Configuring IGMP

This chapter describes how to configure the Internet Group Management Protocol (IGMP) on Cisco NX-OS devices for IPv4 networks.

About IGMP

IGMP is an IPv4 protocol that a host uses to request multicast data for a particular group. Using the information obtained through IGMP, the software maintains a list of multicast group or channel memberships on a per-interface basis. The systems that receive these IGMP packets send multicast data that they receive for requested groups or channels out the network segment of the known receivers.

By default, the IGMP process is running. You cannot enable IGMP manually on an interface. IGMP is automatically enabled when you perform one of the following configuration tasks on an interface:

  • Enable PIM

  • Statically bind a local multicast group

IGMP Versions

The device supports IGMPv2 and IGMPv3, and IGMPv1 report reception.

By default, the software enables IGMPv2 when it starts the IGMP process. You can enable IGMPv3 on interfaces where you want its capabilities.

IGMPv3 includes the following key changes from IGMPv2:

  • Hosts no longer perform report suppression, which means that hosts always send IGMP membership reports when an IGMP query message is received.


Note

The Cisco Nexus® 3550-T switches does not support SSM.

For detailed information about IGMPv2, see RFC 2236.

For detailed information about IGMPv3, see RFC 5790.

IGMP Basics

This figure shows the basic IGMP process of a router that discovers multicast hosts. Hosts 1, 2, and 3 send unsolicited IGMP membership report messages to initiate receiving multicast data for a group or channel.

Figure 1. IGMPv1 and IGMPv2 Query-Response Process

In the figure below, router A, which is the IGMP designated querier on the subnet, sends query messages to the all-hosts multicast group at 224.0.0.1 periodically to discover whether any hosts want to receive multicast data. You can configure the group membership timeout value that the router uses to determine that no members of a group or source exist on the subnet.

The software elects a router as the IGMP querier on a subnet if it has the lowest IP address. As long as a router continues to receive query messages from a router with a lower IP address, it resets a timer that is based on its querier timeout value. If the querier timer of a router expires, it becomes the designated querier. If that router later receives a host query message from a router with a lower IP address, it drops its role as the designated querier and sets its querier timer again.

In this figure, host 1’s membership report is suppressed, and host 2 sends its membership report for group 224.1.1.1 first. Host 1 receives the report from host 2. Because only one membership report per group needs to be sent to the router, other hosts suppress their reports to reduce network traffic. Each host waits for a random time interval to avoid sending reports at the same time. You can configure the query maximum response time parameter to control the interval in which hosts randomize their responses.


Note

IGMPv1 and IGMPv2 membership report suppression occurs only on hosts that are connected to the same port.


Note

IGMPv3 hosts do not perform IGMP membership report suppression.

Messages sent by the designated querier have a time-to-live (TTL) value of 1, which means that the messages are not forwarded by the directly connected routers on the subnet. You can configure the frequency and number of query messages sent specifically for IGMP startup, and you can configure a short query interval at startup so that the group state is established as quickly as possible. Although usually unnecessary, you can tune the query interval used after startup to a value that balances the responsiveness to host group membership messages and the traffic created on the network.


Caution

Changing the query interval can severely impact multicast forwarding.

When a multicast host leaves a group, a host that runs IGMPv2 or later sends an IGMP leave message. To check if this host is the last host to leave the group, the software sends an IGMP query message and starts a timer that you can configure called the last member query response interval. If no reports are received before the timer expires, the software removes the group state. The router continues to send multicast traffic for a group until its state is removed.

You can configure a robustness value to compensate for packet loss on a congested network. The robustness value is used by the IGMP software to determine the number of times to send messages.

Link local addresses in the range 224.0.0.0/24 are reserved by the Internet Assigned Numbers Authority (IANA). Network protocols on a local network segment use these addresses; routers do not forward these addresses because they have a TTL of 1. By default, the IGMP process sends membership reports only for nonlink local addresses, but you can configure the software to send reports for link local addresses.

Prerequisites for IGMP

IGMP has the following prerequisites:

  • You are logged onto the device.

  • For global configuration commands. The default configuration mode shown in the examples in this chapter applies to the default VRF.

