Intel IXM5414E manual Internet Group Management Protocol Igmp snooping

The VLAN classification thus associated with received GMRP PDUs establishes the VLAN context for the received PDU, and identifies the GARP participant instance to which the PDU is directed. GMRP PDUs transmitted by GMRP participants are VLAN-classified according to the VLAN context associated with that participant. GMRP Participants in VLAN networking devices apply the same egress rules that are defined for the transmission port. Therefore:

•GMRP PDUs are transmitted through a given port only if the port is a member of the VLAN concerned.

•GMRP PDUs are transmitted as VLAN-tagged frames or untagged frames, in accordance with the state of the Untagged Set for that port for the VLAN concerned. Where VLAN-tagged frames are transmitted, the VID field of the tag header carries the VLAN Context Identifier value.

Internet Group Management Protocol (IGMP) snooping

Internet Group Management Protocol (IGMP) snooping is a feature that allows a switch to forward multicast traffic intelligently on the switch. Multicast IP traffic is traffic destined to a host group. Host groups are identified by class D IP addresses, which range from 224.0.0.0 to 239.255.255.255. Based on the IGMP query and report messages, the switch forwards traffic only to the ports that request the multicast traffic. This prevents the switch from broadcasting the traffic to all ports and possibly affecting network performance.

Note that the IP address range 224.0.0.1 through 224.0.0.255 is reserved for routing protocols and other low-level topology discovery or maintenance protocols. For example, the address 224.0.0.1 is the “all hosts” address, and 224.0.0.2 indicates all routers on this subnet. Also, only the least significant 23 bits of the IP address are mapped to MAC addresses, so, for example, 225.0.0.123 and 239.128.0.123 and similar IP multicast addresses all map to MAC address 01-00-5E-00-00-7B (for Ethernet). Therefore, a switch using IGMP Snooping may collapse IP multicast group memberships into a single Ethernet multicast group.

A traditional Ethernet network may be physically separated into different network segments to prevent overload of the shared media. Bridges and switches connect these segments. When a packet with a broadcast or multicast destination address is received, the switch will forward a copy into each of the remaining network segments in accordance with IEEE 802.1D. Eventually, the packet is made accessible to all nodes connected to the network.

This approach works well for broadcast packets that are intended to be seen or processed by all connected nodes. In the case of multicast packets, however, this approach can lead to less efficient use of network bandwidth, particularly when the packet is intended for only a small number of nodes. Packets will be flooded onto network segments where no node has any interest in receiving the packet. The problem of wasting bandwidth is even worse when the LAN segment is not shared, for example in full duplex links.

Allowing switches to snoop IGMP packets is one way to solve this problem. The switch uses the information in the IGMP packets as they are being forwarded throughout the network to determine which segments should receive packets directed to particular group addresses.

32 Intel® Blade Server Ethernet Switch Module IXM5414E