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Table Of Contents
Configuring MLD Snooping for IPv6 Multicast Traffic
Joining an IPv6 Multicast Group
Understanding the MLD Snooping Querier
Default MLD Snooping Configuration
MLD Snooping Configuration Guidelines and Restrictions
MLD Snooping Querier Configuration Guidelines and Restrictions
Enabling the MLD Snooping Querier
Configuring a Static Connection to a Multicast Receiver
Configuring a Multicast Router Port Statically
Configuring the MLD Snooping Query Interval
Enabling Fast-Leave Processing
Configuring Explicit Host Tracking
Configuring Report Suppression
Displaying MLD Snooping Information
Displaying Multicast Router Interfaces
Displaying MAC Address Multicast Entries
Displaying MLD Snooping Information for a VLAN Interface
Configuring MLD Snooping for IPv6 Multicast Traffic
This chapter describes how to configure Multicast Listener Discovery (MLD) snooping for IPv6 multicast traffic in Cisco IOS Software Release 12.2SX.
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http://www.cisco.com/en/US/docs/ios/mcl/122sx_mcl.html
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This chapter consists of these sections:
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Understanding MLD Snooping
These sections describe MLD snooping:
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MLD Snooping Overview
MLD snooping allows the switch to examine MLD packets and make forwarding decisions based on their content.
You can configure the switch to use MLD snooping in subnets that receive MLD queries from either MLD or the MLD snooping querier. MLD snooping constrains IPv6 multicast traffic at Layer 2 by configuring Layer 2 LAN ports dynamically to forward IPv6 multicast traffic only to those ports that want to receive it.
MLD, which runs at Layer 3 on a multicast router, generates Layer 3 MLD queries in subnets where the multicast traffic needs to be routed. For information about MLD, see this publication:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios123/123cgcr/ipv6_c/sa_mcast.htm
You can configure the MLD snooping querier on the switch to support MLD snooping in subnets that do not have any multicast router interfaces. For more information about the MLD snooping querier, see the "Enabling the MLD Snooping Querier" section.
MLD (on a multicast router) or, locally, the MLD snooping querier, sends out periodic general MLD queries that the switch forwards through all ports in the VLAN, and to which hosts respond. MLD snooping monitors the Layer 3 MLD traffic.
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MLD Messages
These are the MLD messages:
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Source-Based Filtering
MLD uses source-based filtering, which enables hosts and routers to specify which multicast sources should be allowed or blocked for a specific multicast group. Source-based filtering either allows or blocks traffic based on the following information in MLD messages:
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Because the Layer 2 table is (MAC-group, VLAN) based, with MLD hosts it is preferable to have only a single multicast source per MAC-group.
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Explicit Host Tracking
MLD supports explicit tracking of membership information on any port. The explicit-tracking database is used for fast-leave processing, proxy reporting, and statistics collection. When explicit tracking is enabled on a VLAN, the MLD snooping software processes the MLD report it receives from a host and builds an explicit-tracking database that contains the following information:
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MLD Snooping Proxy Reporting
Because MLD does not have report suppression, all the hosts send their complete multicast group membership information to the multicast router in response to queries. The switch snoops these responses, updates the database and forwards the reports to the multicast router. To prevent the multicast router from becoming overloaded with reports, MLD snooping does proxy reporting.
Proxy reporting forwards only the first report for a multicast group to the router and suppresses all other reports for the same multicast group.
Proxy reporting processes solicited and unsolicited reports. Proxy reporting is enabled and cannot be disabled.
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Joining an IPv6 Multicast Group
Hosts join IPv6 multicast groups either by sending an unsolicited MLD report or by sending an MLD report in response to a general query from an IPv6 multicast router (the switch forwards general queries from IPv6 multicast routers to all ports in a VLAN). The switch snoops these reports.
In response to a snooped MLD report, the switch creates an entry in its Layer 2 forwarding table for the VLAN on which the report was received. When other hosts that are interested in this multicast traffic send MLD reports, the switch snoops their reports and 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 snoops an MLD report.
MLD snooping suppresses all but one of the host reports per multicast group and forwards this one report to the IPv6 multicast router.
The switch forwards multicast traffic for the multicast group specified in the report to the interfaces where reports were received (see Figure 30-1).
Layer 2 multicast groups learned through MLD 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 MLD snooping learning. Multicast group membership lists can consist of both static and MLD snooping-learned settings.
Figure 30-1 Initial MLD Listener Report
Multicast router A sends an MLD 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 an IPv6 multicast group and multicasts an MLD report to the group with the equivalent MAC destination address of 0x0100.5E01.0203. When the switch snoops the MLD report multicast by Host 1, the switch uses the information in the MLD report to create a forwarding-table entry. Table 30-1 shows the forwarding table, which includes the port numbers of Host 1, the multicast router, and the switch.
Table 30-1 MLD Snooping Forwarding Table
0100.5exx.xxxx
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0100.5e01.0203
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The switch hardware can distinguish MLD information packets from other packets for the multicast group. The first entry in the table indicates that only MLD packets should be sent to the CPU, which prevents the switch from becoming overloaded with multicast frames. The second entry indicates that frames addressed to the 0x0100.5E01.0203 multicast MAC address that are not MLD packets (!MLD) should be sent to the multicast router and to the host that has joined the group.
