Catalyst 6500 Release 12.2SY Software Configuration Guide

General IGMP Snooping Restrictions

• Multicast packets are not bridged in a VLAN to local receivers that send IGMP joins when PIM snooping is enabled in the VLAN and IGMP snooping is disabled in the VLAN. ( CSCta03980)
• For more information on IP multicast and IGMP, see RFC 1112 and RFC 2236.
• IGMP snooping supports private VLANs. Private VLANs do not impose any restrictions on IGMP snooping.
• IGMP snooping constrains traffic in MAC multicast groups 0100.5e00.0001 to 0100.5eff.ffff.
• IGMP snooping does not constrain Layer 2 multicasts generated by routing protocols.

IGMP Snooping Querier Restrictions

• The IGMP snooping querier does not support querier elections. Enable the IGMP snooping querier on only one switch in the VLAN. ( CSCsk48795)
• Configure the VLAN in global configuration mode (see Chapter 23, “Virtual Local Area Networks (VLANs)”).
• Configure an IP address on the VLAN interface (see Chapter 32, “Layer 3 Interfaces”). When enabled, the IGMP snooping querier uses the IP address as the query source address.
• If there is no IP address configured on the VLAN interface, the IGMP snooping querier does not start. The IGMP snooping querier disables itself if the IP address is cleared. When enabled, the IGMP snooping querier restarts if you configure an IP address.
• The IGMP snooping querier sends IGMPv3 querier messages. Although the IGMP version of the querier messages is not configurable, the querier is compatible with IGMPv2 hosts.
• When enabled, the IGMP snooping querier starts after 60 seconds with no IGMP traffic detected from a multicast router. If IGMP traffic from a multicast router, or from another IGMP snooping querier in the VLAN, is detected after the IGMP snooping querier has started, the querier will disable itself.
• QoS does not support IGMP packets when IGMP snooping is enabled.

IGMP Snooping Overview

IGMP snooping allows switches to examine IGMP packets and make forwarding decisions based on their content. You can configure the switch to use IGMP snooping in subnets that receive IGMP queries from either IGMP or the IGMP snooping querier. IGMP snooping constrains IPv4 multicast traffic at Layer 2 by configuring Layer 2 LAN ports dynamically to forward IPv4 multicast traffic only to those ports that want to receive it.

Some applications use a single unicast cluster IP address and multicast cluster MAC address. Multicast traffic addressed to a unicast cluster IP address is forwarded to the last-hop router that is configured with the shared multicast MAC address. To support cluster-addressed multicast traffic, assign a static multicast MAC address for the destination IP address of the end host or cluster.

You can configure the IGMP snooping lookup method for each VLAN. Layer 3 IGMP snooping lookup uses destination IP addresses in the Layer 2 multicast table (this is the default). Layer 2 IGMP snooping lookup uses destination MAC addresses in the Layer 2 multicast table.

Note Changing the lookup mode is disruptive. Multicast forwarding is not optimal until all multicast entries are programmed with the new lookup mode. Also, if 32 IP addresses are mapped to a single MAC address, forwarding on the device might be suboptimal.

IGMP, which runs at Layer 3 on a multicast router, generates Layer 3 IGMP queries in subnets where the multicast traffic needs to be routed.

You can configure the IGMP snooping querier on the switch to support IGMP snooping in subnets that do not have any multicast router interfaces. For more information about the IGMP snooping querier, see the “Enabling the IGMP Snooping Querier” section.

IGMP (on a multicast router) or, locally, the IGMP snooping querier, sends out periodic general IGMP queries that the switch forwards through all ports in the VLAN and to which hosts respond. IGMP snooping monitors the Layer 3 IGMP traffic.

Note If a multicast group has only sources and no receivers in a VLAN, IGMP snooping constrains the multicast traffic to only the multicast router ports.

Joining a Multicast Group

Hosts join multicast groups either by sending an unsolicited IGMP join message or by sending an IGMP join message in response to a general query from a multicast router (the switch forwards general queries from multicast routers to all ports in a VLAN).

In response to an IGMP join request, the switch creates an entry in its Layer 2 forwarding table for the VLAN on which the join request was received. When other hosts that are interested in this multicast traffic send IGMP join requests, the switch adds them to the existing Layer 2 forwarding table entry. The switch creates only one entry per VLAN in the Layer 2 forwarding table for each multicast group for which it receives an IGMP join request.

