Multicast - Internet Group Management Protocol

6 important questions on Multicast - Internet Group Management Protocol

What versions of IGMP are there?

- IGMPv1 (Obsolete)
- IGMPv2 (Commonly used)
- IGMPv3 (Used by SSM)

IGMPv2 messages are encapsulated in an IP packet and send to the local router.
What is the IP protocol number used and what is special about the TTL?

The IP Protocol number used is 2
The TTL is set to 1, this way it is only processed by the local router and never forwarded by any router.

What 2 modes does IGMPv3 use when sending a membership report to join a multicast group?

1. Include mode. Includes a list with sources it wants to receive traffic from.
2. Exclude mode. Includes a list with sources it does NOT want to receive traffic from.
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What is IGMP snooping?

It is used by switches. They monitor the traffic for IGMP Joins and keep a table of interfaces and specific IGMP multicast MAC addresses. This way it doesn't always have to flood all multicast traffic but it can send packets only out interfaces a receiver for the specific Multicast group is located.

Which 2 methods can a Cisco switch use to reduce multicast flooding on LAN segments?

1. IGMP Snooping
2. Static MAC address entries (not scalable)

What 2 mechanisms does IGMP snooping use to optimize forwarding of Multicast traffic?

1. Listens for PIM or IGMP messages to identify the router.

2. Listens for IGMP membership reports messages from host to identify ports with interested receivers.

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