IGMP snooping is a Layer 2 switch feature that listens to IGMP messages so that multicast is only sent to ports with interested hosts – instead of being flooded like broadcast.
Facts / key topics
- Standard: not an IETF protocol – a switch feature · depends on IGMP (IPv4) · OSI: Layer 2
- Default behaviour without snooping: flooding
- IPv6 pendant: MLD snooping
- CCNA: the concept and the problem · CCNP: querier, fast leave, design faults
The problem
Multicast frames use a multicast MAC; switches understand only MAC, so without snooping multicast floods to all ports: unnecessary CPU load on hosts, wasted bandwidth, multicast behaving like broadcast. Typical scenarios: IPTV, video conferencing, financial feeds, routing protocols in the access network.
How it solves it
- The switch sniffs IGMP joins/leaves
- It builds a multicast forwarding table
- It forwards multicast only to ports with active members
👉 No routing, no PIM – pure L2 intelligence.
Step by step
- IGMP join: host sends IGMP Membership Report; the switch “hears” it
- L2 multicast table: multicast MAC → specific ports
- Normal operation: frames only to member ports; flooding stops
- Leave/timeout: IGMP Leave → port removed (CCNP: fast leave / robustness)
Key terminology
| Term | Explanation |
|---|---|
| IGMP snooping | Switch listens to IGMP |
| Membership report | IGMP join |
| Leave group | IGMP leave |
| Querier | Device sending IGMP queries |
| Multicast table | L2 mapping group → ports |
Elements
| Element | Description | CCNA | CCNP |
|---|---|---|---|
| IGMP snooping | The base function | ✔ | ✔ |
| Multicast flooding | Default without snooping | ✔ | ✔ |
| IGMP querier | Keeps groups active | ✖ | ✔ |
| Fast leave | Fast port removal | ✖ | ✔ |
| Static group | Manual binding | ✖ | ✔ |
Dependencies & considerations
- Requires IGMP traffic in the VLAN
- If no querier exists → groups expire
- The router is normally the querier; an L2 switch can be querier itself (CCNP)
- VLAN-specific feature
Configuration
! Basic (CCNA)
ip igmp snooping
! Advanced (CCNP)
ip igmp snooping vlan 10
ip igmp snooping querier
ip igmp snooping fast-leave
! Verification
show ip igmp snooping
show ip igmp snooping groups
show mac address-table multicastTroubleshooting
- Symptom: multicast still floods → snooping disabled → enable it
- Symptom: multicast stops after a while → no querier → enable querier
Design & best practices
- Enable IGMP snooping on access switches · one querier per VLAN · fast leave on access ports · avoid blind flooding
Exam tips ⚠️
- IGMP snooping = Layer 2
- The switch listens – it doesn’t send IGMP
- Flooding → think “no snooping”
- IPv6? → same idea, but MLD
Quick summary
- IGMP snooping prevents multicast flooding · the switch learns groups by listening · no routing involved · CCNA: understand the idea · CCNP: design + edge cases