IGMP snooping suppresses all but one of the host join messages per multicast group and forwards this one join message to the multicast router.

The switch forwards multicast traffic for the multicast group specified in the join message to the interfaces where join messages were received (see Figure 43-1).

Layer 2 multicast groups learned through IGMP snooping are dynamic. However, you can statically configure Layer 2 multicast groups using the mac address-table static command. When you specify group membership for a multicast group address statically, the static setting supersedes any IGMP snooping learning. Multicast group membership lists can consist of both static and IGMP snooping-learned settings.

Figure 43-1 Initial IGMP Join Message

Multicast router A sends a general query to the switch, which forwards the query to ports 2 through 5 (all members of the same VLAN). Host 1 wants to join multicast group 224.1.2.3 and multicasts an IGMP membership report (IGMP join message) to the group with the equivalent MAC destination address of 0x0100.5E01.0203. When the CPU receives the IGMP report multicast by Host 1, the CPU uses the information in the IGMP report to set up a forwarding-table entry, that includes the port numbers of Host 1, the multicast router, and the switch internal CPU.

CPU installs snooping forwarding entry based on lookup type, either IP or MAC (by default, it is IP-based). Using IP-based forwarding can avoid group address aliasing problem and optimize per group or per group and source forwarding.

If IP-based is configured, IGMP snooping forwarding table has the following entry. The switch engine matches on the destination IP address of multicast data packets. If they are 224.1.2.3, send them to the host that has joined the group and multicast routers.

If MAC-based is configured, the entry is as follows. In this case, the switch engine matches on the destination MAC address of the packets. The packets with 0100.5e01.0203 are sent to the host that has joined the group and multicast routers.

If another host (for example, Host 4) sends an unsolicited IGMP join message for the same group (Figure 43-2), the CPU receives that message and adds the port number of Host 4 to the forwarding table. Because the forwarding table directs IGMP messages only to the CPU, the message is not flooded to other ports. Any known multicast traffic is forwarded to the group and not to the CPU.

Figure 43-2 Second Host Joining a Multicast Group

Leaving a Multicast Group

• Normal Leave Processing
• Immediate-Leave Processing

Normal Leave Processing

Interested hosts must continue to respond to the periodic general IGMP queries. As long as at least one host in the VLAN responds to the periodic general IGMP queries, the multicast router continues forwarding the multicast traffic to the VLAN. When hosts want to leave a multicast group, they can either ignore the periodic general IGMP queries (called a “silent leave”), or they can send a group-specific IGMPv2 leave message.

When IGMP snooping receives a group-specific IGMPv2 leave message from a host, it sends out a MAC-based general query to determine if any other devices connected to that interface are interested in traffic for the specific multicast group. If IGMP snooping does not receive an IGMP Join message in response to the general query, it assumes that no other devices connected to the interface are interested in receiving traffic for this multicast group, and it removes the interface from its Layer 2 forwarding table entry for that multicast group. If the leave message was from the only remaining interface with hosts interested in the group and IGMP snooping does not receive an IGMP Join in response to the general query, it removes the group entry and relays the IGMP leave to the multicast router. If the multicast router receives no reports from a VLAN, the multicast router removes the group for the VLAN from its IGMP cache.

The interval for which the switch waits before updating the table entry is called the “last member query interval.” To configure the interval, enter the ip igmp snooping last-member-query-interval interval command.

Immediate-Leave Processing

IGMP snooping immediate-leave processing allows IGMP snooping to remove a Layer 2 LAN interface from the forwarding-table entry without first sending out IGMP group-specific queries to the interface. Upon receiving a group-specific IGMPv2 leave message, IGMP snooping immediately removes the interface from the Layer 2 forwarding table entry for that multicast group, unless a multicast router was learned on the port. Immediate-leave processing improves bandwidth management for all hosts on a switched network.

Note Use immediate-leave processing only on VLANs where only one host is connected to each Layer 2 LAN port. If immediate-leave is enabled in VLANs where more than one host is connected to a Layer 2 LAN port, some hosts might be dropped inadvertently. Immediate-leave processing is supported only with IGMP version 2 and 3 hosts.