Guidelines and Limitations for IGMP

IGMP has the following guidelines and limitations:

  • For having low latency, Cisco Nexus® 3550-T switch only supports {Vlan,MAC} lookup for L2 ports. There is no IP based {VLAN,G} or {VLAN,G,S} lookup.

  • Route-Aliasing is expected as routes are installed for optimized {Vlan,MAC} lookup.

  • All unknown multicast packet miss is dropped instead of flood to other routers in the in the same L2 Domain.

  • Multi-access Network with Cisco Nexus® 3550-T switch would not work, there cannot be 2 PIM-Routers in same VLAN segment if one of the PIM enabled routers is Cisco Nexus® 3550-T switch. Cisco Nexus® 3550-T switch cannot act as non-DR.

  • PIM cannot be enabled on L2 Transit node since multicast-lookup miss packets not flooded in VLAN.

  • Cisco Nexus® 3550-T switch should not be used as L3 transit box on SVIs. Though L2 receivers may be present on the transit box on SVI. It can be used as L3 transit on L3 physical ports.

  • No FHR Support – hence no source expected to be connected directly in VLAN where L3 Multicast routing is required.

  • Owing to {Vlan,Mac} lookup, IGMPv2 reports are flooded to the receivers already attached, this results in report-suppression. It is recommended to have hosts configured as IGMPv3.

  • Excluding or blocking a list of sources according to IGMPv3 (RFC 5790) is not supported.

Default Settings for IGMP

This table lists the default settings for IGMP parameters.

Table 1. Default IGMP Parameters

Parameters

Default

IGMP version

2

Startup query interval

30 seconds

Startup query count

2

Robustness value

2

Querier timeout

255 seconds

Query timeout

255 seconds

Query max response time

10 seconds

Query interval

125 seconds

Last member query response interval

1 second

Last member query count

2

Group membership timeout

260 seconds

Report link local multicast groups

Disabled

Enforce router alert

Disabled

Immediate leave

Disabled

Configuring IGMP Parameters

You can configure the IGMP global and interface parameters to affect the operation of the IGMP process.


Note

If you are familiar with the Cisco IOS CLI, be aware that the Cisco NX-OS commands for this feature might differ from the Cisco IOS commands that you would use.

Configuring IGMP Interface Parameters

You can configure the optional IGMP interface parameters described in the table below.

Table 2. IGMP Interface Parameters

Parameter

Description

IGMP version

IGMP version that is enabled on the interface. The IGMP version can be 2 or 3. The default is 2.

Static multicast groups

Multicast groups that are statically bound to the interface. You can configure the groups to join the interface with the (*, G) state. You can specify a route-map policy name that lists the group prefixes, and group ranges to use with the match ip multicast command.

Note 

Although you can configure the (*, G) state, the source tree is built only if you enable IGMPv3.

You can configure a multicast group on all the multicast-capable routers on the network so that pinging the group causes all the routers to respond.

Static multicast groups on OIF

Multicast groups that are statically bound to the output interface. You can configure the groups to join the output interface with the (*, G) state or specify a source IP to join with the (*, G) state. You can specify a route-map policy name that lists the group prefixes, group ranges, and source prefixes to use with the match ip multicast command.

Note 

Although you can configure the (*, G) state, the source tree is built only if you enable IGMPv3.

Startup query interval

Startup query interval. By default, this interval is shorter than the query interval so that the software can establish the group state as quickly as possible. Values range from 1 to 18,000 seconds. The default is 31 seconds.

Startup query count

Number of queries sent at startup that are separated by the startup query interval. Values range from 1 to 10. The default is 2.

Robustness value

Robustness variable that you can tune to reflect expected packet loss on a congested network. You can increase the robustness variable to increase the number of times that packets are resent. Values range from 1 to 7. The default is 2.

Querier timeout

Number of seconds that the software waits after the previous querier has stopped querying and before it takes over as the querier. Values range from 1 to 65,535 seconds. The default is 255 seconds.

Query max response time

Maximum response time advertised in IGMP queries. You can tune the IGMP messages on the network by setting a larger value so that host responses are spread out over a longer time. This value must be less than the query interval. Values range from 1 to 25 seconds. The default is 10 seconds.

Query interval

Frequency at which the software sends IGMP host query messages. You can tune the number of IGMP messages on the network by setting a larger value so that the software sends IGMP queries less often. Values range from 1 to 18,000 seconds. The default is 125 seconds.

