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.
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.
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.