The following sections describe how the switch implements PIM-SSM and IGMP.
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.
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:
You can map one source to multiple groups.
You can allow multiple sources to the same group.
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-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.
After the SSM switch receives an IGMPv2 report for a group that is in the SSM channel table, it joins the specified source immediately.
After the SSM switch receives an IGMPv2 report for a group that uses an enabled static SSM channel table entry, it triggers PIM-SSM processing as if it received an equivalent IGMPv3 report.
After the SSM switch receives an IGMPv2 report for a group out of the SSM range, it processes the report as if it is in PIM-SM mode.
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:
A device is PIM-enabled, running in SSM mode, with IGMPv1 or IGMPv2 configured on the interface.
IGMPv1 and IGMPv2 hosts send IGMPv1 or IGMPv2 reports for groups in SSM range.
The following table identifies the expected behaviors in this scenario.
|
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. |
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
IGMPv3 works without PIM-SSM or SSM-snoop enabled on the interface.
The following rules apply to IGMPv3-enabled interfaces:
Send only IGMPv3 (source-specific) reports for addresses in the SSM range.
Accept IGMPv3 reports.
Drop IGMPv2 reports.
The IGMPv2 report mentioned in SSM and IGMPv2 is processed because it is an IGMPv2 report received on an IGMPv2 interface. If an IGMPv2 interface receives an IGMPv3 report, it drops the report even if PIM-SSM is enabled and the entry is in the SSM channel table. The IGMP versions must match.
Discard IGMP packets with a group address out of the SSM range.
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
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:
An IGMPv2 host (with an IGMPv2 VLAN) must use an existing SSM channel entry if the group is in the SSM range.
If you enable dynamic learning for an IGMPv3 host, the SSM channel automatically learns the group. Otherwise, the SSM channel also needs a static entry.
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.
|
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.
|
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.