If another host (for example, Host 4) sends an unsolicited MLD report for the same group (Figure 30-2), the switch snoops that message and adds the port number of Host 4 to the forwarding table as shown in Table 30-2. Because the forwarding table directs MLD messages only to the switch, the message is not flooded to other ports. Any known multicast traffic is forwarded to the group and not to the switch.
Figure 30-2 Second Host Joining a Multicast Group
Table 30-2 Updated MLD Snooping Forwarding Table
0100.5exx.xxxx
MLD
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0100.5e01.0203
!MLD
1, 2, 5
Leaving a Multicast Group
These sections describe leaving a multicast group:
Normal Leave Processing
Interested hosts must continue to respond to the periodic MLD general queries. As long as at least one host in the VLAN responds to the periodic MLD general 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 MLD general queries (called a "silent leave"), or they can send an MLD filter mode change record.
When MLD snooping receives a filter mode change record from a host that configures the EXCLUDE mode for a group, MLD snooping sends out a MAC-addressed general query to determine if any other hosts connected to that interface are interested in traffic for the specified multicast group.
If MLD snooping does not receive an MLD report in response to the general query, MLD snooping assumes that no other hosts connected to the interface are interested in receiving traffic for the specified multicast group, and MLD snooping removes the interface from its Layer 2 forwarding table entry for the specified multicast group.
If the filter mode change record was from the only remaining interface with hosts interested in the group, and MLD snooping does not receive an MLD report in response to the general query, MLD snooping removes the group entry and relays the MLD filter mode change record 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 MLD 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 ipv6 mld snooping last-member-query-interval interval command.
Fast-Leave Processing
Fast-leave processing is enabled by default. To disable fast-leave processing, turn off explicit-host tracking.
Fast-leave processing 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.
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Understanding the MLD Snooping Querier
Use the MLD snooping querier to support MLD snooping in a VLAN where PIM and MLD 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 MLD 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 MLD querier so that it can send queries.
When enabled, the MLD snooping querier sends out periodic MLD queries that trigger MLD report messages from the switch that wants to receive IP multicast traffic. MLD snooping listens to these MLD reports to establish appropriate forwarding.
You can enable the MLD snooping querier on all the switches in the VLAN, but for each VLAN that is connected to switches that use MLD to report interest in IP multicast traffic, you must configure at least one switch as the MLD snooping querier.
You can configure a switch to generate MLD queries on a VLAN regardless of whether or not IP multicast routing is enabled.
Default MLD Snooping Configuration
Table 30-3 shows the default MLD snooping configuration.
MLD Snooping Configuration Guidelines and Restrictions
When configuring MLD snooping, follow these guidelines and restrictions:
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MLD Snooping Querier Configuration Guidelines and Restrictions
When configuring the MLD snooping querier, follow these guidelines and restrictions:
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Enabling the MLD Snooping Querier
Use the MLD snooping querier to support MLD snooping in a VLAN where PIM and MLD are not configured because the multicast traffic does not need to be routed.
To enable the MLD snooping querier in a VLAN, perform this task:
This example shows how to enable the MLD snooping querier on VLAN 200 and verify the configuration:
Router# interface vlan 200Router(config-if)# ipv6 address 2001:0DB8:0:1::/64 eui-64Router(config-if)# ipv6 mld snooping querierRouter(config-if)# endRouter# show ipv6 mld interface vlan 200 | include querierMLD snooping fast-leave is enabled and querier is enabledConfiguring MLD Snooping
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These sections describe how to configure MLD snooping:
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Enabling MLD Snooping
To enable MLD snooping globally, perform this task:
This example shows how to enable MLD snooping globally and verify the configuration:
Router(config)# ipv6 mld snoopingRouter(config)# endRouter# show ipv6 mld interface vlan 200 | include globallyMLD snooping is globally enabledRouter#To enable MLD snooping in a VLAN, perform this task:
This example shows how to enable MLD snooping on VLAN 25 and verify the configuration:
Router# interface vlan 25Router(config-if)# ipv6 mld snoopingRouter(config-if)# endRouter# show ipv6 mld interface vlan 25 | include snoopingMLD snooping is globally enabledMLD snooping is enabled on this interfaceMLD snooping fast-leave is enabled and querier is enabledMLD snooping explicit-tracking is enabledMLD snooping last member query response interval is 1000 msMLD snooping report-suppression is disabledRouter#Configuring a Static Connection to a Multicast Receiver
To configure a static connection to a multicast receiver, perform this task:
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Router(config)# mac-address-table static mac_addr vlan vlan_id interface type1 slot/port [disable-snooping]
Configures a static connection to a multicast receiver.
Router(config)# no mac-address-table static mac_addr vlan vlan_id
Clears a static connection to a multicast receiver.
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Router(config-if)# end
Exits configuration mode.
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Router# show mac-address-table address mac_addr
Verifies the configuration.
