KB.15.15

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

The switch allows ACL filtering on unicast addresses, but not on multicast addresses. Also, an ACL

does not take effect on a flow if the flow began before the ACL was configured.

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 “Flow capacity”

(page 59).

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.

For more information, see “Operating notes” (page 59) and “Messages related to PIM operation”

(page 61).

IPv4 table operation

The IPv4 table, which contains the active IP multicast addresses the switch is currently supporting,

has 128k entries. However, the IPv4 table also contains IP host entries for every IP source or

destination that the switch has learned, as well as ACL flow entries. Entries in this table are generally

aged-out if they are unused for 5 minutes or more.

60 PIM-DM (Dense Mode)