Catalyst Supervisor Engine 32 PISA IOS Software Configuration Guide, 12.2ZY

Configuring IGMP Snooping for IPv4 Multicast Traffic

This chapter describes how to configure Internet Group Management Protocol (IGMP) snooping for IPv4 multicast traffic on the Catalyst 6500 series switches.

Note•For complete syntax and usage information for the commands used in this chapter, refer to the Catalyst Supervisor Engine 32 PISA Cisco IOS Command Reference, Release 12.2ZY, at this URL:

http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2ZY/command/reference/cmdref.html

•To constrain IPv6 Multicast traffic, see Chapter 26 "Configuring MLDv2 Snooping for IPv6 Multicast Traffic."

This chapter consists of these sections:

•Understanding How IGMP Snooping Works

•Default IGMP Snooping Configuration

•IGMP Snooping Configuration Guidelines and Restrictions

•IGMP Snooping Querier Configuration Guidelines and Restrictions

•Enabling the IGMP Snooping Querier

•Configuring IGMP Snooping

Understanding How IGMP Snooping Works

These sections describe IGMP snooping:

•IGMP Snooping Overview

•Joining a Multicast Group

•Leaving a Multicast Group

•Understanding the IGMP Snooping Querier

•Understanding IGMP Version 3 Support

IGMP Snooping Overview

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.

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. For information about IGMP, see Chapter 25 "Configuring IPv4 Multicast Layer 3 Switching."

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 the IGMP snooping querier (on the supervisor engine) 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 27-1).

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 27-1 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, as shown in Table 27-1, that includes the port numbers of Host 1, the multicast router, and the switch internal CPU.

The switch hardware can distinguish IGMP information packets from other packets for the multicast group. The first entry in the table tells the switching engine to send only IGMP packets to the CPU. This prevents the CPU from becoming overloaded with multicast frames. The second entry tells the switching engine to send frames addressed to the 0x0100.5E01.0203 multicast MAC address that are not IGMP packets (!IGMP) to the multicast router and to the host that has joined the group.

If another host (for example, Host 4) sends an unsolicited IGMP join message for the same group (Figure 27-2), the CPU receives that message and adds the port number of Host 4 to the forwarding table as shown in Table 27-2. 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 27-2 Second Host Joining a Multicast Group

Leaving a Multicast Group

These sections describe leaving a multicast group:

•Normal Leave Processing

•Fast-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.

Fast-Leave Processing

IGMP snooping fast-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. Fast-leave processing improves bandwidth management for all hosts on a switched network.

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

Understanding 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 IGMP 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.

You can enable the IGMP snooping querier on all the Catalyst 6500 series switches in the VLAN, but for each VLAN that is connected to switches that use IGMP to report interest in IP multicast traffic, you must configure at least one switch as the IGMP snooping querier.

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

Understanding IGMP Version 3 Support

IGMP snooping supports IGMP version 3. IGMP version 3 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 IGMP version 3 snooping on a Catalyst 6500 series switch, the system maintains IGMP version 3 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

Because the Layer 2 table is (MAC-group, VLAN) based, with IGMPv3 hosts it is preferable to have only a single multicast source per MAC-group.

Note Source-based filtering for IGMP version 3 reports is not supported in hardware. The states are maintained only in software and used for explicit host tracking and statistics collection. The source-only entries are deleted every 5 minutes and relearned to ensure that they are still valid.

IGMPv3 Fast-Leave Processing

IGMP version 3 fast-leave processing is enabled by default. To disable IGMP version 3 fast-leave processing you must turn off explicit-host tracking.

Fast-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 fast-leave processing is enabled, 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

Because IGMPv3 does not have report suppression, all the hosts send their complete membership information to the router in response to queries. The switch receives these responses, updates the database and forwards the reports to the router. To prevent the router from becoming overloaded with reports, you can configure the switch for proxy-reporting mode. In proxy reporting mode, the switch forwards only the first report for a channel to the router and suppresses all other reports for the same channel.

With IGMPv3 proxy reporting, the switch does proxy reporting for unsolicited reports, as well as for reports received in the general query interval. Proxy reporting is turned on by default. When you disable proxy reporting, the switch works in transparent mode and updates the IGMP snooping database as it receives reports and forwards this information to the upstream router, which can then explicitly track all reporting hosts.

To support a mix of IGMPv2 and IGMPv3 hosts, the switch converts the IGMPv2 report into a EXCLUDE mode report. You must configure the switch to support both IGMPv2 and IGMPv3 hosts.

