Internet Group Management Protocol

Receivers use IGMP to inform last-hop routers of the groups they would like to become members of. The last-hop multicast routers use the IGMP messages to track group memberships on each of its directly connected LAN segments. The router forwards traffic from the requested group to the LAN segment that contains members of the group. If there are any intermediary switches between the client workstation and router, you can configure Cisco Group Multicast Protocol (CGMP) or IGMP snooping to avoid duplicating the multicast group across Layer 2 networks. There are currently three versions of IGMP: IGMPv1, IGMPv2, and IGMPv3.

Internet Group Management Protocol, Version 1

With IGMPv1, there are only two messages: IGMP Membership Queries and IGMP Membership Reports. Hosts can join multicast groups, using Membership Reports, but cannot explicitly leave the groups that they are members of. To determine if there are any receivers available for a group, routers send IGMP Membership Queries for the group in question to the all-hosts group 224.0.0.1 on its local LAN segments every 60 seconds. If the router does not receive an IGMP Membership Report for three consecutive queries (3 minutes), it stops forwarding traffic to that local LAN segment.

In Figure 5-4, the router first sends a general query to the all-hosts group. When the hosts receive the query from the router, they each start a local countdown timer. The countdown timer is a random time value between the range of 0 and 10 seconds for IGMPv1. When its countdown timer expires, the host sends a membership report to the all-hosts group with a Time To Live (TTL) of 1. If a host receives a report for the desired group within its countdown timer, it suppresses its own report because the router requires only one report per LAN segment.

Figure 5-4. IGMPv1 Operation

Note

Hosts set the TTL in the IP header to 1 to contain the IGMP report to the local LAN segment. Recall that, when a router receives a packet with a TTL of 1, it processes and then discards the packet.

In Figure 5-4, Client B is the first to send a report, and the others therefore suppress their respective reports. The router sends periodic queries to the all-hosts group to ensure that there are still receivers on the LAN, and the process described previously is repeated.

To enable IGMPv1 on last-hop routers, use the interface configuration command ip igmp version 1.

Internet Group Management Protocol, Version 2

IGMPv2, adds Leave messages for receivers to terminate group membership in a timely fashion. The timeout-based group membership termination method described previously for IGMPv1 proves inefficient for groups whose membership is volatile. High-bandwidth groups also suffer from this method because bandwidth continues streaming to LANs with no receivers until the timeout period expires.

With IGMPv2, when a host wants to leave a group, it sends an IGMP Leave message to the all-routers group 224.0.0.2. When the router receives the Leave message, it sends a group-specific query to make sure that there are no other members of the group on the LAN before terminating the stream. You can configure the number of queries the router sends before terminating the stream by using the interface configuration command ip igmp last-member-query-count. You can also configure the time between group-specific queries using the interface configuration command ip igmp last-member-query-interval.

In Figure 5-5, Client B sends an IGMPv2 Leave message, but hosts A and C still require the multicast traffic group for group 224.2.2.2. Therefore, they send group membership reports in response to the router's query.

Figure 5-5. IGMPv2 Operation

Note

IGMPv2 is backward compatible to IGMPv1.

To enable IGMPv2 on the last-hop router, use the interface configuration command ip igmp version 2.

Internet Group Management Protocol, Version 3

IGMPv3 adds source-filtering capabilities to the IGMP protocol. Source filtering enables receivers to specify the list of senders from which they want to receive multicast traffic.

Note

IGMPv3 is backward compatible to IGMPv1 and IGMPv2 and is necessary for SSM, which will be discussed later in this Chapter.

You can configure IGMPv3 on receivers and last-hop routers in your network by using one of the following host signaling mechanisms:

• IGMPv3 Host Signaling IGMP receivers announce membership into multicast groups using either of the following two membership report messages:

  - INCLUDE messagesSpecifies which multicast groups and sources the host wants to receive.

  - EXCLUDE messagesSpecifies which multicast groups and sources the host does not want to receive.

  Figure 5-6 specifies the IGMPv3 operation. The last-hop router sends a general query to its LAN segment, and Clients A and B respond for (S, G) groups that they want to receive data from. IGMPv3 is backwards-compatible with IGMPv2 and therefore allows receivers to use IGMP Leave messages.

  Figure 5-6. IGMPv3 Operation

  

  Enable IGMPv3 on the last-hop router with the interface configuration command ip igmp version 3.

• IGMP v3lite Hosting Signaling This is a Cisco-developed protocol used to transition from ISM to SSM applications. IGMP v3lite enables receiving hosts that do not yet support IGMPv3 in their operating systems to signal last-hop routers of their interest in (S, G) multicast traffic. The IGMP v3lite software is available as an extension to the receiving host's operating system with the Host Side IGMP Library (HSIL). This software supplies applications with a subset of the IGMPv3 application programming interface (API) that is required to write IGMPv3 applications.

  You can enable IGMP v3lite on the last-hop router with the interface configuration command ip igmp v3lite.

• URL Rendezvous Directory (URD) Besides IGMP v3lite, you can also use the URD protocol to enable non-IGMPv3 supporting receivers with IGMPv3 capabilities. You need to enable only URD on the last hop router with the command ip urd. To specify the group and sources to the last-hop router, you need to open a web browser with the IP address of the last-hop router, TCP port 465, group multicast IP address, and desired sources of the multicast. For example, you should enter the following URL in the URL field of your browser to receive multicast traffic of group 232.1.1.1 from sources 10.2.2.2, 10.3.3.3, and 10.4.4.4, if your last-hop router is 10.1.1.1.

   http://10.1.1.1:465/ path?group=232.1.1.1&source=10.2.2.2&source=10.3.3.3&source=10.3.3.3 

  The router will inspect the URL and remember the group and sources requested when the receiver application sends the IGMPv1 or IGMPv2 (*, G) Join message to the router. After the router receives the HTTP request from your browser, either of these Join messages will trigger the router to send PIM (S, G) Joins to each source your specify in the URL.

  You can enable URD on the last-hop router with the interface configuration command ip urd.