VLT
®
AutomationDrive EtherNet/IP
MG.90.J1.02 – VLT is a registered Danfoss trademark
19
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.
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 100/200/300’s are running in a
ring/redundant line topology.
