Cisco Nexus 1000V - Multicast IGMP; Information About Multicast; Multicast IGMP Snooping

CHAPTER
19-1
Cisco Nexus 1000V Troubleshooting Guide, Release 5.2(1)SV3(1.1)
OL-31593-01
19
Multicast IGMP
This chapter describes how to identify and resolve problems that relate to multicast Internet Group
Management Protocol (IGMP) snooping and includes the following sections:
• Information About Multicast, page 19-1
• Problems with Multicast IGMP Snooping, page 19-2
Information About Multicast
IP multicast is a method of forwarding the same set of IP packets to a number of hosts within a network.
You can use multicast in an IPv4 network to provide efficient delivery of data to multiple destinations.
Multicast involves both a method of delivery and discovery of senders and receivers of multicast data,
which is transmitted on IP multicast addresses called groups. A multicast address that includes a group
and source IP address is often referred to as a channel.
Multicast IGMP Snooping
IGMP snooping software examines Layer 2 IP multicast traffic within a VLAN to discover the ports
where interested receivers reside. Using the port information, IGMP snooping can reduce bandwidth
consumption in a multi-access LAN environment to avoid flooding the entire VLAN. The IGMP
snooping feature tracks which ports are attached to multicast-capable routers to help the routers forward
IGMP membership reports. The IGMP snooping software responds to topology change notifications.
In general, IGMP snooping works as follows:
• Ethernet switches, such as Cisco Catalyst 6000 Series switches, parse and intercept all IGMP
packets and forward them to a CPU, such as a supervisor module, for protocol processing.
• Router ports are learned using IGMP queries. The switch returns IGMP queries, it remembers which
port the query comes from, and marks the port as a router port.
• IGMP membership is learned using IGMP reports. The switch parses IGMP report packets and
updates its multicast forwarding table to keep track of IGMP membership.
• When the switch receives multicast traffic, it check its multicast table and forwards the traffic only
to those ports interested in the traffic.
• IGMP queries are flooded to the whole VLAN.
• IGMP reports are forwarded to the uplink port (the router ports).
• Multicast data traffic is forwarded to uplink ports (the router ports).