Users Guide

Table Of Contents
958 Layer-2 Multicast Features
desirable as it reduces the network load by sending packets only to other
hosts/switches/routers that have indicated an interest in receiving the
multicast.
If L2 snooping is not enabled, multicast packets are flooded in the ingress
VLAN.
What Are the Multicast Bridging Features?
The Dell EMC Networking N-Series switches support multicast forwarding
and multicast flooding. For multicast traffic, the switch uses a database called
the Layer-2 Multicast Forwarding Database (MFDB) to make forwarding
decisions for packets that arrive with a multicast destination MAC address. By
limiting multicasts to only certain ports in the switch, traffic is prevented
from going to parts of the network where that traffic is unnecessary.
When a packet enters the switch, the destination MAC address is combined
with the VLAN ID, and a search is performed in the Layer-2 MFDB. If no
match is found, then the packet is flooded. If a match is found, then the
packet is forwarded only to the ports that are members of that multicast
group within the VLAN.
Multicast traffic destined to well-known (reserved) multicast IP addresses
(control plane traffic) is always flooded to all ports in the VLAN. The well-
known IP multicast addresses are 224.0.0.x for IPv4 and FF0x:: for IPv6.
By default IGMP/MLD snooping is enabled and multicast data traffic is
flooded to all ports in the VLAN if no multicast router ports have been
identified. Once a multicast router port is identified, multicast data traffic is
forwarded to the multicast router ports. The MFDB is populated by snooping
the membership reports sent to the multicast routers. This causes multicast
data traffic to be forwarded to any hosts joining the multicast group. Enabling
multicast routing on the switch internally enables an mrouter port, and
snooping will forward multicast to hosts joining the group instead of flooding
it in the VLAN.
It is possible to statically define an mrouter port. This causes IGMP/MLD
snooping to forward multicast data traffic to hosts from which it has received
membership reports. This behavior exists even if the mrouter port is not
enabled.