IGMP Snooping

This module describes how to enable and configure the Ethernet Virtual Connection (EVC)-based IP Multicast Internet Group Management Protocol (IGMP) Snooping feature both globally and on bridge domains.

Information About IGMP Snooping

IGMP Snooping

IGMP snooping is the process of listening to IGMP network traffic between hosts and routers. It sends multicast traffic only to the interfaces that are subscribed to a particular multicast group and thus restricts flooding of multicast traffic. Switch maintains a map of the links and the associated IP multicast streams.

IGMP snooping is designed to prevent hosts on a local network from receiving traffic for a multicast group that have not explicitly joined. IGMP snooping takes place internally on switches and is not a protocol feature. Hence, it is especially useful for bandwidth-intensive IP multicast applications such as IPTV.

Cisco NCS 520 Series Routers dynamically configure layer 2 interfaces so that multicast traffic is forwarded to those interfaces only that are associated with the multicast devices. Thus, these routers use IGMP snooping to constrain the flooding of multicast traffic.

IGMP snooping requires the router to snoop on the IGMP transmissions between the host and the router to keep a track of the multicast groups and member ports. The router receives an IGMP report from a host for a particular multicast group and then adds the host port number to the forwarding table entry. The router receives an IGMP Leave signal and removes the host port from the table entry. It also periodically deletes entries if it does not receive IGMP membership reports from the multicast clients.

IGMP snooping is supported on Metro Access licenses.

Prerequisites for IGMP Snooping

  • IGMP snooping is implemented based on layer 2 multicast frames.

  • Basic IGMP v3 snooping support (BISS) is supported.

  • POP operation for all vlan tags should be configured on EFP.

  • Bridge domain (BD) interfaces from 1 to 4094 support IGMP snooping.

  • IGMP static joins are not supported.

Restrictions for IGMP Snooping

  • IGMP snooping is implemented based on layer 2 multicast frames. So, any unregistered multicast traffic is flooded to all ports in a bridge-domain (BD), even when IGMP snooping is enabled.

  • Any multicast traffic that does not have IGMP group entry is flooded, irrespective of the presence of a mrouter.

  • When multicast traffic is received for a group that has IGMP receivers, traffic is forwarded to that IGMP-learnt port only and not to other mrouter port on the same BD.

  • Do not use overlapping addresses. Only the last 24 bits in the DMAC field can be used to create unique group entries.

  • L2 Multicast reserved MAC addresses are punted to CPU.

  • IGMP with ACL is not supported.

  • When multicast traffic is sent over ring protocols like REP or G8032 or MST, packet drops for a maximum of 60 seconds is expected during various cut over scenarios.

  • Static mrouter configuration is not supported.

  • Multicast routing is not supported.

  • Since IGMP snooping is based on L2 MAC, TTL validation check is disabled.

  • Maximum number of of IGMP entries supported is 100.

  • IGMP snooping querier functionality is not supported.

How to Configure IGMP Snooping

Enabling IGMP Snooping

Procedure

  Command or Action Purpose
Step 1

enable

Example:

Device> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal

Enters global configuration mode.

Step 3

ip igmp snooping

Example:

Device(config)# ip igmp snooping

Globally enables IGMP snooping after it has been disabled.

Step 4

bridge-domain bridge-id

Example:

Device(config)# bridge-domain 100

(Optional) Enters bridge domain configuration mode.

Step 5

ip igmp snooping

Example:

Device(config-bdomain)# ip igmp snooping

(Optional) Enables IGMP snooping on the bridge domain interface being configured.

  • Required only if IGMP snooping was previously explicitly disabled on the specified bridge domain.

Step 6

end

Example:

Device(config-bdomain)# end

Returns to privileged EXEC mode.

Disabling IGMP Snooping Globally

Procedure

  Command or Action Purpose
Step 1

enable

Example:

Device> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal

Enters global configuration mode.

Step 3

no ip igmp snooping

Example:

Device(config)# no ip igmp snooping

Disables IGMP snooping on the router.

Step 4

exit

Example:

Device(config)# exit

Exits global configuration mode and returns to privileged EXEC mode.

Disabling IGMP Snooping on a Bridge Domain

Procedure

  Command or Action Purpose
Step 1

enable

Example:

Device> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal

Enters global configuration mode.

Step 3

bridge-domain bridge-id

Example:

Device(config)# bridge-domain 4000

Enters bridge domain configuration mode.