1 type = fastethernet, gigabitethernet, or tengigabitethernet
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 5/7Configuring a Multicast Router Port Statically
To configure a static connection to a multicast router, perform this task:
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Router(config)# interface vlan vlan_ID
Selects the VLAN interface.
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Router(config-if)# ipv6 mld snooping mrouter interface type1 slot/port
Configures a static connection to a multicast router.
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Router(config-if)# end
Exits configuration mode.
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Router# show ipv6 mld snooping mrouter
Verifies the configuration.
1 type = fastethernet, gigabitethernet, or tengigabitethernet
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)# ipv6 mld snooping mrouter interface fastethernet 5/6Router(config-if)#Configuring the MLD 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.
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To configure the interval for the MLD snooping queries sent by the switch, perform this task:
This example shows how to configure the MLD snooping query interval:
Router(config-if)# ipv6 mld snooping last-member-query-interval 1000Router(config-if)# exitRouter# show ipv6 mld interface vlan 200 | include lastMLD snooping last member query response interval is 1000 msEnabling Fast-Leave Processing
To enable fast-leave processing in a VLAN, perform this task:
This example shows how to enable fast-leave processing on the VLAN 200 interface and verify the configuration:
Router# interface vlan 200Router(config-if)# ipv6 mld snooping fast-leaveConfiguring fast leave on vlan 200Router(config-if)# endRouter# show ipv6 mld interface vlan 200 | include fast-leaveMLD snooping fast-leave is enabled and querier is enabledRouter#Enabling SSM Safe Reporting
To enable source-specific multicast (SSM) safe reporting, perform this task:
This example shows how to SSM safe reporting:
Router(config)# interface vlan 10Router(config-if)# ipv6 mld snooping ssm-safe-reportingConfiguring Explicit Host Tracking
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To enable explicit host tracking on a VLAN, perform this task:
This example shows how to enable explicit host tracking:
Router(config)# interface vlan 25Router(config-if)# ipv6 mld snooping explicit-trackingRouter(config-if)# endRouter# show ipv6 mld snooping explicit-tracking vlan 25Source/Group Interface Reporter Filter_mode------------------------------------------------------------------------10.1.1.1/226.2.2.2 Vl25:1/2 16.27.2.3 INCLUDE10.2.2.2/226.2.2.2 Vl25:1/2 16.27.2.3 INCLUDEConfiguring Report Suppression
To enable report suppression on a VLAN, perform this task:
This example shows how to enable explicit host tracking:
Router(config)# interface vlan 25Router(config-if)# ipv6 mld snooping report-suppressionRouter(config-if)# endRouter# Router# show ipv6 mld interface vlan 25 | include report-suppressionMLD snooping report-suppression is enabledDisplaying MLD Snooping Information
These sections describe displaying MLD snooping information:
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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:
Router# show ipv6 mld snooping mrouter vlan_ID
Displays multicast router interfaces.
This example shows how to display the multicast router interfaces in VLAN 1:
Router# show ipv6 mld snooping mrouter vlan 1vlan ports-----+----------------------------------------1 Gi1/1,Gi2/1,Fa3/48,RouterRouter#Displaying MAC Address Multicast Entries
To display MAC address multicast entries for a VLAN, perform this task:
Router# show mac-address-table multicast vlan_ID [count]
Displays MAC address multicast entries for a VLAN.
This example shows how to display MAC address multicast entries for VLAN 1:
Router# show mac-address-table multicast vlan 1vlan mac address type qos ports-----+---------------+--------+---+--------------------------------1 0100.5e02.0203 static -- Gi1/1,Gi2/1,Fa3/48,Router1 0100.5e00.0127 static -- Gi1/1,Gi2/1,Fa3/48,Router1 0100.5e00.0128 static -- Gi1/1,Gi2/1,Fa3/48,Router1 0100.5e00.0001 static -- Gi1/1,Gi2/1,Fa3/48,Router,SwitchRouter#This example shows how to display a total count of MAC address entries for a VLAN:
Router# show mac-address-table multicast 1 countMulticast MAC Entries for vlan 1: 4Router#Displaying MLD Snooping Information for a VLAN Interface
To display MLD snooping information for a VLAN interface, perform this task:
This example shows how to display explicit tracking information on VLAN 25:
Router# show ipv6 mld snooping explicit-tracking vlan 25Source/Group Interface Reporter Filter_mode------------------------------------------------------------------------10.1.1.1/226.2.2.2 Vl25:1/2 16.27.2.3 INCLUDE10.2.2.2/226.2.2.2 Vl25:1/2 16.27.2.3 INCLUDEThis example shows how to display the multicast router interfaces in VLAN 1:
Router# show ipv6 mld snooping mrouter vlan 1vlan ports-----+----------------------------------------1 Gi1/1,Gi2/1,Fa3/48,RouterThis example shows IGMP snooping statistics information for VLAN 25:
Router# show ipv6 mld snooping statistics interface vlan 25Snooping staticstics for Vlan25#channels:2#hosts :1Source/Group Interface Reporter Uptime Last-Join Last-Leave10.1.1.1/226.2.2.2 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 -