Note•Source-based filtering for IGMP version 3 reports is not supported in hardware. The states are maintained only in software and used for explicit host tracking and statistics collection. The source-only entries are deleted every 5 minutes and relearned to ensure that they are still valid.

•Turning off explicit host tracking disables fast-leave processing and proxy reporting.

Explicit Host Tracking

IGMPv3 supports explicit tracking of membership information on any port. The explicit-tracking database is used for fast-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•Turning off explicit host tracking disables fast-leave processing and proxy reporting.

•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.

Default IGMP Snooping Configuration

Table 27-3 shows the default IGMP snooping configuration.

IGMP Snooping Configuration Guidelines and Restrictions

When configuring IGMP snooping, follow these guidelines and restrictions:

•To support Cisco Group Management Protocol (CGMP) client devices, configure the PISA as a CGMP server. Refer to the Cisco IOS IP Configuration Guide, Release 12.2, "Configuring IP Multicast Routing," at this URL:

http://www.cisco.com/en/US/docs/ios/12_2/ip/configuration/guide/1cfmulti.html

•For more information on IP multicast and IGMP, refer to 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 Configuration Guidelines and Restrictions

When configuring the IGMP snooping querier, follow these guidelines and restrictions:

•Configure the VLAN in global configuration mode (see Chapter 12 "Configuring VLANs").

•Configure an IP address on the VLAN interface (see Chapter 19 "Configuring 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 supports IGMP version 2.

•When enabled, the IGMP snooping querier does not start if it detects IGMP traffic from a multicast router.

•When enabled, the IGMP snooping querier starts after 60 seconds with no IGMP traffic detected from a multicast router.

•When enabled, the IGMP snooping querier disables itself if it detects IGMP traffic from a multicast router.

•QoS does not support IGMP packets when IGMP snooping is enabled.

•You can enable the IGMP snooping querier on all the Catalyst 6500 series switches in the VLAN. One switch is elected as the querier.

Note When you are in configuration mode you can enter EXEC mode commands by entering the do keyword before the EXEC mode command.

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:

Router# interface vlan 200 

Router(config-if)# ip address 172.20.52.106 255.255.255.248

Router(config-if)# igmp snooping querier 

Router(config-if)# end 

Router# show ip igmp interface vlan 200 | include querier 

IGMP snooping querier is enabled on this interface

Router#

Configuring IGMP Snooping

Note To use IGMP snooping, configure a Layer 3 interface in the subnet for multicast routing (see Chapter 25 "Configuring IPv4 Multicast Layer 3 Switching") or enable the IGMP snooping querier in the subnet (see the "Enabling the IGMP Snooping Querier" section).

IGMP snooping allows Catalyst 6500 series switches to examine IGMP packets and make forwarding decisions based on their content.

These sections describe how to configure IGMP snooping:

•Enabling IGMP Snooping

•Configuring a Static Connection to a Multicast Receiver

•Configuring a Multicast Router Port Statically

•Configuring the IGMP Snooping Query Interval

•Enabling IGMP Fast-Leave Processing

•Configuring Source Specific Multicast (SSM) Mapping

•Configuring IGMPv3 Explicit Host Tracking

•Displaying IGMP Snooping Information

Note Except for the ip igmp snooping command, all IGMP snooping commands are supported only on VLAN interfaces.

Enabling IGMP Snooping

To enable IGMP snooping globally, perform this task:

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

Router(config)# ip igmp snooping 

Router(config)# end 

Router# show ip igmp interface vlan 200 | include globally 

  IGMP snooping is globally enabled

Router#

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:

Router# interface vlan 25 

Router(config-if)# ip igmp snooping 

Router(config-if)# end 

Router# show ip igmp interface vl25 | include snooping

  IGMP snooping is globally enabled

  IGMP snooping is enabled on this interface 

  IGMP snooping fast-leave is disabled and querier is disabled

  IGMP snooping explicit-tracking is enabled on this interface 

  IGMP snooping last member query interval on this interface is 1000 ms

Router#

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:

Router(config)# mac-address-table static 0050.3e8d.6400 vlan 12 interface fastethernet 5/7 

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:

Router(config-if)# ip igmp snooping mrouter interface fastethernet 5/6 

Router(config-if)#

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 fast-leave processing and the IGMP query interval are configured, fast-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:

Router(config-if)# ip igmp snooping last-member-query-interval 200

Router(config-if)# exit

Router# show ip igmp interface vlan 200 | include last 

  IGMP snooping last member query interval on this interface is 200 ms

Enabling IGMP Fast-Leave Processing

To enable IGMP fast-leave processing in a VLAN, perform this task:

This example shows how to enable IGMP fast-leave processing on the VLAN 200 interface and verify the configuration:

Router# interface vlan 200 

Router(config-if)# ip igmp snooping fast-leave 

Configuring fast leave on vlan 200

Router(config-if)# end 

Router# show ip igmp interface vlan 200 | include fast-leave 

IGMP snooping fast-leave is enabled on this interface

Router(config-if)#

Configuring Source Specific Multicast (SSM) Mapping

Note Do not configure SSM mapping in a VLAN that supports IGMPv3 multicast receivers.