Step 4

no ip igmp snooping

Example:

Device(config-bdomain)# no ip igmp snooping

Disables IGMP snooping on the bridge domain.

Step 5

end

Example:

Device(config-bdomain)# end

Returns to privileged EXEC mode.

Verifying IGMP Snooping

Use these commands to verify IGMP Snooping on the router.

  • show ip igmp snooping

    This command displays the IGMP snooping configuration globally on the router. The following is a sample output from the command: 

    Router# show ip igmp snooping

Global IGMP Snooping configuration:
-------------------------------------------
IGMP snooping Oper State     : Enabled
IGMPv3 snooping (minimal)    : Enabled
Report suppression           : Enabled
TCN solicit query            : Enabled
Robustness variable          : 3
Last member query count      : 2
Last member query interval   : 200
Check TTL=1                  : Yes
Check Router-Alert-Option    : No

Vlan 1:
--------
IGMP snooping Admin State           : Enabled
IGMP snooping Oper State            : Enabled
IGMPv2 immediate leave              : Disabled
Report suppression                  : Enabled
Robustness variable                 : 3
Last member query count             : 2
Last member query interval          : 200
Check TTL=1                         : Yes
Check Router-Alert-Option           : Yes
.
.
.

  • show ip igmp snooping [bd bd-id]

    This command displays configuration for IGMP snooping by bridge domain. The following is a sample output from the command: 

    Router# show ip igmp snooping bd 100

Global IGMP Snooping configuration:
-------------------------------------------
IGMP snooping Oper State     : Enabled
IGMPv3 snooping (minimal)    : Enabled
Report suppression           : Enabled
TCN solicit query            : Enabled
Robustness variable          : 3
Last member query count      : 2
Last member query interval   : 200
Check TTL=1                  : Yes
Check Router-Alert-Option    : No

Vlan 100:
--------
IGMP snooping Admin State           : Enabled
IGMP snooping Oper State            : Enabled
IGMPv2 immediate leave              : Disabled
Report suppression                  : Enabled
Robustness variable                 : 3
Last member query count             : 2
Last member query interval          : 200
Check TTL=1                         : Yes
Check Router-Alert-Option           : Yes
Query Interval                      : 0
Max Response Time                   : 10000

  • show ip igmp snooping groups count

    This command displays snooping information for groups. This is a sample output from the command: 

    Router# show ip igmp snooping groups count

Total number of groups:   4
Total number of  (S,G):   0

  • show ip igmp snooping mrouter

    This command displays multicast ports, globally or by bridge domain.. This is a sample output from the command: 

    Router# show ip igmp snooping mrouter

Vlan    ports
----    -----
100    Gi0/3/4-efp1(dynamic)
 10    Gi0/4/5-tefp1(dynamic)
100    Po64-efp100(dynamic)

  • show ip igmp snooping querier

    This command displays the IGMP querier information globally or by a bridge domain. This is a sample output from the command: 

    Router# show ip igmp snooping querier

Vlan      IP Address               IGMP Version   Port
-------------------------------------------------------------
100       10.0.0.2                 v2            Gi0/3/4-efp1
10        10.0.0.2                 v2            Gi0/4/5-tefp1
100       30.1.1.12                v2            Po64-efp100

  • show ip igmp snooping group

    This command displays the IGMP snooping information about multicast groups by VLAN. This is a sample output from the command: 

    Router# show ip igmp snooping  group

Flags: I -- IGMP snooping, S -- Static, P -- PIM snooping, A -- ASM mode
Vlan      Group/source             Type        Version     Port List
-----------------------------------------------------------------------
100       226.0.1.1                I           v2          Gi0/1/1-efp100  
10        225.1.1.1                I           v2          Gi0/4/2-tefp1
100       235.1.1.3                I           v2          Po64-efp1

Additional References

Related Documents

Related Topic Document Title

Cisco IOS commands

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/mcl/allreleasemcl/all-book.html

Standards and RFCs

Standard/RFC Title

No specific Standards and RFCs are supported by the features in this document.

—

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Feature Information for IGMP Snooping

The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.

Table 1. Feature Information for IGMP Snooping

Feature Name

Releases

Feature Information

IGMP Snooping

Cisco IOS XE Release 16.12.1

This feature was introduced on the NCS 520 Routers.