Cisco Nexus 1000V Troubleshooting Guide, Release 5.2(1)SV3(1.1)

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).

Troubleshooting Guidelines

Follow these guidelines when troubleshooting multicast IGMP issues:

• Use the show ip igmp snooping command to verify that IGMP snooping is enabled.
• Make sure that the upstream switch has IGMP configured.
• Use the show ip igmp snooping groups command to verify if the Cisco Nexus 1000V switch is configured correctly and is ready to forward multicast traffic. In the displayed output of the command, look for the letter R under the port heading. The R indicates that the Virtual Supervisor Module (VSM) has learned the uplink router port from the IGMP query that was sent by the upstream switch, and means that the Cisco Nexus 1000V is ready to forward multicast traffic.

IGMP Snooping Debugging Commands

You can enable debugging commands for IGMP snooping:

Step 1 Enable logs files on the module that hosts the preferred VMs/Veths.

Step 2 (Optional) Clear existing log data.

Step 3 Start collecting log data.

Step 4 Wait for the IGMP queries and reports to hit the VEM ports.

Step 5 Stop and verify the log data.

Multicast IGMP Snooping Troubleshooting Commands

You can use the commands in this section to troubleshoot problems related to multicast IGMP snooping.

• show cdp neighbor

Displays if IGMP uses the packet VLAN to forward IGMP packets to the VSM, which is the same mechanism that CDP uses. However, if you have disabled the CDP protocol on the upstream switch using the no cdp enable command, the show cdp neighbor command will not display any information.

Example 19-1 show cdp neighbor Command

• show ip igmp groups

Displays whether IGMP snooping is enabled on the VLAN.

Example 19-2 show ip igmp snooping vlan Command

• show ip igmp snooping groups

Example 19-3 debug ip igmp snooping vlan Command

Note Even if you enable the debug command for IGMP snooping, log details are not available for multicast groups and their members.

Example 19-4 module vem module-number execute vemcmd show vlan Command

On the VSM, use the following command:

• module vem 3 execute vemcmd show igmp 1784

In Example 19-2, global IGMP snooping is enabled on VLAN 1784 (the disabled global state takes precedence)

Multicast group table values are as follows:

Group 227.0.0.1, Multicast LTL: 10363

Group */*, Multicast LTL: 10358

• module vem 3 execute vemcmd show igmp 1784 de

In Example 19-2, global IGMP snooping is enabled on VLAN 1784 (the disabled global state takes precedence)

Multicast group table values are as follows:

Group 227.0.0.1, Multicast LTL: 10363

Members: 59, 1039

Group */*, Multicast LTL: 10358

Members: 1039

Querier Info -

IP Address: 184.184.0.12

Uptime: 241955 seconds

Version: 3

Timeout: 8 seconds

• module vem module-number execute vemcmd show vlan

In Example 19-4, the output shows that LTL 18 corresponds to vmnic3, and LTL 47 corresponds to VM fedora8, interface eth0.

The multicast group table for 224.1.2.3 shows the interfaces that the VEM forwards to when it receives multicast traffic for group 224.1.2.3. If fedora8 has multicast group 224.1.2.3 on its eth0 interface, LTL 47 should be in the multicast group table for 224.1.2.3.

LTL 18 is also in multicast group 224.1.2.3, which means it is a VM and generates multicast traffic to 224.1.2.3. The traffic is forwarded to vmnic3, which is the uplink to the upstream switch.

The multicast group table entry for 0.0.0.0 serves as a default route. If any multicast group traffic does not match any of the multicast group, the address uses the default route, which means that the traffic is forwarded to an upstream switch through vmnic3.

Problems with Multicast IGMP Snooping

The following are symptoms, possible causes, and solutions for problems with multicast IGMP snooping.