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Detailed explanation of MLDsnooping technology for IPv6 multicast

Detailed explanation of MLDsnooping technology for IPv6 multicast

MLDsnooping is the abbreviation for Multicast Listener Discovery Snooping. It is an IPv6 multicast restriction mechanism running on Layer 2 devices, used to manage and control IPv6 multicast groups.

The Layer 2 device running MLD Snooping analyzes the received MLD messages, establishes a mapping relationship between ports and MAC multicast addresses, and forwards IPv6 multicast data based on this mapping relationship. When the Layer 2 device is not running MLD Snooping, IPv6 multicast data packets are broadcast at Layer 2. When the Layer 2 device is running MLD Snooping, it is known that multicast data packets of IPv6 multicast groups will not be broadcast at Layer 2. Broadcast, and be multicast to designated receivers at Layer 2. 

MLDsnooping uses Layer 2 multicast to forward information only to receivers in need, which can bring the following benefits:

1. Reduce the broadcast message in the second layer network and save the network bandwidth;

2. Enhance the security of IPv6 multicast information;

3. It is convenient to realize the separate billing for each host.

The specific processing methods for different MLD actions by switches running MLD Snooping are as follows:

1. General group query

The MLD querier periodically sends MLD general query messages to all hosts and routers (FF02::1) in the local network segment to query which IPv6 multicast group members are on the network segment. When receiving an MLD general query message, the switch forwards it through all ports in the VLAN except the receiving port, and performs the following processing on the receiving port of the message:

If the dynamic router port is already included in the router port list, reset its aging timer. If the dynamic router port is not yet included in the router port list, add it to the router port list and start its aging timer.

2. Report membership

When an IPv6 multicast group member host receives an MLD query message, it will reply with an MLD membership report message. If a host wants to join an IPv6 multicast group, it will actively send an MLD membership report message to the MLD querier to declare to join the IPv6 multicast group. When receiving an MLD membership report message, the switch forwards it through all router ports in the VLAN, parses the IPv6 multicast group address that the host wants to join from the message, and performs a command on the receiving port of the message. Treat as follows:

If there is no forwarding entry corresponding to the IPv6 multicast group, create a forwarding entry, add the port as a dynamic member port to the outgoing port list, and start its aging timer;

If the forwarding entry corresponding to the IPv6 multicast group already exists, but the port is not included in the outgoing port list, the port is added to the outgoing port list as a dynamic member port, and its aging timer is started;

If the forwarding entry corresponding to the IPv6 multicast group already exists and the dynamic member port is already included in the outgoing port list, the aging timer is reset.

3. Leave the multicast group

When a host leaves an IPv6 multicast group, it will send an MLD leave group message to notify the multicast router that it has left an IPv6 multicast group. When the switch receives an MLD leave group message from a dynamic member port, it first determines whether the forwarding table entry corresponding to the IPv6 multicast group to leave exists, and the outgoing port list of the forwarding table entry corresponding to the IPv6 multicast group Whether the receiving port is included in.

4. MLD SnoopingProxying

By configuring the MLD Snooping Proxying (MLD Snooping proxy) function on the edge device, the number of MLD report and leave messages received by the upstream device can be reduced, and the overall performance of the upstream device can be effectively improved. A device configured with MLD Snooping Proxying function (called MLD Snooping proxy device) is equivalent to a host in the view of its upstream device, and equivalent to a querier from its downstream host.

Although the MLD Snooping proxy device is equivalent to a host from its upstream device, the MLD membership report suppression mechanism on the host will not take effect on the MLD Snooping proxy device.

How the MLD snooping agent device processes MLD messages:

1. General group query message: After receiving the general group query message, it is forwarded to all ports in the VLAN except the receiving port; at the same time, a report message is generated according to the locally maintained group membership and sent to all router ports.

2. MLD last listener query message/MLD specific source group query message: If there are member ports in the forwarding entry corresponding to the group, the report message of the group will be returned to all router ports.

3. MLD report message:

1) If there is no forwarding entry corresponding to the group, create a forwarding entry, add the receiving interface as a dynamic member port to the outgoing interface list, start its aging timer, and then send the report of the group to all router ports Message

2) If the forwarding entry corresponding to the group already exists and the dynamic member port is included in the outgoing interface list, reset its aging timer;

3) If the forwarding entry corresponding to the group already exists, but the receiving interface is not included in the outgoing interface list, the interface is added to the outgoing interface list as a dynamic member port, and its aging timer is started.

4. MLD leave message: Send a group-specific query message for the group to the receiving interface. Only when the last member port in the forwarding entry corresponding to a multicast group is deleted, the leave message of the group will be sent to all router ports.

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