Information about the IGMP Snooping Querier

Use the IGMP snooping querier to support IGMP snooping in a VLAN where PIM and IGMP are not configured because the multicast traffic does not need to be routed.

In a network where IP multicast routing is configured, the IP multicast router acts as the IGMP querier. If the IP-multicast traffic in a VLAN only needs to be Layer 2 switched, an IP-multicast router is not required, but without an IP-multicast router on the VLAN, you must configure another switch as the IGMP querier so that it can send queries.

When enabled, the IGMP snooping querier sends out periodic IGMPv3 queries that trigger IGMP report messages from the switch that wants to receive IP multicast traffic. IGMP snooping listens to these IGMP reports to establish appropriate forwarding.

Configure one switch as the IGMP snooping querier in each VLAN that is supported on switches that use IGMP to report interest in IP multicast traffic.

You can configure a switch to generate IGMP queries on a VLAN regardless of whether or not IP multicast routing is enabled.

Information about IGMP Version 3 Support

• IGMP Version 3 Support Overview
• IGMPv3 Immediate-Leave Processing
• Proxy Reporting
• Explicit Host Tracking

IGMP Version 3 Support Overview

IGMP snooping supports IGMP version 3 (IGMPv3). IGMPv3 uses source-based filtering, which enables hosts and routers to specify which source addresses should be allowed or blocked for a specific multicast group. When you enable IGMPv3 snooping, the switch maintains IGMPv3 states based on messages it receives for a particular group in a particular VLAN and either allows or blocks traffic based on the following information in these messages:

• Source lists
• Allow (include) or block (exclude) filtering options

When a host wants to receives multicast traffic only from specific sources, it can send IGMPv3 joins with source filtering. For example, when host1 on port 3/11 sends join to group 224.1.2.3 from sources 10.1.1.1 and host2 on port 3/12 sends join to the same group but from a different source 20.1.1.1, the following entries are installed in forwarding table when IP-based lookup is configured:

The second entry constrain group traffic from source 10.1.1.1 to host1 only and the third entry constrain traffic from source 20.1.1.1 to Host2. The first entry drops group traffic from any other sources since there is no receiver interesting in other sources.

IGMPv3 Immediate-Leave Processing

IGMPv3 immediate-leave processing is active if explicit-host tracking is enabled. The ip igmp snooping immediate-leave command that enables IGMP version 2 immediate-leave processing does not affect IGMPv3 immediate-leave processing.

Immediate-leave processing with IGMPv3 is implemented by maintaining source-group based membership information in software while also allocating LTL indexes on a MAC GDA basis.

When immediate-leave processing is active, hosts send BLOCK_OLD_SOURCES{src-list} messages for a specific group when they no longer want to receive traffic from that source. When the switch receives such a message from a host, it parses the list of sources for that host for the given group. If this source list is exactly the same as the source list received in the leave message, the switch removes the host from the LTL index and stops forwarding this multicast group traffic to this host.

If the source lists do not match, the switch does not remove the host from the LTL index until the host is no longer interested in receiving traffic from any source.

Proxy Reporting

IGMP supports proxy reporting for IGMPv1 and IGMPv2 messages to handle group-specific queries. These queries are not sent downstream, but the switch does respond to them directly. When the switch receives a group-specific query, the switch terminates the query and sends an IGMP proxy report if there is a receiver for the group. There is no proxy reporting for IGMPv3 messages. For IGMPv3, a group-specific query or a group source-specific query is flooded to all VLAN member ports. The database for the IGMPv3 membership report is built based on the reports received.

Host reports responding to a specific query can be suppressed by the report suppression feature. Report suppression is supported for IGMPv1, IGMPv2, and IGMPv3 messages. With report suppression enabled (by default), when the switch receives a general query, the switch starts a suppression cycle for reports from all hosts to each group or channel (S,G). Only the first report to the discovered multicast routers is forwarded; the rest of the reports are suppressed. For IGMPv1 and IGMPv2, the time of suppression is the report response time indicated in the general query message. For IGMPv3, suppression occurs for the entire general query interval.

Note The states are maintained only in software and used for explicit host tracking and statistics collection.

