IGMP SNOOPING
- General query The
IGMP querier periodically sends IGMP general query messages to all hosts and routers (224.0.0.1) in the local network segment to query which multicast group members are on the network segment.
When receiving an IGMP general query message, the switch forwards it through all ports in the VLAN except the receiving port, and processes the receiving port of the message as follows:
If the dynamic is included in the router port list Router port, 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. -
The
host that reports the membership will send an IGMP membership report message to the IGMP querier:
when a member host of the multicast group receives an IGMP query message, it will reply with an IGMP membership report message.
If a host wants to join a multicast group, it will actively send an IGMP membership report message to the IGMP querier to declare to join the multicast group.
When receiving an IGMP membership report message, the switch forwards it through all router ports in the VLAN, resolves the multicast group address that the host wants to join from the message, and does the following on the receiving port of the message Processing:
If there is no forwarding entry corresponding to the 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 multicast group already exists If the corresponding forwarding table entry, 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 there is already a corresponding multicast group Forward the entry, and the dynamic member port is already included in the outgoing port list, reset its aging timer.
The switch will not forward IGMP membership report messages through non-router ports, because according to the IGMP membership report suppression mechanism on the host, if there are member hosts of the multicast group under the non-router port, these hosts will receive The report message suppresses its own report, so that the switch cannot know that there are member hosts of the multicast group under these ports. (IGMP SNOOPING turns off the last report mechanism by default) - Leaving a multicast group
A host running IGMPv1 will not send an IGMP leave group message when leaving a multicast group, so the switch cannot immediately learn the host leaving information. However, since the host will no longer send IGMP membership report messages after leaving the multicast group, when the aging timer of its corresponding dynamic member port expires, the switch will remove the forwarding table entry corresponding to the port from the forwarding table. delete.
When a host running IGMPv2 or IGMPv3 leaves a multicast group, it will send an IGMP leave group message to notify the multicast router that it has left a multicast group. When the switch receives an IGMP leave group message from a dynamic member port, it first judges whether the forwarding entry corresponding to the multicast group to leave exists, and whether the forwarding entry corresponding to the multicast group is in the outgoing port list Include the receiving port:
If there is no forwarding entry corresponding to the multicast group, or the port is not included in the outgoing port list of the forwarding entry corresponding to the multicast group, the switch will not forward the packet to any port, but will It is directly discarded;
if there is a forwarding entry corresponding to the multicast group and the port is included in the outgoing port list of the forwarding entry corresponding to the multicast group, the switch will forward the packet through all router ports in the VLAN. At the same time, because it does not know whether there are other members of the multicast group under the receiving port, the switch will not immediately delete the port from the outgoing port list of the forwarding entry corresponding to the multicast group, but reset it. Its aging timer.
When the IGMP querier receives the IGMP leave group message, it resolves the address of the multicast group that the host wants to leave from it, and sends an IGMP specific group query message to the multicast group through the receiving port. After the switch receives the IGMP specific group query message, it forwards it through all router ports in the VLAN and all member ports of the multicast group. For the receiving port of the IGMP leave group message (assumed to be a dynamic member port), the switch is within its aging time:
if it receives an IGMP membership report message from the host in response to the specific group query from this port, it means that the port is still There are members of the multicast group, so its aging timer is reset;
If the host does not receive an IGMP membership report message from the port in response to a specific group query, it means that there are no members of the multicast group under the port. After the aging time expires, it will be removed from the multicast group. Delete from the outgoing port list corresponding to the forwarding entry.
IGMP SNOOPING PROXY
After the IGMP Snooping function is configured, the switch forwards the Query messages of the upstream IGMP querier and the Report and Leave messages of the downstream host intact. When there are a large number of user hosts in the network, redundant IGMP messages bring processing pressure to upstream devices. After configuring the IGMP Snooping Proxy function, the switch can replace the upstream Layer 3 device to send IGMP Query messages to the downstream, and can also replace the downstream host to send IGMP Report and IGMP Leave messages to the upstream Layer 3 device, effectively saving the upstream Layer 3 device and this device. The bandwidth between. The device configured with the proxy function only sends a Report message upstream when a member of the multicast group joins and needs to establish a multicast entry or responds to an IGMP query message; or the last member of the multicast group has left and the multicast entry needs to be deleted Send a Leave message upstream.
When IGMP Snooping Proxy is configured on the Switch, the Switch can terminate upstream IGMP query messages, and construct query messages to send to downstream hosts; terminate IGMP report/leave messages from downstream hosts, and construct unified report/leave messages by itself The text is sent upstream.
When IGMP is not enabled on the Layer 3 device, for example, only a static multicast group is configured, and no querier will send Query messages. In this way, even if the device is enabled with IGMP Snooping, the group membership cannot be established and maintained. With the IGMP Snooping Proxy function, the switch can send Query messages, which is a querier relative to the downstream host.
The difference between igmp proxy and igmp snooping mode is that the proxy acts as a "proxy", that is, it acts as a multicast router for lower-layer hosts, it acts as a host for upper-layer multicast routers, and sends igmp report to lower-layer users. Instead of transparently transmitting the message to the upper-layer network, it uses the identity of the host to apply for a multicast program to the upper-layer multicast router. It also intercepts the query message sent by the upper-layer multicast router. Send query messages to the host as a multicast router; while snooping is only a snooping, listening to the igmp messages sent by the lower-layer host and the upper-layer multicast router, and transparently transmitting at the same time. Of course, whether it is igmp proxy or igmp snooping, the device establishes forwarding entries according to the igmp report message sent by the lower-layer host, so as to realize controllable multicast.
IGMP Snooping and IGMP Snooping Proxy have the same functions but different principles. IGMP Snooping only obtains relevant user information by listening to IGMP messages, while IGMP Snooping Proxy intercepts and suppresses the IGMP request of the end user and performs its own related processing before forwarding it to the upper router.
Compared with devices that only implement IGMP Snooping, the Layer 2 network device that implements IGMP Snooping Proxy assumes the role of Server on the user side, and periodically queries user information; on the network side, it assumes the role of Client. When a query is received Report the current user information. Therefore, compared with IGMP Snooping, it can not only achieve the purpose of suppressing the flooding of Layer 2 network multicast, but also effectively reduce the protocol messages in the network and reduce the network load. However, since it needs to maintain and calculate the status change of the table entry and also needs the function of the proxy router, the implementation complexity and processing overhead of the device are correspondingly increased.
Self-thinking:
igmp proxy establishes multicast routing table entries on the device to guide multicast traffic forwarding, and the device needs to enable the multicast routing table. , It is a multicast receiver for the upstream router, and a multicast queryer for the downstream pc, and will not transmit packets transparently. Not only can it achieve the purpose of suppressing the flooding of the Layer 2 network multicast, but also can effectively reduce the protocol messages in the network and reduce the network load, but it will increase its own load.
The igmp snooping proxy is listening and intercepting. It establishes the Layer 2 multicast forwarding table entry. The difference is that the igmp snooping proxy switch receives the corresponding (*, G) leave message if there is a corresponding Entry, and the interface is also a member interface of the entry, then