Last member query response interval

Interval in which the software sends a response to an IGMP query after receiving a host leave message from the last known active host on the subnet. If no reports are received in the interval, the group state is deleted. You can use this value to tune how quickly the software stops transmitting on the subnet. The software can detect the loss of the last member of a group or source more quickly when the values are smaller. Values range from 1 to 25 seconds. The default is 1 second.

Last member query count

Number of times that the software sends an IGMP query, separated by the last member query response interval, in response to a host leave message from the last known active host on the subnet. Values range from 1 to 5. The default is 2.

Setting this value to 1 means that a missed packet in either direction causes the software to remove the multicast state from the queried group or channel. The software may wait until the next query interval before the group is added again.

Group membership timeout

Group membership interval that must pass before the router decides that no members of a group or source exist on the network. Values range from 3 to 65,535 seconds. The default is 260 seconds.

Report link local multicast groups

Option that enables sending reports for groups in 224.0.0.0/24. Link local addresses are used only by protocols on the local network. Reports are always sent for nonlink local groups. The default is disabled.

Immediate leave

Option that minimizes the leave latency of IGMPv2 group memberships on a given IGMP interface because the device does not send group-specific queries. When immediate leave is enabled, the device removes the group entry from the multicast routing table immediately upon receiving a leave message for the group. The default is disabled.

Note 

Use this command only when there is one receiver behind the interface for a given group.

Procedure

  Command or Action Purpose
Step 1

configure terminal

Example:

switch# configure terminal
switch(config)#

Enters global configuration mode.

Step 2

interface interface

Example:

switch(config)# interface ethernet 1/1
switch(config-if)#

Enters interface configuration mode.

Note 

Use the commands listed from step-3 to configure the IGMP interface parameters.
Step 3

ip igmp version value

Example:

switch(config-if)# ip igmp version 3

Sets the IGMP version to the value specified. Values can be 2 or 3. The default is 2.

The no form of the command sets the version to 2.

Step 4

ip igmp join-group {group [source source] }

Example:

switch(config-if)# ip igmp join-group 230.0.0.0

Configures an interface on the device to join the specified group or channel. The device accepts the multicast packets for CPU consumption only.

Caution 

The device CPU must be able to handle the traffic generated by using this command. Because of CPU load constraints, using this command, especially in any form of scale, is not recommended. Consider using the ip igmp static-oif command instead.

Step 5

ip igmp static-oif {group [source source]}

Example:

switch(config-if)# ip igmp static-oif 230.0.0.0

Statically binds a multicast group to the outgoing interface, which is handled by the device hardware. If you specify only the group address, the (*, G) state is created. If you specify the source address, the (*, G) state is created.

Note 

A source tree is built for the (*, G) state only if you enable IGMPv3.

Step 6

ip igmp startup-query-interval seconds

Example:

switch(config-if)# ip igmp startup-query-interval 25

Sets the query interval used when the software starts up. Values can range from 1 to 18,000 seconds. The default is 31 seconds.

Step 7

ip igmp startup-query-count count

Example:

switch(config-if)# ip igmp startup-query-count 3

Sets the query count used when the software starts up. Values can range from 1 to 10. The default is 2.

Step 8

ip igmp robustness-variable value

Example:

switch(config-if)# ip igmp robustness-variable 3

Sets the robustness variable. Values can range from 1 to 7. The default is 2.

Step 9

ip igmp querier-timeout seconds

Example:

switch(config-if)# ip igmp querier-timeout 300

Sets the querier timeout that the software uses when deciding to take over as the querier. Values can range from 1 to 65,535 seconds. The default is 255 seconds.

Step 10

ip igmp query-timeout seconds

Example:

switch(config-if)# ip igmp query-timeout 300

Sets the query timeout that the software uses when deciding to take over as the querier. Values can range from 1 to 65,535 seconds. The default is 255 seconds.

Note 

This command has the same functionality as the ip igmp querier-timeout command.

Step 11

ip igmp query-max-response-time seconds

Example:

switch(config-if)# ip igmp query-max-response-time 15

Sets the response time advertised in IGMP queries. Values can range from 1 to 25 seconds. The default is 10 seconds.

