Release 15.1SY Supervisor Engine 2T Software Configuration Guide

IGMP Filtering Overview

Note IGMP, which runs at Layer 3 on a multicast router, generates Layer 3 IGMP queries in subnets where the multicast traffic needs to be routed. For information about IGMP, see Chapter14, “Information About IPv4 Multicast Layer 3 Features”

IGMP snooping is a protocol that learns and maintains multicast group membership at the Layer 2 level. IGMP snooping looks at IGMP traffic to decide which ports should be allowed to receive multicast traffic from certain sources and for certain groups. This information is used to forward multicast traffic to only interested ports. The main benefit of IGMP snooping is to reduce flooding of packets. For information about IGMP snooping, see “Information About IGMP Filtering” section.

IGMP filtering allows users to configure filters on a switch virtual interface (SVI), a per-port, or a per-port per-VLAN basis to control the propagation of IGMP traffic through the network. By managing the IGMP traffic, IGMP filtering provides the capability to manage IGMP snooping, which in turn controls the forwarding of multicast traffic.

When an IGMP packet is received, IGMP filtering uses the filters configured by the user to determine whether the IGMP packet should be discarded or allowed to be processed by the existing IGMP snooping code. With a IGMP version 1 or version 2 packet, the entire packet is discarded. With a IGMPv3 packet, the packet is rewritten to remove message elements that were denied by the filters.

The IGMP filtering feature is SSO compliant.

IGMP traffic filters control the access of a port to multicast traffic. Access can be restricted based on the following:

• Which multicast groups or channels can be joined on a port. Channels are joined by IGMPv3 hosts that specify both the group and the source of the multicast traffic.
• Maximum number of groups or channels allowed on a specific port or interface (regardless of the number of hosts requesting service).
• IGMP protocol versions (for example, disallow all IGMPv1 messages).

When you enter an IGMP filtering command, a user policy is applied to a Layer 3 SVI interface, a Layer 2 port, or a particular VLAN on a Layer 2 trunk port. The Layer 2 port may be an access port or a trunk port. The IGMP filtering features will work only if IGMP snooping is enabled (either on the interface or globally).

IGMP filtering is typically used in access switches connected to end-user devices.

There are three different types of IGMP filters: IGMP group and channel access control, several IGMP groups and channels limit, and an IGMP minimum version. These filters are configurable and operate differently on different types of ports:

• Per SVI
• Per port
• Per VLAN basis on a trunk port

You can configure filters separately for each VLAN passing through a trunk port.

IGMP Filter Precedence

• Access Mode
• Trunk Mode

Access Mode

In access mode, filters can be configured on both the port and the SVI. When an IGMP packet is received on a port in access mode, the port filter is checked first. If the port filter exists, it is applied and the SVI filter is ignored. If no per-port filter exists, the SVI filter is used.

This hierarchy is applied separately for each type of filter. For example, a limit filter configured on the port overrides the default limit filter on the SVI, but has no affect on any of the other filters.

Trunk Mode

With ports in trunk mode, a filter can be configured for an SVI corresponding to one of the VLANs on the trunk port, another filter configured on the trunk port itself, and a third filter configured on one of the Layer 2 VLANs passing through the trunk. When an IGMP packet is received, the trunk-per-VLAN specific filter will be checked first. If this filter exists, it is applied. The main trunk port filter and SVI filter will be ignored. If no trunk-per-VLAN filter exists, the main trunk port filter will be used. If neither of these filters exist, the SVI filter for the VLAN will be used as a final default for ports in trunk mode.

Configuring IGMP Group and Channel Access Control

Filtering on the IGMP group or channel allows the user to control which IGMP groups or channels can be joined on a port or on a per VLAN basis on a trunk port.

To configure filtering on the IGMP group or channel use the following CLI command:

To allow or deny several groups or channels, you must configure multiple access control entries in the access control list. Depending on whether the ACL is configured as permit or deny, the corresponding group or channel is allowed or denied. The ACL specified may be either a simple or extended ACL.

