless there may be joins occurring on
that VLAN. But if IGMP is enabled on a VLAN, you must also enable PIM if you want that VLAN to participate in
multicast routing.
IP address removed
If you remove the IP address for a VLAN, the switch automatically removes the PIM configuration for that VLAN.
Troubleshooting
Symptom: Noticeable slowdown in some multicast traffic
If the switch is supporting more than 1022 active flows, this generates the message
Unable to learn HW IP
multicast groups, table FULL
in the Event Log, because there is no room in the hardware MRT to add
another multicast group. Software will route any multicast packets sent to multicast groups that are not in the
hardware MRT, but it will be slower, and packets may be dropped if the data rate is greater than 3000 packets per
second. See
NOTE:
The PIM protocol uses oneMRT entry for every IP multicast S/G pair that it is routing. An entry is not
used if the multicast flow is bridged and not routed. Entries in this table are automatically aged-out if
they are unused for a period of time.
Heavy memory usage
Heavy use of PIM (many S/G flows over many VLANs), combined with other memory-intensive features, can
oversubscribe memory resources and impact overall performance. If available memory is exceeded, the switch
drops any new multicast flows and generates appropriate Event Log messages. Corrective actions can include:
• Reducing the number of VLANs on the switches by moving some VLANs to another device.
• Freeing up system resources by disabling another, non-PIM feature.
• Moving some hosts to another device.
Chapter 3 PIM-DM (Dense Mode)
79