Explicit Host Tracking

IGMPv3 supports explicit tracking of membership information on any port. The explicit-tracking database is used for immediate-leave processing for IGMPv3 hosts, proxy reporting, and statistics collection. When explicit tracking is enabled on a VLAN, the IGMP snooping software processes the IGMPv3 report it receives from a host and builds an explicit-tracking database that contains the following information:

• The port connected to the host
• The channels reported by the host
• The filter mode for each group reported by the host
• The list of sources for each group reported by the hosts
• The router filter mode of each group
• For each group, the list of hosts requesting the source

Note ● When explicit tracking is enabled and the switch is working in proxy-reporting mode, the router may not be able to track all the hosts behind a VLAN interface.

Enabling the IGMP Snooping Querier

Use the IGMP snooping querier to support IGMP snooping in a VLAN where PIM and IGMP are not configured because the multicast traffic does not need to be routed. To enable the IGMP snooping querier in a VLAN, perform this task:

This example shows how to enable the IGMP snooping querier on VLAN 200 and verify the configuration:

Enabling IGMP Snooping

• Enabling IGMP Snooping Globally
• Enabling IGMP Snooping in a VLAN

Enabling IGMP Snooping Globally

To enable IGMP snooping globally, perform this task:

This example shows how to enable IGMP snooping globally and verify the configuration:

Enabling IGMP Snooping in a VLAN

To enable IGMP snooping in a VLAN, perform this task:

This example shows how to enable IGMP snooping on VLAN 25 and verify the configuration:

Configuring the IGMP Snooping Lookup Method

To configure the IGMP snooping lookup method for a VLAN, perform this task:

Note Changing the lookup mode is disruptive. Multicast forwarding is not optimal until all multicast entries are programmed with the new lookup mode. Also, if 32 IP addresses are mapped to a single MAC address, forwarding on the device might be suboptimal.

Configuring a Static Connection to a Multicast Receiver

To configure a static connection to a multicast receiver, perform this task:

When you configure a static connection, enter the disable-snooping keyword to prevent multicast traffic addressed to the statically configured multicast MAC address from also being sent to other ports in the same VLAN.

This example shows how to configure a static connection to a multicast receiver:

The above static mac command can be used when the lookup type in the VLAN is MAC-base. Irrespective of lookup type, the following commands can be used to configure static connection to a multicast receiver for a group or a group and from a specific source.

Configuring a Multicast Router Port Statically

To configure a static connection to a multicast router, perform this task:

The interface to the router must be in the VLAN where you are entering the command, the interface must be administratively up, and the line protocol must be up.

This example shows how to configure a static connection to a multicast router:

Configuring the IGMP Snooping Query Interval

You can configure the interval for which the switch waits after sending a group-specific query to determine if hosts are still interested in a specific multicast group.

Note When both IGMP immediate-leave processing and the IGMP query interval are configured, immediate-leave processing takes precedence.

To configure the interval for the IGMP snooping queries sent by the switch, perform this task:

This example shows how to configure the IGMP snooping query interval:

Enabling IGMP Snooping Immediate-Leave Processing

Fast-leave configuration applies to IGMP version 2 hosts only. To enable IGMP snooping fast-leave processing in a VLAN, perform this task:

This example shows how to enable IGMP snooping immediate-leave processing for IGMP version 2 hosts on the VLAN 200 interface, and how to verify the configuration:

Configuring IGMPv3 Snooping Explicit Host Tracking

To enable IGMPv3 snooping explicit host tracking on a VLAN, perform this task:

This example shows how to enable IGMPv3 snooping explicit host tracking:

Displaying IGMP Snooping Information

• Displaying Multicast Router Interfaces
• Displaying MAC Address Multicast Entries
• Displaying IGMP Snooping Information for a VLAN Interface

Displaying Multicast Router Interfaces

When you enable IGMP snooping, the switch automatically learns to which interface the multicast routers are connected. To display multicast router interfaces, perform this task:

This example shows how to display the multicast router interfaces in VLAN 1:

Displaying MAC Address Multicast Entries

To display MAC address multicast entries for a VLAN, perform this task:

This example shows how to display MAC address multicast entries for VLAN 1:

This example shows how to display a total count of MAC address entries for a VLAN:

Displaying IGMP Snooping Information for a VLAN Interface

To display IGMP snooping information for a VLAN interface, perform this task:

This example shows how to display IGMP snooping information on the VLAN 200 interface:

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