IGMP Snooping and Unknown Multicast
Traffic
IGMP snooping floods unknown multicast data packets on the member ports of VLAN. This unknown
data traffic is the traffic sent to multicast groups that are not learned by means of
IGMP membership reports or static IGMP group configuration.
Restricting the unknown multicast is achieved by redirecting the unknown multicast traffic
(traffic destined to multicast MAC 01:00:5E:XX:XX:XX) to a multicast group identifier
(MGID) that has no replication ports. Unknown multicast traffic is dropped because there
are no ports to send the traffic.
Traffic Forwarding Scenarios
The following scenarios describe how multicast data traffic is forwarded when the restrict
unknown multicast feature is enabled.
- Multicast traffic learned from IGMP reports and static groups
- IGMP v2 groups are programmed in LEM table as (*,G,V) entries.
- IGMP v3 groups are programmed in KAPS as (S,G,V) entries.
- The multicast data traffic is forwarded based on a match from LEM or KAPS
table, which provides an MGID. The multicast traffic is replicated to an
MGID that has egress ports.
- Unknown multicast traffic
- All the unknown multicast data traffic is restricted or dropped instead of
flooded on the VLAN members.
- Mrouter port behavior
- Mrouter ports that are learned by means of receiving IGMP queries, PIM hello
messages, or static configuration are added as part of the restrict unknown
multicast MGID. Unknown multicast data traffic is then flooded only to the
mrouter ports of the VLAN.
TCAM Profile
The restrict unknown multicast functionality is supported in default TCAM profile.
Limitations
- The MGIDs used for redirecting the unknown
multicast traffic are allocated from the IGMP MGID space. The maximum number for
IGMP groups without restrict unknown multicast is 16384.
- When you enable restrict unknown multicast on a
VLAN or bridge domain, the IGMP groups maximum scaling number is reduced by 1.
Note
The maximum number of
15872 IGMP groups can be learned, if restrict unknown multicast is enabled
on 512 VLANs or bridge domains.
