DELL N3000 User Manual

806

Configuring L2 Multicast Features

the switch sees a multicast router in the VLAN, it forwards the group to the

multicast router and does not flood in the VLAN. There is a user option to

cause the switch to flood multicast sources in the VLAN if no multicast

clients are present.
By default, dynamically discovered multicast routers are aged out every five

minutes. The user can control whether or not multicast routers age out. If all

multicast routers age out, the switch floods the VLAN with any received

multicast groups.
Multicast routers send an IGMP query every 60 seconds. This query is

intercepted by the switch and forwarded to all ports in the VLAN. All hosts

that are members of the group answer that query. The switch intercepts the

replies and forwards only one report per group from all of the received

responses.
In summary:

• IGMP snooping controls the flooding/forwarding behavior for multicast

groups. Multicast data is flooded in the VLAN until a multicast router port

is identified.

• IGMP snooping is enabled by default
• IGMP snooping forwards multicast sources to multicast routers by default
• Reserved multicast IP addresses (224.0.0.x) are always flooded to all ports

in the VLAN

• Unregistered multicast traffic may be flooded in the VLAN by a user

configuration option.

NOTE:

It is strongly recommended that operators enable MLD snooping if IGMP

snooping is enabled and vice-versa. This is because both IGMP snooping and

MLD snooping utilize the same forwarding table and not enabling both may

cause unwanted pruning of protocol packets utilized by other protocols, e.g.

OSPFv3.

NOTE:

IGMP snooping (and IGMP querier) validates IGMP packets. As part of

the validation, IGMP checks for the router alert option. If other devices in the

network do not send IGMP packets with the router alert option, IGMP snooping

(and snoooping querier) will discard the packet. Use the no ip igmp snooping

router-alert-check command to disable checking for the router alert option.