Application and support of SSM mapping on an S series switch


The source-specific multicast (SSM) model conserves multicast addresses and is more secure than the any-source multicast (ASM) model. Only IGMPv3 supports SSM. A host running IGMPv3 can specify multicast source addresses in Report messages. Some hosts can only run IGMPv1 or IGMPv2. To enable these hosts to receive the SSM service, multicast devices need to offer the IGMP snooping SSM mapping function.
IGMP snooping SSM mapping is a Layer 2 SSM mapping feature used on IPv4 multicast networks. After static SSM mapping entries are configured on a Layer 2 device, the device can convert (*, G) information in IGMPv1 and IGMPv2 Report messages to (S, G) information to provide the SSM service for IGMPv1 and IGMPv2 hosts. S indicates the multicast source, G indicates the multicast group, and the asterisk (*) indicates any multicast source.
S series switches excluding the S1700 support IGMP snooping SSM mapping.

Other related questions:
(S, G) entry is not generated on the S series switch configured with IGMP SSM mapping
The switch interface is enabled with SSM mapping and IGMP, and is configured with a static SSM mapping policy. After the interface receives IGMPv1 or IGMPv2 Report messages, no corresponding (S, G) entry exists in the FIB table. A: When a querier receives an IGMPv1 or IGMPv2 (*, G) Report message, the querier checks G. When G is in the SSM group address range and the host is configured with G-related SSM mapping rules, the switch converts (*, G) to (G, Include, (S1, S2, S3�?), implementing SSM mapping. Possible causes are as follows: G in the (*, G) Report message is not in the SSM group address range. Run the display this command in the PIM view to check the configuration. If the command output shows the ssm-policy basic-acl-number or ssm-policy acl-name acl-name command, the SSM group address range is defined on the device. Run the display acl command to check the ACL configuration. Ensure that G is in the SSM group address range. By default, the SSM group address range is - The SSM mapping rules related to G in the (*, G) Report message are not configured. Run the display igmp ssm-mapping interface interface-type interface-number command to check the SSM mapping rules configured on the device and check whether the source address of G is specified. Check whether the group address configured with SSM mapping rules is in the SSM group address range.

Can S series switches use SSM mapping to convert the version of IGMP messages
Although SSM mapping takes effect only for IGMPv3 messages in a VLAN, the switch does not convert IGMPv2 messages into IGMPv3 messages before sending the messages to router ports. You can configure IGMP snooping proxy or IGMP snooping Report suppression on the switch to enable the switch to send IGMPv3 messages to the upstream device.

Implementation and application of fast leave of S series switches
For an S series switch, the fast leave function is configured using the igmp-snooping prompt-leave command (for Layer 2 multicast) or the igmp prompt-leave command (for Layer 3 multicast). If this function is not configured, the switch sends a Group-Specific Query message in response to an IGMP Leave message to check whether there are other users in the group specified in the Leave message. After fast leave is configured, the switch directly deletes the interface that receives the IGMP Leave message from the multicast forwarding entry, and does not check whether there are still users in the group. The fast leave function is applicable only to interfaces with only one user attached. If fast leave is configured on an interface connecting to multiple users, multicast traffic of all users under the interface is interrupted when one user switches to another channel.

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