Table of Contents
Advertisement
Chapter 14
Configuring IGMP Snooping
S e n d f e e d b a c k t o n x 5 0 0 0 - d o c f e e d b a c k @ c i s c o . c o m
IGMPv3
The IGMPv3 snooping implementation on the switch forwards IGMPv3 reports to allow the upstream
multicast router do source-based filtering.
By default, the software tracks hosts on each VLAN port. The explicit tracking feature provides a fast
leave mechanism. Because every IGMPv3 host sends membership reports, a report suppression feature
limits the amount of traffic the switch sends to other multicast capable routers. When report suppression
is enabled, and no IGMPv1 or IGMPv2 hosts requested the same group, the software provides proxy
reporting. The proxy feature builds group state from membership reports from the downstream hosts and
generates membership reports in response to queries from upstream queriers.
Even though the IGMPv3 membership reports provide a full accounting of group members on a LAN
segment, when the last host leaves, the software sends a membership query. You can configure the
parameter last member query interval. If no host responds before the timeout, the software removes the
group state.
IGMP Snooping Querier
When there is no multicast router in the VLAN to originate the queries, you must configure an IGMP
snooping querier to send membership queries.
When an IGMP snooping querier is enabled, it sends out periodic IGMP queries that trigger IGMP report
messages from hosts that want to receive IP multicast traffic. IGMP snooping listens to these IGMP
reports to establish appropriate forwarding.
IGMP Forwarding
The control plane of the Cisco Nexus 5000 Series switch is able to detect IP addresses but forwarding
occurs using the MAC address only.
When a host connected to the switch wants to join an IP multicast group, it sends an unsolicited IGMP
join message, specifying the IP multicast group to join. Alternatively, when the switch receives a general
query from a connected router, it forwards the query to all interfaces, physical and virtual, in the VLAN.
Hosts wanting to join the multicast group respond by sending a join message to the switch. The switch
CPU creates a multicast forwarding table entry for the group if it is not already present. The CPU also
adds the interface where the join message was received to the forwarding table entry. The host associated
with that interface receives multicast traffic for that multicast group.
The router sends periodic multicast general queries and the switch forwards these queries through all
ports in the VLAN. Interested hosts respond to the queries. If at least one host in the VLAN wants to
receive multicast traffic, the router continues forwarding the multicast traffic to the VLAN. The switch
forwards multicast group traffic to only those hosts listed in the forwarding table for that multicast
group.
When hosts want to leave a multicast group, they can either silently leave, or they can send a leave
message. When the switch receives a leave message from a host, it sends a group-specific query to
determine if any other devices connected to that interface are interested in traffic for the specific
multicast group. The switch then updates the forwarding table for that MAC group so that only those
hosts interested in receiving multicast traffic for the group are listed in the forwarding table. If the router
receives no reports from a VLAN, it removes the group for the VLAN from its IGMP cache.
OL-16597-01
Cisco Nexus 5000 Series Switch CLI Software Configuration Guide
Information About IGMP Snooping
14-3
Advertisement
Chapters
Table of Contents
Troubleshooting
