HP IPv6 6200yl, IPv6 2900, IPv6 5400zl, IPv6 3500yl manual Multicast Listener Discovery MLD Snooping

Multicast Listener Discovery (MLD) Snooping

Introduction to MLD Snooping

A network node establishes itself as an MLD host by issuing a multicast “join” request (also called a multicast “report”) for a specific multicast address when it starts an application that listens to multicast traffic. The switch to which the node is connected sees the join request and forwards traffic for that multicast address to the node’s port.

Queries. The querier is a multicast router or a switch that periodically asks MLD hosts on the network to verify their multicast join requests. There is one querier for each VLAN, and all switches on the VLAN listen to the responses of MLD hosts to multicast queries, and forward or block multicast traffic accordingly.

All of the ProCurve switches described by this guide have the querier function enabled by default. If there is another device on the VLAN that is already acting as querier, the switch defers to that querier. If there is no device acting as querier, the switch enters an election state and negotiates with other devices on the network (if any) to determine which one will act as the querier.

The querier periodically sends general queries to MLD hosts on each multicast address that is active on the VLAN. The time period that the querier waits between sending general queries is known as the query interval; the MLD standard sets the default query interval to 125 seconds.

Network nodes that wish to remain active as MLD hosts respond to the queries with join requests; in this way they continue to assert their presence as MLD hosts. The switch through which any given MLD host connects to the VLAN sees the join requests and continues forwarding traffic for that multicast address to the MLD host’s port.

Leaves. A node acting as an MLD host can be disconnected from a multicast address in two ways:

■It can stop sending join requests to the querier. This might happen if the multicast application quits or the node is removed from the network. If the switch goes for slightly more than two query intervals without seeing a join request from the MLD host, it stops sending multicast traffic for that multicast address to the MLD host’s port.

■It can issue a “leave” request. This is done by the application software running on the MLD host. If the MLD host is the only node connected to its switch port, the switch sees the leave request and stops sending multicast packets for that multicast address to that port. (If there is more than one node connected to the port the situation is somewhat more complicated, as explained below under “Fast leaves and forced fast leaves”.)

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