Installation guide

Network Infrastructure for EtherNet/IP™
Designing the Infrastructure
4-46
IGMP Snooping
The Internet Group Management Protocol is very important because it reduces the load of traffic
traversing through the network. It also relieves the hosts from processing frames that are not needed.
In the producer-consumer model used by EtherNet/IP, IGMP snooping limits unnecessary traffic from the
producing I/O, such that it only reaches the devices that consume that data. Messages delivered to a given
device (say Device A) that were intended for another device (say Device B) will consume resources,
negatively affecting the performance of Device A.
Networks with multiple multicasting devices will suffer performance issues if IGMP snooping or other
multicast limiting schemes are not implemented. The number of devices that can be supported in a given
network varies with the type of traffic that is present.
When using IGMP snooping, at least one
IGMP querier is needed somewhere on the network. A router or
switch can act as the IGMP querier if the device supports this functionality. An IGMP querier periodically
queries the multicast receiving devices as to their multicast interest. Once queried, a multicast recipient
will respond with the multicasts it wishes to receive by sending an
IGMP Join Report for each multicast
group in which it has interest.
The Ethernet switch snoops on the IGMP traffic coming to that switch and keeps track of multicast
groups and member ports. When the switch receives an IGMP Join Report from a host for a particular
multicast group, the switch adds the host port number to the associated multicast forwarding table entry.
When it receives an
IGMP Leave Group message from a host, it removes the host port from the table
entry. After it relays the IGMP queries, it deletes entries periodically if it does not receive any IGMP
membership reports from the multicast clients. A Layer 3 device (router) normally performs the querying
function, but it can also be implemented by another network element designed to take over this
functionality.
When IGMP snooping is enabled in a network with Layer 3 devices, the multicast router sends out
periodic
IGMP general queries to all VLANs. The switch responds to the router queries with only one
join request per MAC multicast group, and the switch creates one entry per VLAN in the Layer 2
forwarding table for each MAC group from which it receives an IGMP join request. All hosts interested
in this multicast traffic send join requests and are added to the forwarding table entry.
Layer 2 multicast groups learned through IGMP snooping are dynamic. However, in a managed switch, it
is possible to statically configure
MAC multicast groups. If a group membership for a multicast group
address is specified statically, this setting supersedes any automatic manipulation by IGMP snooping.
Multicast Group Membership Lists can consist of both user-defined and IGMP snooping-learned
settings.
If a port spanning-tree, a port group, or a VLAN ID change occurs, the IGMP snooping-learned multicast
groups from this port on the VLAN are deleted.