As a part of the Internet protocol suite TCP/IP, Internet Group Management Protocol (IGMP) is responsible for membership management of IPv4 multicast groups. By allowing hosts to dynamically join and leave a multicast group, IGMP streamlines network deployment and management. Transit routers now support IGMP. When creating a multicast network using transit routers, you no longer need to manually specify multicast sources and members. Transit routers can dynamically adjust these components based on IGMP, which enables quick addition to or removal of resources from virtual private clouds (VPCs). This helps enhance management and deployment efficiency.
The IGMP mode is currently in beta testing and is exclusively available to users on the whitelist.
Scenarios
IGMP supports the creation of multicast networks for VPCs in the same region. For inter-region multicast networks, you must statically add inter-region resources to a group.
Create a multicast network for same-region VPCs in IGMP mode
Create a multicast network for inter-region VPCs in IGMP and static mode
Limits
Transit routers use the Any-Source Multicast model and do not support the Source-Specific Multicast model. We do not recommend that you configure any source filters on host applications.
Transit routers use IGMPv2 and are capable of processing IGMPv3 JOIN and LEAVE messages.
Upon receiving an IGMPv3 JOIN message from a host, a transit router issues an IGMPv2 QUERY message, which prompts the host to revert to IGMPv2 behavior.
To create a multicast network in the IGMP mode, the IGMP feature must be enabled for the multicast domain:
For a multicast domain without IGMP enabled, you can only statically create multicast sources and members.
For a multicast domain with IGMP enabled, you can create multicast sources and members in both static and IGMP modes.
If you manually set a host as a multicast member and the host later joins the group through IGMP, the IGMP mode does not override the state mode. Conversely, if a host joins through IGMP, it cannot be manually set as a multicast source or member thereafter.
Once enabled for a multicast domain, the IGMP feature cannot be disabled.
How IGMP works
Hosts send IGMP JOIN messages to join a multicast group, typically with 2 to 3 retries. If these messages are lost, which is unlikely to happen, a host does not join the group. You need to use a host-specific method to re-initiate the JOIN request.
Transit routers monitor hosts from the receipt of the first IGMPv2 JOIN message and stop tracking upon receiving an IGMPv2 LEAVE message. Transit routers query all multicast members every two minutes with an IGMPv2 QUERY message. Members must reply to a JOIN message to keep their status. Failure to respond three consecutive times results in removal from the group. When a router receives an IGMPv2 Leave message from a member, it immediately removes the member from further multicast processing.
Transit routers send query packets to all IGMP members to track membership. IGMP protocol messages are permitted if security group rules or access control policies are configured for IGMP hosts.
Upon joining a group through IGMP, hosts become both multicast sources and members of the transit router.