IGMP for transit router
Managing multicast group membership manually — registering each source and member as your VPC resources change — is error-prone and slow to respond to network topology changes. Internet Group Management Protocol (IGMP) support on transit routers removes this overhead: hosts join and leave multicast groups dynamically, and transit routers track membership in real time and adjust forwarding automatically.
Key concepts
| Concept | Description |
|---|---|
| Multicast domain | A logical boundary that segments a multicast network. |
| Multicast group | A set of hosts that send and receive the same multicast traffic, identified by a group IP address. |
| Multicast source | A host that sends multicast traffic to a group. In static mode, sources are manually registered. In IGMP mode, any host that joins via IGMP becomes both a source and a member. |
| Multicast member | A host that receives multicast traffic. Members can be registered statically or added dynamically when IGMP is enabled for a multicast domain. |
Use cases
IGMP supports multicast networks for VPCs in the same region. For inter-region multicast networks, statically add inter-region resources to the 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
How it works
Transit routers manage multicast membership through a JOIN/QUERY/LEAVE cycle.
Joining a group
A host sends IGMP JOIN messages to join a multicast group, typically with 2 to 3 retries. If all JOIN messages are lost — which is unlikely — the host does not join the group. Re-initiate the JOIN request using a host-specific method. Once a host joins via IGMP, it becomes both a multicast source and a member of the transit router.
Membership tracking
After the transit router receives the first IGMPv2 JOIN message from a host, it begins tracking that host. Every two minutes, the transit router sends an IGMPv2 QUERY message to all members. Each member must reply with a JOIN message to maintain its membership. If a member fails to respond to three consecutive queries, the transit router removes it from the group.
Leaving a group
When a host sends an IGMPv2 LEAVE message, the transit router immediately removes it from multicast processing and stops tracking it.
Limitations
IGMP version and multicast model
Transit routers use IGMPv2 and the Any-Source Multicast (ASM) model. Source-Specific Multicast (SSM) is not supported. Avoid configuring source filters on host applications.
Transit routers can process IGMPv3 JOIN and LEAVE messages. Upon receiving an IGMPv3 JOIN message, the transit router sends an IGMPv2 QUERY, which prompts the host to fall back to IGMPv2.
IGMP mode and static mode behavior
The behavior of a multicast domain depends on whether IGMP is enabled.
| Attribute | IGMP disabled (default) | IGMP enabled |
|---|---|---|
| How members join | Statically — manual registration only | Statically or dynamically via IGMP |
| How sources are registered | Statically — manual registration only | Statically or dynamically via IGMP |
Mode interaction rules:
If a host is manually registered as a member and later joins via IGMP, the IGMP mode does not override the static registration.
If a host joins via IGMP, it cannot be manually registered as a multicast source or member afterward.
Security group and access control requirements
Configure security group rules or access control policies to permit IGMP protocol messages for IGMP hosts.
IGMP cannot be disabled
Once IGMP is enabled for a multicast domain, it cannot be disabled.