Summary

There are three ways of disseminating information: unicasts, broadcasts, and multicasts. With a unicast, a packet is sent to each destination individually. With a broadcast, a single packet is generated and every destination receives it. With a multicast, a single packet is sent to a group of devices. Class D addresses are used in IP for multicasting: 224.0.0.0 239.255.255.255. These addresses are broken into groups, such as reserved link local, globally scoped, source specific, GLOP, and limited score addresses. 224.0.0.1 is the all-host group and has the IP TTL set to 1. 224.0.0.2 is the all-router group for a segment.

IGMP is used by clients and RPs to determine which clients want to see specific multicast feeds. There are three versions of IGMP: v1, v2, and v3. v2 supports an active querier, which forwards multicasts to a segment and is elected based on the lowest IP address. v2 also supports client leave messages. v3 supports source filtering, allowing the client to specify a list of sources that it wants to receive a multicast feed from. Cisco has created a v3lite version of IGMP, which provides a transition for those clients whose operating system doesn't support v3.

Multicast routing protocols are used to route multicast packets between a source and end stations. A shared distribution tree has a rendezvous point that's used as the central point of dissemination of multicast traffic: A single-tree structure is used for all multicast traffic. Source-based distribution trees have a separate tree structure for each multicast application, which allows for optimal paths but causes more overhead on the RPs.

Cisco uses PIM to route multicast traffic. PIM supports both sparse and dense modes. Dense mode is typically used in LAN environments where most stations need to see the multicast traffic: Multicasts are flooded and the distribution tree is pruned back based on IGMP messages (not) received from end stations. Sparse mode is used when not many clients are participating in multicasts or they're geographically dispersed. The distribution tree starts out empty and is built based on IGMP messages received from clients.

To enable multicasting, use the ip multicast-routing command. To enable PIM on an interface, use the ip pim dense-mode|sparse-mode|sparse-dense-mode command. To examine the multicast routing table, use the show ip mroute command.