- Overview
- Tools Used in Troubleshooting
- Installation
- Upgrade
- Licenses
- High Availability
- VSM and VEM Modules
- Ports
- Port Profiles
- Port Channels and Trunking
- Layer 2 Switching
- VLANs
- Private VLANs
- NetFlow
- Access Control Lists
- Quality of Service
- SPAN
- Multicast IGMP Snooping
- DHCP, DAI, and IPSG
- System
- Network Segmentation Manager
- Ethanalyzer
- Before Contacting Technical Support
Multicast IGMP Snooping
This chapter describes how to identify and resolve problems that relate to multicast Internet Group Management Protocol (IGMP) snooping.
Information About Multicast IGMP Snooping
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 both IPv4 and IPv6 networks 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.
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.
IGMP snooping works as follows:
- Ethernet switches, such as Catalyst 6500 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 by 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 by 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 checks 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).
Problems with Multicast IGMP Snooping
The operation of multicast IGMP snooping depends on the correct configuration of the upstream switch. Because the IGMP process needs to know which upstream port connects to the router that supports IGMP routing, you must turn on IP multicast routing on the upstream switch by entering the
ip multicast-routing command.
This example shows how to turn on global multicast routing, configure an SVI interface, and turn on the PIM routing protocol:
Troubleshooting Guidelines
Follow these guidelines when troubleshooting multicast IGMP issues:
- Verify that IGMP snooping is enabled by entering the show ip igmp snooping command.
- Make sure the upstream switch has IGMP configured.
- Verify that the Cisco Nexus 1000V switch is configured correctly and is ready to forward multicast traffic by entering the show ip igmp snooping groups command. 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.
Multicast IGMP Snooping Troubleshooting Commands
You can use the commands in this section to troubleshoot problems related to multicast IGMP snooping.
Example 18-1 show cdp neighbor command
Example 18-2 show ip igmp snooping vlan command
Example 18-3 debug ip igmp snooping vlan command
On the VSM, use the following command:
In Example 18-4, 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 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 that 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 groups, the address uses the default route, which means that the traffic is forwarded to an upstream switch through vmnic3.
Example 18-4 module vem module-number execute vemcmd show vlan Command
Problems with Multicast IGMP Snooping
The following are symptoms, possible causes, and solutions for problems with multicast IGMP snooping.