Foundation Summary

The Foundation Summary provides a convenient review of many key concepts in this chapter. If you are already comfortable with the topics in this chapter, this summary might help you recall a few details. If you just read this chapter, this review should help solidify some key facts. If you are doing your final prep before the exam, the following lists and tables are a convenient way to review the day before the exam.

Hosts use IGMP to communicate to a router to join a multicast group. The following are key points about IGMPv1:

• The host sends a Membership Report that includes group address to join group.

• Users lack the ability to leave a group dynamically.

• The querier router sends a query to 224.0.0.1 every 60 seconds to check for members.

• Only one host needs to respond with a membership report to maintain the forwarding.

• Hosts can join multicast groups at any time.

• Querier election is handled by the routing protocol.

• Routers age out a multicast group if no membership reports are received for three consecutive query intervals.

The following are key points about IGMPv2:

• IGMPv2 is backward compatible.

• The version is changed with the command

  Router(config-if)# ip igmp version {1 | 2 | 3}

• Queries can be sent as general queries to the all-hosts address (like v1) or group-specific queries.

• Hosts can leave a group dynamically using a Leave Group message. The querier responds with a group-specific query asking if any other members remain.

• Querier election occurs—all routers start as queriers and transition to non-queriers if they hear another querier with a higher IP address.

• It has a query-interval response time—queries include a field to tell members how long they have to respond.

• Current membership records can be displayed with the command

  Router# show ip igmp group

• The last reporter field from show ip igmp group is significant for leave operations.

The following are key components of IGMPv3:

• Added support for multicast source filtering

• Backward-compatibility with v2 and v1

• The following command to see IGMP parameters (including version):

  show ip igmp interface

Switch support for multicasting is important. The following are key points:

• Switches associate MAC addresses with ports using the source-addresses of sent traffic.

• Frames to unknown MAC addresses are flooded out all ports.

• Multicast addresses are only used as the destination address. Switches flood multicasts.

The three ways to forward multicast specifically are

• Static MAC table entries— Labor intensive and not scalable.

• CGMP— Cisco proprietary. Routers receive IGMP and pass CGMP to the switch with the MAC of the requestor and the multicast group requested.

• IGMP snooping— Enables switch to recognize IGMP and act on it.