Internet Group Management Protocol

A host uses IGMP to register group memberships with the local querier router to receive datagrams sent to this router targeted to a group with a specific IP multicast address.

A router uses IGMP to learn the existence of group members on networks to which it directly attaches. The router periodically sends a general query message to each of its local networks. A host that is a member of a multicasting group identifies itself by sending a response.

IGMP queries

When multiple IGMP routers operate on a network, one router is elected to send queries. This elected querier periodically sends host membership queries (also known as general queries) to the attached local subnets. The switch supports queries from all three versions of IGMP.

IGMP host reports

A host that receives a membership query from a local router can respond with a host membership report, one for each multicast group that joins. A host that receives a query delays its reply by a random interval and listens for a reply from other hosts in the same host group. For example, consider a network that includes two host members—host A and host B—of the same multicast group. The router sends out a host membership query on the local network. Both host A and host B receive the query and listen on the network for a host membership report. The delay timer for host B expires first, so it responds to the query with a membership report. Hearing the response, host A does not send a report of its own for the same group.

Each query from a router to a host includes a maximum response time field. IGMP inserts a value n into this field specifying the maximum time in tenths of a second within which the host must issue a reply. The host uses this value to calculate a random value between 0 and n tenths of a second for the period that it waits before sending a response. This calculation is true for IGMP versions 2 and 3. For IGMP version 1, this field is 0 but defaults to a value of 100, that is, 10 seconds.

If at least one host on the local network specifies that it is a member of a group, the router forwards to that network all datagrams that bear the multicast address for the group.

Upon initialization, the host can immediately issue a report for each of its supported multicast groups. The router accepts and processes these asynchronous reports the same as requested reports.

After hosts and routers are in a steady state, they communicate in a way that minimizes the exchange of queries and reports. The designated routers establish a path between the IP multicast stream source and the end stations and periodically query the end stations about whether to continue participation. As long as a client continues to participate, all clients, including nonparticipating end stations on the switch port, receive the IP multicast stream.

Host leave messages

If an IGMPv2 host leaves a group and it is the host that issues the most recent report, it also issues a leave group message. The multicast router on the network issues a group-specific query to determine whether other group members exist on the network. If no host responds to the query, the router assumes that no members belonging to that group exist on that interface.

Fast leave feature

The switch supports a fast leave feature that is useful for multicast-based television distribution applications. Fast leave relies on an alternative leave process where the switch stops sending traffic for the group immediately after it receives a leave message, without issuing a query to check if other group members exist on the network. Fast leave alleviates the network from additional bandwidth demand after a customer changes television channels.

The switch provides several fast leave processes for IP multicast:

• immediate leave with one user for each interface

• immediate leave with several users for each interface

• standard IGMP leave based on a Last Member Query Interval (LMQI), which you can configure in tenths of seconds

Fast leave modifies the IGMP leave processing mechanism on an IGMP interface. After the system receives an IGMP leave message on a fast leave enabled interface, the switch does not send a group-specific query and immediately stops sending traffic to the leaving member (IGMP host) port. Without fast leave, traffic continues to forward until the group times out. This situation wastes bandwidth if no receiver that requires the group traffic exists.

Fast leave mode provides two options of the fast leave mechanism—single-user mode and multiple-users mode:

• Single-user mode: In this mode, the port stops receiving traffic immediately after a group member on that port sends a leave message. Use the single-user mode if each interface port connects to only one IGMP host.

• Multiple-users mode: Use this mode if the interface port connects to multiple IGMP hosts. In this case, the port stops receiving traffic after all members leave the IGMP group. The switch removes the leaving IGMP member and, if more group members exist on that port, the switch continues sending traffic to the port.

  When operating in multiple-users mode, the switch must use the correct membership information. To support multiple-users mode, multicast receivers on the same interface cannot use IGMP report suppression. If you must use IGMP report suppression, do not use this mode. Instead, use the LMQI (configurable in units of 1/10ths of seconds) to provide a faster leave process while still sending group-specific queries after the interface receives a leave message.

Fast leave mode applies to all fast-leave enabled IGMP interfaces.

IGMP snoop

The switch provides IP multicast capability and can support all three versions of IGMP to prune group membership for each port within a VLAN. This feature is IGMP snoop.

Important

IGMP snoop can optimize only local multicast data flow. IGMP snoop does not manage the forwarding state of the multicast tree. You cannot configure a port as a static receiver in an IGMP snoop-enabled VLAN that does not contain at least one dynamic receiver port and forward multicast data.

Use the IGMP snoop feature to optimize the multicast data flow, for a group within a VLAN, to only those ports that are members of the group. The switch builds a database of group members by listening to IGMP reports from each port. The switch suppresses the reports heard by not forwarding them to ports other than the one receiving the report, thus forcing the members to continuously send their own reports. The switch relays group membership from the hosts to the multicast routers and forwards queries from multicast routers to all port members of the VLAN. Furthermore, the switch forwards multicast data only to the participating group members and to the multicast routers within the VLAN.

The multicast routing functionality can coexist with IGMP snoop on the same switch, but you can configure only one of IGMP snoop or an IP multicast routing protocol, excluding IGMP, on the same VLAN.