To configure SSM mapping, refer to this publication:

http://www.cisco.com/en/US/docs/ios-xml/ios/ipmulti_igmp/configuration/12-2sx/imc_ssm_mapping.html

Configuring IGMPv3 Explicit Host Tracking

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

This example shows how to enable explicit host tracking:

Router(config)# interface vlan 25

Router(config-if)# ip igmp snooping explicit-tracking

Router(config-if)# end

Router# show ip igmp snooping explicit-tracking vlan 25 

Source/Group                    Interface    Reporter        Filter_mode

------------------------------------------------------------------------

10.1.1.1/226.2.2.2              Vl25:1/2     16.27.2.3       INCLUDE    

10.2.2.2/226.2.2.2              Vl25:1/2     16.27.2.3       INCLUDE 

Displaying IGMP Snooping Information

These sections describe displaying IGMP snooping information:

•Displaying Multicast Router Interfaces

•Displaying MAC Address Multicast Entries

•Displaying IGMP Snooping Information for a VLAN Interface

•Displaying IGMP Snooping Statistics

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:

Router# show ip igmp snooping mrouter vlan 1

vlan            ports

-----+----------------------------------------

  1          Gi1/1,Gi2/1,Fa3/48,Router

Router#

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:

Router# show mac-address-table multicast vlan 1

vlan   mac address     type    qos             ports

-----+---------------+--------+---+--------------------------------

  1  0100.5e02.0203  static   --  Gi1/1,Gi2/1,Fa3/48,Router

  1  0100.5e00.0127  static   --  Gi1/1,Gi2/1,Fa3/48,Router

  1  0100.5e00.0128  static   --  Gi1/1,Gi2/1,Fa3/48,Router

  1  0100.5e00.0001  static   --  Gi1/1,Gi2/1,Fa3/48,Router,Switch

Router#

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

Router# show mac-address-table multicast 1 count

Multicast MAC Entries for vlan 1:    4

Router#

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:

Router# show ip igmp interface vlan 43 

Vlan43 is up, line protocol is up

  Internet address is 43.0.0.1/24

  IGMP is enabled on interface

  Current IGMP host version is 2

  Current IGMP router version is 2

  IGMP query interval is 60 seconds

  IGMP querier timeout is 120 seconds

  IGMP max query response time is 10 seconds

  Last member query count is 2

  Last member query response interval is 1000 ms

  Inbound IGMP access group is not set

  IGMP activity:1 joins, 0 leaves

  Multicast routing is enabled on interface

  Multicast TTL threshold is 0

  Multicast designated router (DR) is 43.0.0.1 (this system)

  IGMP querying router is 43.0.0.1 (this system)

  Multicast groups joined by this system (number of users):

      224.0.1.40(1)

  IGMP snooping is globally enabled

  IGMP snooping is enabled on this interface 

  IGMP snooping fast-leave is disabled and querier is disabled

  IGMP snooping explicit-tracking is enabled on this interface 

  IGMP snooping last member query interval on this interface is 1000 ms

Router# 

Displaying IGMP Snooping Statistics

The show ip igmp snooping statistics interface vlan_ID command displays the following information:

•The list of ports that are members of a group

•The filter mode

•The reporter-address behind the port

•The last-join and last-leave information collected since the last time a clear ip igmp snooping statistics command was entered

To display IGMP snooping statistics, perform this task:

This example shows IGMP snooping statistics information for interface VLAN 25:

Router# show ip igmp snooping statistics interface vlan 25

Snooping statistics for Vlan25

#channels:2

#hosts   :1

Source/Group            Interface       Reporter      Uptime        Last-Join   Last-Leave

10.1.1.1/226.2.2.2      Gi1/2:Vl25      16.27.2.3     00:01:47      00:00:50      -     

10.2.2.2/226.2.2.2      Gi1/2:Vl25      16.27.2.3     00:01:47      00:00:50      - 

Router#