Filtering by IGMP group or channel is configurable on a Layer 3 SVI as a default filter for all ports in access mode under that SVI and for the corresponding VLAN on all trunk ports carrying that VLAN. This filter is also configurable on a Layer 2 port. If the port is in access mode, this filter will override any default SVI filter. If the port is in trunk mode, this filter will act as a default for all VLANs on that trunk and will override the SVI filter for each corresponding VLAN.

The vlan keyword can apply the filter only to IGMP packets arriving on the specified Layer 2 VLAN if the port is a trunk port. This per-VLAN filter (configured using the vlan keyword) will override any interface level filter and any SVI filter for the same VLAN.

Configuring IGMP Group and Channel Limits

Limiting the number of IGMP groups or channels allows you to control how many IGMP groups or channels can be joined on a port or on a per-VLAN basis on a trunk port.

To limit the number of IGMP groups or channels, use the following interface command CLI:

A maximum of n groups or channels are allowed on the port or interface. The except keyword allows you to specify groups or channels that are exempt from the configured limit. The ACL used with the except keyword may be either a simple or extended ACL.

If joins are received for (*,G1) and (S1,G1) on the same interface, these are counted as two separate joins. If the limit on an interface has been set to 2, and joins are received for (*,G1) and (S1,G1), all other joins (for groups or channels different from these two) will then be discarded.

This filter is configurable on a Layer 3 SVI as a default filter for all ports in access mode under that SVI and for the corresponding VLAN on all trunk ports carrying that VLAN. This filter is also configurable on a Layer 2 port. If the Layer 2 port is in access mode, this filter will override any default SVI filter. If the Layer 2 switch port is in trunk mode, this filter will act as a default for all VLANs on that trunk and will override the SVI filter for each corresponding VLAN. The vlan keyword allows the user to apply the filter only to IGMP packets arriving on the specified Layer 2 VLAN if the Layer 2 switch port is a trunk port. This per-VLAN filter, configured using the vlan keyword, will override any interface level filter and any SVI filter for the same VLAN.

Configuring IGMP Version Filtering

Filtering on the IGMP protocol allows you to configure the minimum version of IGMP hosts allowed on the SVI. For example, you may want to disallow all IGMPv1 hosts (such as, allow a minimum IGMP version of 2) or all IGMPv1 and IGMPv2 hosts (such as, allow a minimum IGMP version of 3). This filtering applies only to membership reports.

To configure filtering on the IGMP protocol, use the following CLI command:

This filter is only configurable on a Layer 3 SVI as a default filter for all ports in access mode under that SVI and for the corresponding VLAN on all trunk ports.

Clearing IGMP Filtering Statistics

To clear IGMP filtering statistics, perform one of these tasks:

Displaying IGMP Filtering Configuration

To display IGMP filtering rules, perform this task:

This example shows how to display the default filters configured on the SVI:

This example shows how to display the filters configured for all ports in access mode under this SVI and for all trunk ports carrying the corresponding VLAN:

This example shows how to display the filters configured for all ports in access mode under this SVI:

This example shows how to display the default trunk port filters:

This example shows how to display the per-VLAN filters for all VLANs on this trunk:

This example shows how to display the per-VLAN filters for a specific VLAN on this trunk:

Note If the port is in the shutdown state, filter status will not be displayed because it cannot be determined whether the port is in trunk mode or access mode. In this situation, you can use the show running-config interface xxxx command to view the configuration.

Displaying IGMP Filtering Statistics

Statistics are maintained on an interface basis for ports in access mode and on a per-VLAN basis for ports in trunk mode.

To display IGMP filtering statistics, perform this task:

This example shows how to display statistics for each port in access mode under the SVI:

This example shows how to display statistics for a specific port in access mode:

This example shows how to display statistics for Gigabit Ethernet port 3/47 in access mode with no default SVI filter and no port filter:

This example shows how to display statistics for all VLANs under a trunk:

This example shows how to display statistics for a specific VLAN under a trunk:

This example shows how to display statistics for a specific VLAN under a trunk port with no trunk and no VLAN filter:

Note If the port is in the shutdown state, filter statistics will not be displayed because it cannot be determined whether the port is in trunk mode or access mode.