31.5 Notes on using IGMP snooping/MLD snooping

<Structure of this section>

(1) Coexistence with other functions

See "22.3 Coexistence of Layer 2 switch function and other functions".

(2) Control packet flooding

Because multicast traffic that is subject to suppression by IGMP snooping or MLD snooping is data traffic, flooding needs to be performed within a VLAN so that the routing protocol and other control packets can be received by all routers and all hosts. Therefore, the Switch forwards packets with destination IP addresses contained in the address ranges shown in the table below to all ports on the VLAN. Packets with destination IP addresses outside the address ranges shown in the following table are forwarded according to learning results for IGMP snooping or MLD snooping.

Table 31-7: Flooding of control packets
Protocol	Address range
IGMP snooping	224.0.0.0/24
MLD snooping	ff02::/16

Note that multicast group addresses that duplicate multicast MAC addresses for control packets cannot be used. The following table describes multicast group addresses that cannot be used for addresses outside the address ranges shown in the above table.

Table 31-8 Multicast group addresses that cannot be used with MAC addressing control method
Protocol	Multicast group address
IGMP snooping	224.128.0.0/24
	225.0.0.0/24
	225.128.0.0/24
	226.0.0.0/24
	226.128.0.0/24
	227.0.0.0/24
	227.128.0.0/24
	228.0.0.0/24
	228.128.0.0/24
	229.0.0.0/24
	229.128.0.0/24
	230.0.0.0/24
	230.128.0.0/24
	231.0.0.0/24
	231.128.0.0/24
	232.0.0.0/24
	232.128.0.0/24
	233.0.0.0/24
	233.128.0.0/24
	234.0.0.0/24
	234.128.0.0/24
	235.0.0.0/24
	235.128.0.0/24
	236.0.0.0/24
	236.128.0.0/24
	237.0.0.0/24
	237.128.0.0/24
	238.0.0.0/24
	238.128.0.0/24
	239.0.0.0/24
	239.128.0.0/24

When addresses shown in the above table are used for multicast group addresses, multicast data bound for corresponding multicast group addresses will be forwarded to all ports in the VLAN.

When setting a trunk port, make sure that it does not receive any untagged control packets. Set a native VLAN in the configuration if untagged control packets are to be handled by the trunk port.

(3) Configuring Multicast Router Ports

(a) In a redundant configuration

When Spanning Tree Protocols are used for a redundant configuration and the connection with the router might change due to topology changes by a Spanning Tree Protocol, a multicast router port must be set for all ports that might connect with the router.

(b) Connection between Layer 2 switches

On VLANs that contain only multiple Layer 2 switches, a multicast router port must be set for ports connecting to Layer 2 switches handling multicast traffic transmission hosts.

When a redundant configuration is used, a multicast router port must be set for all ports that might connect to Layer 2 switches handling transmission hosts.

(4) Connecting to IGMP Version 3 Hosts

One of the following needs to be performed when the Switch is connected to an IGMPv3 host:

Use IPv4 multicast for the corresponding VLAN and set the IGMP version to 3.
Set an IP address so that the corresponding router connected to the IGMPv3 router becomes the representative querier.

Use a configuration in which IGMPv3 messages from IGMPv3 hosts are not split into fragments.

(5) Connecting with MLD Version 2 Hosts

One of the following needs to be performed when the Switch is connected to an MLDv2 host:

Use IPv6 multicast for the corresponding VLAN and set the MLD version to 2.
Set an IP address so that the corresponding router connected to the MLDv2 router becomes the representative querier.

Use a configuration in which IGMPv2 messages from IGMPv2 hosts are not split into fragments.

(6) Re-learning entries by executing an operation command

In addition to the operation commands for IGMP and MLD snooping, if the commands below are executed, any learned entries are cleared and then relearned. After these operation commands are executed, multicast communication will be cut off temporarily.

The running-config command is overwritten by the copy command
Executing the restart vlan command

(7) Using IPv4 multicast functionality at the same time

(a) Setting the configuration command swrt_multicast_table

To use IPv4 multicast function and IGMP snooping at the same time, set the configuration command swrt_multicast_table and use IPv4 multicast for the applicable VLAN.

(b) Temporary communication stop when IGMP snooping setting is added

When an IGMP snooping setting is added to a VLAN using IPv4 multicast, multicast communication stops temporarily. After the IGMP snooping setting is configured, multicast communication is restarted when an IGMP Report (membership request) is received.

(c) Using with the Static Group Join Function

For VLANs using the static group participation functionality for IPv4 multicast, some IGMP Reports (membership requests) from hosts might not be sent. When this functionality is used with IGMP snooping and IGMP Reports (membership requests) are not sent, set a multicast router port for ports needed for multicast communication on VLANs using static group participation functionality because multicast communication cannot be performed.

(d) IPv4 multicast packet flooding

The following IPv4 multicast packets are forwarded to all ports in the received VLAN in a VLAN that uses IPv4 multicast and IGMP snooping simultaneously.

IPv4 multicast packets received before IPv4 multicast registers multicast forwarding entries (including negative caches)
IPv4 multicast packet with a TTL of 1

(e) Flooding of IPv4 multicast packets received on non-upstream interfaces

When IPv4 multicast and IGMP snooping are used simultaneously for multicast forwarding, if a IPv4 multicast packet is received on a VLAN other than the upstream interface of the registered multicast forwarding entry, the corresponding IPv4 multicast packet is forwarded to all ports in the received VLAN.

(8) Using IPv6 multicast functionality at the same time

(a) Setting the configuration command swrt_multicast_table

To use IPv6 multicast function and MLD snooping at the same time, set the configuration command swrt_multicast_table and use IPv6 multicast for the applicable VLAN.

(b) Temporary communication stop when MLD snooping setting is added

When an MLD snooping setting is added to a VLAN using IPv6 multicast, multicast communication stops temporarily. After the MLD snooping setting is configured, multicast communication is restarted when an MLD Report (membership request) is received.

(c) Using with the Static Group Join Function

For VLANs using the static group participation functionality for IPv6 multicast, some MLD Reports (membership requests) from hosts might not be sent. When this functionality is used with MLD snooping and MLD Reports (membership requests) are not sent, set a multicast router port for ports needed for multicast communication on VLANs using static group participation functionality because multicast communication cannot be performed.

(d) IPv6 multicast packet flooding

The following IPv6 multicast packets are forwarded to all ports in the received VLAN in a VLAN that uses IPv6 multicast and MLD snooping simultaneously.

IPv6 multicast packets received before IPv6 multicast registers multicast forwarding entries (including negative caches)
IPv6 multicast packets with a hop limit of 1

(e) Flooding of IPv6 multicast packets received on non-upstream interfaces

When IPv6 multicast and MLD snooping are used simultaneously for multicast forwarding, if a IPv6 multicast packet is received on a VLAN other than the upstream interface of the registered multicast forwarding entry, the corresponding IPv6 multicast packet is forwarded to all ports in the received VLAN.

(9) IGMP Fast-Leave Function

When IGMP instant leave is used and an IGMPv2 Leave or IGMPv3 Report (leave request) message is received, multicast communication to the corresponding port stops immediately. Therefore, when this functionality is used, we recommend that you place only one recipient terminal for each multicast group on the connection port.

When multiple recipient terminals in the same multicast group are placed on a connection port, multicast communication to other recipients stops temporarily. In this case, multicast communication is restarted when an IGMP Report (membership request) message is received from the recipient.

(10) Using IGMP snooping in Stacking

If your stack uses IGMP snooping, you must use IP addressing control method. Therefore, set the configuration command swrt_multicast_table and use IPv4 multicast for the applicable VLAN.