32.3.3 Connection with the multicast router

<Structure of this section>

(1) Configuring Multicast Router Ports

Multicast packets must be forwarded not only to the receiver but also to neighboring multicast routers. Therefore, when you connect the Switch to a multicast router and use IGMP snooping, configure the port that connects to the multicast router (hereinafter referred to as the multicast router port) to forward multicast packets to the multicast router.

There are two ways to configure multicast router ports on the Switch:

Configuring multicast router ports in the configuration
Automatically configuring a port on which a multicast router is detected as a multicast router port

In both cases, the operation as a multicast router port is the same. Configuration settings and automatic settings can also be used for the same port or channel group.

(2) Auto-learning of multicast router ports

Detects multicast routers by receiving monitored packets on VLAN for which the configuration command ip igmp snooping mrouter discovery is set. A port or channel group that detects a multicast router automatically configures it as a multicast router port.

(a) Monitored packets

The following table describes the packets monitored for multicast router detection.

Table 32-3: Packets monitored
Monitored packets*	Detection-target multicast router
IGMPv1 Membership Query messaging	IGMPv1 enabled multicast router
IGMPv2 General Query messaging	IGMPv2 enabled multicast router
IGMPv3 General Query messaging	IGMPv3 enabled multicast router
IPv4 PIM-Hello message	PIM enabled multicast router

#: Fragmented messages are not monitored.

(b) Retention time for multicast router ports

The retention time of the automatically configured multicast router port varies depending on the monitoring target. If a monitored packet is received from a port that has been automatically configured again within the specified retention time, the retention time for the corresponding auto-configured multicast router port is updated.

The following table describes the retention time for multicast router ports.

Table 32-4: Retention times for multicast router ports
Monitored packets	Retention time
IGMPv1 Membership Query Message	Robustness Variable^#1NGQuery Interval^#2+ Query Response Interval^#3 /2+X^#4
IGMPv2 General Query Message
IGMPv3 General Query Message #5	Robustness VariableNGQuery Interval+ Query Response Interval/2+X^#4
IPv4 PIM-Hello Message	Holdtime option-value of the received PIM-Hello message6. 105 seconds if Holdtime option is not present.

#1: Robustness Variable is fixed at 2.
#2: The default value is 125 seconds. If ip igmp snooping query-interval of the configuration command is set, the value specified by the corresponding command.
#3: The default value is 10 seconds. If Query Interval is less than or equal to 10 seconds, (Query Interval-1) seconds.
#4: The duration specified in the configuration command ip igmp snooping mrouter discovery extension. If a value is changed while it has already been learned, the new value is applied when the hold time is updated.
#5: For IGMPv3 Query messages, Robustness Variable,Query Interval,Query Response Interval is taken from the last received Query message. 0. The seconds part is rounded up to one second.
#6: It is not detected when Holdtime of PIM-Hello messaging is 0. Also, the corresponding multicast router port automatically configured by PIM monitoring is deleted without updating the hold time if a VLAN that has already been automatically configured by Holdtime monitoring receives a PIM-Hello message with a VLAN of 0.

(c) Removing Multicast Router Ports

If the auto-configured multicast router port does not detect the multicast router again on the corresponding port or channel group during the hold time, the hold time expires and is automatically deleted.

The following table lists the conditions for deleting multicast router ports that are set automatically when the retention time is not expired and what is to be deleted:

When an operation-command clear igmp-snooping all is executed

Removes all auto-configured multicast router ports for all VLAN.
When an operation-command clear igmp-snooping mrouter is executed

Deletes all multicast router ports that are automatically configured on the corresponding VLAN.
When IGMP snooping/MLD snooping programming is restarted using the operation command restart snooping

Removes all auto-configured multicast router ports for all VLAN.
When IGMP snooping is disabled in the configuration

Deletes all multicast router ports that are automatically configured on the corresponding VLAN.
When the configuration command ip igmp snooping mrouter discovery igmp is deleted

Deletes all multicast router ports that are automatically configured for IGMP monitoring on the relevant VLAN.
When the configuration command ip igmp snooping mrouter discovery pim is deleted

Deletes all multicast router ports that are automatically configured for PIM monitoring on the relevant VLAN.
When a port or channel group is removed from a VLAN

Deletes the multicast router ports automatically configured on the port or channel group from VLAN containing the removed port or channel group.
When a port is added to a channel group, the port is automatically set as a multicast router port.

Deletes the multicast router port that was automatically configured on the port from VLAN containing the port.
When a PIM-Hello messagewith a Holdtime of 0 seconds is received with PIM monitoring set automatically

Deletes the corresponding multicast router port for which PIM monitoring is set automatically.

It is not deleted in the following cases:

When the applicable port or channel group is linked down
When the applicable port or channel group becomes Blocking due to Spanning Tree or other reasons
When the corresponding VLAN is in Down or Disable status

(d) Output and Suppress Operation Messages

This function outputs operation messages related to multicast router detection. To suppress the outputting of operation messages, set no ip igmp snooping mrouter logging of the configuration command.

(3) Relaying IGMP Messaging

IGMP messages are received by multicast routers and receivers because they are sent and received between multicast routers and receivers. The Switch forwards IGMP messages as shown in the following table.

TABLE 32-5 Behavior for each IGMPv1/IGMPv2 message
IGMP message type	Transfer port within the VLAN	Remarks
Membership Query	Forwarded to all ports.
Version 2 Membership Report	Forwarded only to multicast router ports.
Leave Group	Not forwarded to any port when group members still exist for other ports. Forwarded to multicast router ports when no group members exist for other ports.	^#
Version 1 Membership Report	Forwarded only to multicast router ports.

#: Relay operation is performed when a querier is set for the local device and another device is a representative querier. If the local device is a representative querier, IGMPv2 Leave messaging is not forwarded. If no querier has been set, forwarding is always performed to multicast router ports. However, if a port that has not received an IGMPv1, IGMPv2, or IGMPv3 Report (membership request) message receives an IGMPv2 Leave message, the IGMPv2 Leave message is not forwarded regardless of the querier settings.

TABLE 32-6 Behavior for each IGMPv3 message
IGMPv3 message type		Transfer port within the VLAN	Remarks
Version3 Membership Query		Forwarded to all ports.
Version 3 Membership Report	Membership Request Report	Forwarded only to multicast router ports.
Version 3 Membership Report	Leave Request Report	Not forwarded to any port when group members still exist for other ports. Forwarded to multicast router ports when no group members exist for other ports.	^#

#: Relay operation is performed when a querier is set for the local device and another device is a representative querier. If the local device is a representative querier, IGMPv3 Report (detachment req) messageis not forwarded. If no querier has been set, forwarding is always performed to multicast router ports. However, if a port that has not received an IGMPv1, IGMPv2, or IGMPv3 Report (membership request) message receives an IGMPv3 Report (leave request) message, the IGMPv3 Report (leave request) message is not forwarded regardless of the querier settings.