Step 12

ip igmp query-interval interval

Example:

switch(config-if)# ip igmp query-interval 100

Sets the frequency at which the software sends IGMP host query messages. Values can range from 1 to 18,000 seconds. The default is 125 seconds.

Step 13

ip igmp last-member-query-response-time seconds

Example:

switch(config-if)# ip igmp last-member-query-response-time 3

Sets the query interval waited after sending membership reports before the software deletes the group state. Values can range from 1 to 25 seconds. The default is 1 second.

Step 14

ip igmp last-member-query-count count

Example:

switch(config-if)# ip igmp last-member-query-count 3

Sets the number of times that the software sends an IGMP query in response to a host leave message. Values can range from 1 to 5. The default is 2.

Step 15

ip igmp group-timeout seconds

Example:

switch(config-if)# ip igmp group-timeout 300

Sets the group membership timeout for IGMPv2. Values can range from 3 to 65,535 seconds. The default is 260 seconds.
Step 16

ip igmp report-link-local-groups

Example:

switch(config-if)# ip igmp report-link-local-groups

Enables sending reports for groups in 224.0.0.0/24. Reports are always sent for nonlink local groups. By default, reports are not sent for link local groups.

Step 17

ip igmp report-policy policy

Example:

switch(config-if)# ip igmp report-policy my_report_policy

Configures an access policy for IGMP reports that is based on a route-map policy.

Step 18

ip igmp access-group policy

Example:

switch(config-if)# ip igmp access-group my_access_policy

Configures a route-map policy to control the multicast groups that hosts on the subnet serviced by an interface can join.

Note 

Only the match ip multicast group command is supported in this route map policy. The match ip address command for matching an ACL is not supported.

Step 19

ip igmp immediate-leave

Example:

switch(config-if)# ip igmp immediate-leave

Enables the device to remove the group entry from the multicast routing table immediately upon receiving a leave message for the group. Use this command to minimize the leave latency of IGMPv2 group memberships on a given IGMP interface because the device does not send group-specific queries. The default is disabled.

Note 

Use this command only when there is one receiver behind the interface for a given group.

Step 20

(Optional) copy running-config startup-config

Example:


switch(config)# copy running-config startup-config
 (Optional)

Copies the running configuration to the startup configuration.

Restarting the IGMP Process

You can restart the IGMP process and optionally flush all routes.

Procedure

  Command or Action Purpose
Step 1

restart igmp

Example:

switch# restart igmp

Restarts the IGMP process.

Step 2

configure terminal

Example:


switch# configure terminal
switch(config)#

Enters global configuration mode.

Step 3

ip igmp flush-routes

Example:

switch(config)# ip igmp flush-routes

Removes routes when the IGMP process is restarted. By default, routes are not flushed.

Step 4

(Optional) show running-configuration igmp

Example:

switch(config)# show running-configuration igmp
 (Optional)

Shows the running-configuration information.

Step 5

(Optional) copy running-config startup-config

Example:

switch(config)# copy running-config startup-config
 (Optional)

Copies the running configuration to the startup configuration.

Verifying the IGMP Configuration

To display the IGMP configuration information, perform one of the following tasks:

Command

Description

show ip igmp interface [interface] [brief]

Displays IGMP information about all interfaces or a selected interface.

show ip igmp groups [{source [group]}] | {group [source]}] [interface] [summary]

Displays the IGMP attached group membership for a group or interface.

show ip igmp route [{source [group]}] | {group [source]}] [interface] [summary]

Displays the IGMP attached group membership for a group or interface.

show ip igmp local-groups

Displays the IGMP local group membership.

show running-configuration igmp

Displays the IGMP running-configuration information.

show startup-configuration igmp

Displays the IGMP startup-configuration information.

Configuration Examples for IGMP

The following example shows how to configure the IGMP parameters: 


configure terminal
  
  interface ethernet 1/1
    ip igmp version 3
    ip igmp join-group 230.0.0.0
    ip igmp startup-query-interval 25
    ip igmp startup-query-count 3
    ip igmp robustness-variable 3
    ip igmp querier-timeout 300
    ip igmp query-timeout 300
    ip igmp query-max-response-time 15
    ip igmp query-interval 100
    ip igmp last-member-query-response-time 3
    ip igmp last-member-query-count 3
    ip igmp group-timeout 300
    ip igmp report-link-local-groups