Implementation of SSM and IGMP

The following sections describe how the switch implements PIM-SSM and IGMP.

SSM range

The standard SSM range is 232/8, but you can extend the range to include an IP multicast address. Although you can configure the SSM range, you cannot configure it for all multicast groups (224/4 or 224.0.0.0/240.0.0.0 or 224.0.0.0/255.0.0.0).

You can extend the SSM range to configure existing applications without changing their group configurations.

SSM channel table

You can use the SSM channel to manually configure (S,G) entries that map existing groups to their sending source. These table entries apply to the whole switch, not for each interface, and both IGMPv2 and IGMPv3 hosts use the SSM channel table.

The following rule applies to an SSM channel table for an individual switch:

Important

Important

Different switches can use different mappings for groups to sources, for example, different channels map differently even if they are on the same network.

SSM and IGMPv2

SSM-configured switches can accept reports from IGMPv2 hosts on IGMPv2 interfaces if the group uses an SSM channel table entry. However, the IGMPv2 host groups must exist in the SSM range defined on the switch, which is 232/8 by default.

Deleting or Disabling an ssm-map with IGMPv1 or IGMPv2

Before you disable or delete an ssm-map, always send IGMPv1 or IGMPv2 leave messages from hosts that operate using IGMPv1 or IGMPv2. If you do not perform this action, receiving and processing reports in SSM range on an IGMP interface enabled with IGMPv1 or IGMPv2 can lead to unexpected behavior.

Consider the following configuration scenario:

The following table identifies the expected behaviors in this scenario.

Table 1. Expected behaviors for ssm-map configuration

Action

Expected behavior

You do not configure an ssm-map for the group in SSM range.

IGMPv1 and IGMPv2 reports are not processed.

You do configure an ssm-map for the group in SSM range.

IGMPv1 and IGMPv2 reports are processed and the group in SSM range is learned.

SSM and IGMPv3

The switch supports IGMPv3 for SSM. With IGMPv3, a host can selectively request or filter traffic from sources within the multicast group. IGMPv3 is an interface-level configuration.

Important

Important

IGMPv3 works without PIM-SSM or SSM-snoop enabled on the interface.

The following rules apply to IGMPv3-enabled interfaces:

The switch implements IGMPv3 in one of two modes: dynamic and static.

In dynamic mode, the switch learns about new (S,G) pairs from IGMPv3 reports and adds them to the SSM channel table. If you do not enable dynamic mode and an IGMPv3-enabled interface receives a report that includes a group not listed in the SSM channel table, it ignores the report.

In static mode, you can statically configure (S,G) entries in the SSM channel table. If an IGMPv3-enabled interface receives a report that includes a group not listed in the SSM channel table, it ignores the report. The interface also ignores the report if the group is in the table, but the source or mask does not match what is in the table.

Important

Important

After you enable IGMPv3, changes to the query interval and robustness values on the querier switch propagate to other switches on the same VLAN through IGMP query.

Both IGMPv2 and IGMPv3 hosts use the SSM channel table:

The following table summarizes how a switch in PIM-SSM mode works with IGMP if you disable IGMPv3 compatibility. In the following table, references to matching a static SSM channel entry assumes that the entry is enabled. If an entry is disabled, it is treated as though it is disallowed.

Table 2. PIM-SSM interaction with IGMPv2 and v3 with IGMPv3 compatibility disabled

Host

VLAN

SSM range

Action

IGMPv2 host

IGMPv3 VLAN

In or out of range

Drop report.

IGMPv3 host

IGMPv2 VLAN

In or out of range

Drop report.

IGMPv2 host

IGMPv2 VLAN

In range

If the report matches an existing static SSM channel entry, create (S,G).

If the report does not match an existing static SSM channel entry, drop it.

IGMPv2 host

IGMPv2 VLAN

Out of range

Ignore the SSM channel table and process the report as if it is in PIM-SM mode.

IGMPv3 host

IGMPv3 VLAN

Out of range

Process the report.

IGMPv3 host

IGMPv3 VLAN

In range

Dynamic enabled. Create (S,G).

IGMPv3 host

IGMPv3 VLAN

In range

Dynamic disabled and matches an existing SSM channel entry. Create (S,G).

IGMPv3 host

IGMPv3 VLAN

In range

Dynamic disabled and does not match an existing SSM channel entry. Drop report.

The following table summarizes how a switch in PIM-SSM mode works with IGMP if you enable IGMPv3 compatibility.

Table 3. PIM-SSM interaction with IGMPv2 and v3 with IGMPv3 compatibility enabled

Host

VLAN

SSM range

Action

IGMPv2 Host

IGMPv3 VLAN

In range

If the report matches an existing static SSM channel entry, create (S,G).

If the report does not match an existing static SSM channel entry, drop it.

IGMPv2 Host

IGMPv3 VLAN

Out of range

Process the report as in PIM-SM mode.

If an IGMPv3 group report enters the VLAN port and the port must discard one or more of the groups in that packet after the application of IGMP access controls, the port drops the entire packet and does not forward it on to other ports of the VLAN.

If an IGMPv3 interface receives an IGMPv2 or v1 query, the interface backs down to IGMPv2 or v1. As a result, the interface flushes all senders and receivers on the interface.