Multicast group trace for IGMP snoop

Use this feature to monitor the multicast group trace for an IGMP snoop-enabled switch . You can view the multicast group trace from CLI.

Multicast group trace tracks the data flow path of the multicast streams. Group trace tracks information such as the multicast group address, the source address, ingress VLAN and port, and egress VLAN and port.

IGMP proxy

If a switch receives multiple reports for the same multicast group, it does not transmit each report to the multicast upstream router. Instead, the switch consolidates the reports into a single report and forwards the one report. If you add another multicast group or the system receives a query since it last transmitted the report upstream, the system forwards the report onto the multicast router ports. This feature is IGMP proxy.

IGMP versions

The switch supports IGMPv1, IGMPv2, and IGMPv3. IGMPv1 and IGMPv2 are backward compatible and can exist together on a multicast network. The following list describes the purpose for each version:

• IGMPv1 provides the support for IP multicast routing. IGMPv1 specifies the mechanism to communicate IP multicast group membership requests from a host to its locally attached routers. For more information, see RFC1112.

• IGMPv2 extends the features in IGMPv1 by quickly reporting group membership termination to the routing protocol. This feature is important for multicast groups with highly volatile group membership. For more information, see RFC2236.

• IGMPv3 supports the PIM Source Specific Multicast (SSM) protocol, PIM-SM, and snooping. A host can selectively request or filter traffic from individual sources within a multicast group or from specific source addresses sent to a particular multicast group. Multicast routing protocols use this information to avoid delivering multicast packets from specific sources to networks where there are no interested receivers. For more information, see RFC3376.

For the switch implementation of PIM-SSM, each group can use multiple sources.

The following list identifies group records that a report message includes:

• current-state record

• source-list-change record

• filter-mode-change record

A current-state record is sent by a system in response to a query received on an interface. It reports the current reception state of that interface, with respect to a single multicast address.

After you enable IGMPv3, the following actions occur:

• After you change the version on an interface to or from IGMPv3, the switch experiences a disruption to existing multicast traffic on that interface but traffic does recover. Do not make this change when the system passes multicast traffic.

IGMP states

Each source record is of the form source address or source timer. If all sources within a given group are desired, an empty source record list is kept with filter-mode set to EXCLUDE. This means hosts on this network want all sources for this group to be forwarded. This is the IGMPv3 equivalent to a IGMPv1 or IGMPv2 group join.

Group timer

A group timer represents the time for the filter-mode to expire and switch to INCLUDE mode and is used only when a group is in EXCLUDE mode.

Group timers are updated according to the types of group records received. If a group timer is expiring when a router filter-mode for the group is EXCLUDE means, there are no listeners on the attached network in EXCLUDE mode. At this point, a router will transition to INCLUDE filter-mode.

Source timer

If a source timer expires with a router filter-mode for the group of INCLUDE, the router concludes that traffic from this particular source is no longer desired on the attached network, and deletes the associated source record.

If a source record has a running timer with a router filter-mode for the group of EXCLUDE, it means that at least one system desires the source. It should therefore be forwarded by a router on the network. If a source timer expires with a router filter-mode for the group of EXCLUDE, the router informs the routing protocol that there is no receiver on the network interested in traffic from this source. The records are deleted when the group timer expires in the EXCLUDE router filter-mode.

Processing IGMP messages for groups in SSM range

IGMP messages are processed for groups in SSM range in the following scenarios:

1. IGMPv3 interface enabled; PIM-sparse or snooping enabled

   • IGMPv3 reports that contain group records with groups within SSM range are processed with no restrictions.

   • IGMPv2 reports for groups within SSM range translate to IGMPv3 reports with one group record and type IS_EXCLUDE{NULL}. These reports are processed with no restriction as an IGMPv3 report.

   • IGMPv2 leave for groups within SSM range translate to IGMPv3 reports with one group record and type TO_INCLUDE{NULL}. These reports are processed with no restriction as an IGMPv3 report.

2. IGMPv3 interface enabled; PIM-SSM or ssm-snooping enabled

   • IGMPv3 reports that contain group records with groups within SSM range received from members in the EXCLUDE mode are discarded (eg. IS_EXCLUDE and TO_EXCLUDE messages).

   • IGMPv2 reports for groups within SSM range translate to IGMPv3 reports with one group record and type ALLOW{S1,S2,...}. The source list is obtained from the global ssm-map. If there are no sources in the global ssm-map, the message is discarded. These reports are processed with no restriction as an IGMPv3 report.

   • IGMPv2 leave for groups within SSM range translate to IGMPv3 reports with one group record and type BLOCK{S1,S2,...}. The source list is obtained from the global ssm-map. If there are no sources in the global ssm-map, the message is discarded. These reports are processed with no restriction as an IGMPv3 report.

Note

In order to accept v2 messages, you must enable the compatibility mode on the IGMPv3 interface.

IGMPv3 source-specific forwarding rules

After a multicast router receives a datagram from a source destined to a particular group, the router must decide to forward the datagram to the attached network. The multicast routing protocol uses IGMPv3 information to forward datagrams to all required sources or groups on a subnetwork.

