Configuring IGMP

Configuring IGMP

Configuring IGMP on Cisco routers is very easy — you don’t have to do anything. When a multicast routing protocol is enabled on a router interface, IGMP is automatically enabled. A number of commands exist to tailor IGMP to suit your environment. IGMP interface commands can be listed by entering interface configuration mode and typing

router(config-if)aip igmp ?

access-group

IGMP group-access group

helper-address

IGMP helper address

join-group

IGMP join multicast group

querier-timeout

IGMP previous querier timeout

query-interval

IGMP host query interval

query-max-response-time

IGMP max query response value

version

IGMP version

By default, all hosts on a subnet are allowed to join all multicast groups. The groups that hosts on a subnet can join are controlled using the interface command:

ip igmp access-group access-list-number [version].

access-list-number

IP standard access-list number (1—99)

version

Optional. Changes the IGMP version number. Default is 2.

Example

Configure the ethernet 0 interface on a router such that hosts can only join multicast groups 239.0.0.0 through 239.255.255.255.

interface ethernet 0

ip igmp access-group 1

access-list 1 permit 239.0.0.0 0.255.255.255

To enable stub multicast routing, use the ip igmp helper-address in conjunction with the ip pim neighbor-filter command. This IGMP command causes the router to forward IGMP Host Reports and Leave Group messages received on the interface to the IP address specified. An example of this command and stub multicast routing is contained in Chapter 7, “Protocol Independent Multicast — Sparse Mode.”

ip igmp helper-address ip-address

IP address where IGMP Host Reports and Leave Group messages are forwarded

Example

See Chapter 7.

A router interface can be configured as though there are always receivers for a multicast group present on the interface. One reason to do this is to be able to ping all multicast routers. Sending a ping to a multicast group causes all routers that have joined that group to respond. To configure a router in order to join a multicast group on an interface, use the interface configuration command:

ip igmp join-group group-address

Multicast group IP address

Example

Configure interface ethernet 0 to join the multicast group 225.250.250.1.

interface ethernet 0

ip igmp join-group 225.250.250.1

The default IGMP query interval on an interface is 60 seconds. Every 60 seconds the router sends IGMP host-query messages on the interface. To modify this default value, use the interface command:

ip igmp query-interval seconds

seconds

Number of seconds between host-query messages. The value can be between 0 and 65535.

Example

Change the query interface on interface serial 0 to 3 minutes.

interface serial 0

ip igmp query-interval 180

Be very careful with this command. If the query interval is longer than the query timeout value, then IGMP is effectively broken on the interface. All neighbor routers should be configured with the same value.

The default Maximum Response Time that is advertised in IGMP queries is 10 seconds. This value can be modified using the interface command:

ip igmp query-max-response-time seconds

seconds

Maximum Response Time that is advertised in IGMP queries

Example

Configure the Maximum Response Time on interface ethernet 0 to 15 seconds.

interface ethernet 0

ip igmp query-max-response-time 15

A Non-Querier router on a multi-access network becomes the Querier if the current Querier times out. The default value for the time out is twice the Query Interval. To modify the Query Timeout Value, use the interface command:

ip igmp query-timeout seconds

seconds

Number of seconds a Non-Querier router will wait before taking over as Querier if the current Querier times out

Example

Change the Query Timeout Value to 60 seconds on interface serial 1

interface serial

ip igmp query-interval 30

ip igmp query-timeout 60

The ip igmp join-group command can be used to statically configure a router to join a multicast group. When this command is used, packets for the configured group are handled at the process level. To fast-switch the packets for a static group, use the interface command:

ip igmp static-group group-address

Group IP multicast address

Example

Configure interface ethernet 0 to join the multicast group 225.250.250.1.

interface ethernet 0

ip igmp static-group 225.250.250.1

When PIM is enabled on an interface, IGMP version 2 is automatically enabled. To change the version, use the interface command:

ip igmp version {2 | 1 }

Example

Configure the ethernet 0 interface to use IGMP version 1. If version 1 is configured on an interface, then the commands ip igmp query-max-response-time and ip igmp query-timeout cannot be used because they are version 2-specific.

interface ethernet 0

ip igmp version 1

Entries in the router’s IGMP cache can be deleted using the Exec command:

clear ip igmp group [group-name | group-address |interface-type interface-number]

group-name

Optional. Multicast group name. Defined either in DNS or by the ip host command

group-address

Optional. Multicast group address

interface-type

Specify the interface (ethernet 0, serial 0, and so on)

Examples

To clear a particular group, use clear ip igmp group 225.250.250.1.

To clear all groups on an interface, use clear ip igmp group ethernet 0.

To clear all groups, use clear ip igmp group.