Internet Group Management Protocol enabling hosts to join and leave multicast groups
Internet Group Management Protocol (IGMP)
- IGMP enables hosts to join and leave multicast groups - tells routers which multicast traffic to forward on specific network segments
- Operates at Layer 3 between hosts and their directly connected multicast routers
- Uses multicast addresses in range 224.0.0.0 to 239.255.255.255 (Class D addressing)
- Only works on local network segment - PIM handles routing between subnets
IGMP Versions and Operations
| Version |
Key Features |
Query Timer |
Leave Process |
| IGMPv1 |
Basic join/leave |
60-90 seconds |
Host stops responding |
| IGMPv2 |
Explicit leave messages |
125 seconds |
Leave Group message |
| IGMPv3 |
Source filtering (SSM) |
125 seconds |
Advanced leave |
Message Types
- Membership Query - Router asks “who wants multicast traffic?”
- Membership Report - Host says “I want this group’s traffic”
- Leave Group - Host says “I’m done with this group”
IGMP Process Flow
- General Query: Router periodically asks all hosts what groups they want
- Join Process: Host sends unsolicited Membership Report when application requests multicast
- Report Suppression: Hosts use random timer before responding to avoid flooding
- Leave Process: IGMPv2+ hosts send Leave Group message
Notes
- IGMP snooping on switches prevents multicast flooding
- IGMP operates only between host and first-hop router
- IGMPv1 has no leave mechanism - routers must wait for membership timeout