The following table describes the forwarding suggestions that IGMPv3 makes to the routing protocol. The table also identifies the action taken after the source timer expires, based on the filter mode of the group.

IGMPv3 explicit host tracking

IGMPv3 explicit host tracking enables the IGMP to track all the source and group members. To track all the source and group members, the sources that are in the include mode hold a list of members who want to receive traffic from that source.

The members that are in the exclude mode are on hold on the reporter list under the port data. By default, IGMPv3 explicit host tracking is disabled.

Important

If explicit host tracking is enabled, you cannot downgrade the IGMPv3 interface to IGMPv1 or IGMPv2.

IGMPv3 fast leave

When a BLOCK message is received for a source, you must check if the member that sent this message is the last reporter for the source. If it is the last reporter, delete the source. Else, delete the member. No group and source specific queries are sent.

When a LEAVE message is received, you must check if the member that sent this message is the last reporter for the group. If it is the last reporter, switch to INCLUDE mode if sources are available (if no sources are available the port is deleted). Else, delete the member. No group and source specific queries or group specific queries are sent.

Important

To use the IGMPv3 fast leave feature, you must first enable the explicit host tracking feature.

Synchronization of IGMPv3 over SMLT

The implementation of IGMPv3 offers support for IGMPv3 over SMLT. The Virtual-IST (vIST) peers must be in sync with the IGMPv3 reports received over SMLT links to ensure effective performance. The vIST protocol ensures the infrastructure to send such information from one vIST peer to the other.

The synchronization of IGMPv3 members and their advertised sources is different from IGMPv1 and IGMPv2. Because of IGMPv3 compatibility mode, you must consider the IGMP member version. If you have version 1 or 2 members, you must synchronize the IGMP information as IGMPv1 or IGMPv2 reports, so the peer can build an accurate database. In particular, if members with version 1 or 2 exist, the group filter mode is exclude and the exclude source list is empty. Also no v1 or v2 member will be present on any source from include list.

Each member sends IGMP reports in the same manner for all IGMP versions. The sending mechanism depends on the SMLT state.

After a vIST peer receives an IGMPv3 report over an SMLT link, it must pass the message to its peer. If the SMLT state is up, the vIST peer sends the message encapsulated in an vIST IGMPv3 message. If the SMLT state is down, the vIST peer sends the message as a plain IGMPv3 report.

In both cases the IGMPv3 message is not altered and the receiving vIST peer processes it as expected in SMLT conditions (translating the receiving port to SMLT port if applicable).

Note

If you enable compatibility mode and the member sends an IGMPv1 or IGMPv2 report, the message is either a vIST IGMPv1 or v2 encapsulated Message or a plain IGMPv1 or IGMPv2 report.

After SMLT up or down events occur, the vIST peer must synchronize its IGMPv3 database to its peer, taking into account the new state of the SMLT link.

If you enable IGMP explicit host tracking, each include source stores information for each member that advertises that particular source in an include list. This information is synchronized with the vIST peer.

If you do not enable explicit host tracking, each source from include list contains only information related to the last member that sent an IGMPv3 report. Only this information is synchronized with the vIST peer.

Backward compatibility

IGMPv3 for PIM-SSM is backward compatible with IGMPv2. You can configure the switch to operate in v3-only mode or in v2-v3 compatibility mode. If you configure the switch to use v3-only mode, it ignores all v2 and v1 messages except the query message.

If you configure the switch to operate in v2-v3 compatibility mode, the switch supports all IGMPv1, v2, and v3 messages. The switch parses the group address of the messages. If the group address is out of SSM range and it is a v3 message, the switch drops the message; if it is a v2 message, PIM-SM or IGMP snoop processes handle the message.

After the switch receives an IGMPv2 leave message and the group address in it is within SSM range, the switch sends the group-and-source specific query. If the group address is not within the SSM range, the switch sends the group specific query.

According to RFC3376, the multicast router with IGMPv3 can use one of two methods to handle older query messages:

• If an older version of IGMP is present on the router, the querier must use the lowest version of IGMP present on the network.

• If a router that is not explicitly configured to use IGMPv1 or IGMPv2 hears an IGMPv1 query or IGMPv2 general query, it logs a rate-limited warning.

You can configure if the switch dynamically downgrades the version of IGMP to handle older query messages. If the switch downgrades, the host with IGMPv3 only capability does not work. If you do not configure the switch to downgrade the version of IGMP, the switch logs a warning.

In v2-v3 compatibility mode, an IGMPv2 host can only join if you configure a static entry in SSM map and if the interface operates in PIM-SSM mode or IGMP SSM-Snoop mode.

You can use the compatibility mode with Split MultiLink Trunking (SMLT). One core switch sends an SMLT message to the other core switch after it receives an IGMPv3 message. This action synchronizes the IGMP host information.

Implementation of IGMP

You can enable and disable multicast routing on an interface basis. If you disable multicast routing on an interface, the interface does not generate IGMP queries. If the switch or interface is in IGMP router behavior mode, for example, PIM enabled, you cannot configure IGMP snoop. The switch still learns the group membership and snoops multicast receivers on the switch VLAN or ports.