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
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.
Redundancy
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.
Spanning tree
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
frequency converters are running in a ring/redundant line
topology.
How to Configure MCA 121 EtherNet/IP
16 MG.90.J3.02 - VLT
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