Ip Traffic - Danfoss VLT MCA 121 EtherNet/IP Manual

MCA 121 EtherNet/IP 4 How to Configure

4.1.4 IP traffic

The use of Ethernet based network for industrial automation purposes, calls for careful and thorough network design. Especially the use of active network
components like switches and routers requires detailed know-how about the behaviour of IP traffic.
Some important issues:
Multicast
Multicast traffic; is traffic that is addressed to a number of recipients. Each host processes the received multicast packet to determine if it is the target
for the packet. If not, the IP package is discarded. This causes an excessive network load of each node in the network since they are flooded with multicast
packages. The nature of EtherNet/IP traffic is that all Originator-to-Target traffic is Unicast (point-to-point) but Target-to-Originator traffic is optional
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Multicast. This enables that several listen only -connections can be made to a single host.
In switched networks hosts also have the risk of becoming flooded with multicast traffic. A switch usually forwards traffic by MAC address tables build by
looking into the source address field of all the frames it receives.
A multicast MAC address is never used as a source address for a packet. Such addresses do not appear in the MAC address table, and the switch has no
method for learning them, so it will just forward all multicast traffic to all connected hosts.
IGMP
IGMP (Internet Group Management Protocol) is an integrated part of IP. It allows hosts to join or leave a multicast host group. Group membership
information is exchanged between a specific host and the nearest multicast router.
For EtherNet/IP networks it is essential that the switches used, supports IGMP Snooping. IGMP Snooping enables the switch to "listen in" on the IGMP
conversation between hosts and routers. By doing this the switch will recognise which hosts are members of which groups, thus being able to forward
multicast traffic only to the appropriate hosts.
Spanning Tree Protocol (STP)
For an Ethernet network to function properly, only one active path can exist between two nodes. Spanning-Tree Protocol is a link management protocol
that provides path redundancy while preventing undesirable loops in the network.
When loops occur, some switches see stations appear on both sides of it self. This condition confuses the forwarding algorithm and allows for duplicate
frames to be forwarded.
To provide path redundancy, Spanning-Tree Protocol defines a tree that spans all switches in an extended network. Spanning-Tree Protocol forces certain
redundant data paths into a standby (blocked) state. If one network segment in the Spanning-Tree Protocol becomes unreachable, or if Spanning-Tree
Protocol costs change, the spanning-tree algorithm reconfigures the spanning-tree topology and re-establishes the link by activating the standby path.
Spanning-Tree Protocol operation is necessary if the FC 200/FC 300's are running in a ring/redundant line topology.
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