Danfoss MCA 121 Operating Instructions Manual

Also see for VLT MCA 121 EtherNet/IP: Instructions Manual

Danfoss MCA 121 Operating Instructions Manual Manual pdf 17 page image

4.1.4 IP trafficThe use of Ethernet based network for industrialautomation purposes, calls for careful and thoroughnetwork design. Especially the use of active networkcomponents like switches and routers requires detailedknow-how about the behaviour of IP traffic.Some important issues:MulticastMulticast traffic; is traffic that is addressed to a number ofrecipients. Each host processes the received multicastpacket to determine if it is the target for the packet. If not,the IP package is discarded. This causes an excessivenetwork load of each node in the network since they areflooded with multicast packages. The nature of EtherNet/IPtraffic is that all Originator-to-Target traffic is Unicast(point-to-point) but Target-to-Originator traffic is optionalMulticast. This enables that several listen only -connectionscan be made to a single host.In switched networks hosts also have the risk of becomingflooded with multicast traffic. A switch usually forwardstraffic by MAC address tables build by looking into thesource address field of all the frames it receives.A multicast MAC address is never used as a source addressfor a packet. Such addresses do not appear in the MACaddress table, and the switch has no method for learningthem, so it will just forward all multicast traffic to allconnected hosts.IGMPIGMP (Internet Group Management Protocol) is anintegrated part of IP. It allows hosts to join or leave amulticast host group. Group membership information isexchanged between a specific host and the nearestmulticast router.For EtherNet/IP networks it is essential that the switchesused, supports IGMP Snooping. IGMP Snooping enablesthe switch to “listen in" on the IGMP conversation betweenhosts and routers. By doing this the switch will recognisewhich hosts are members of which groups, thus being ableto forward multicast traffic only to the appropriate hosts.RedundancyFor an Ethernet network to function properly, only oneactive path can exist between two nodes. Spanning-TreeProtocol is a link management protocol that provides pathredundancy while preventing undesirable loops in thenetwork.When loops occur, some switches see stations appear onboth sides of it self. This condition confuses the forwardingalgorithm and allows for duplicate frames to be forwarded.Spanning treeTo provide path redundancy, Spanning-Tree Protocoldefines a tree that spans all switches in an extendednetwork. Spanning-Tree Protocol forces certain redundantdata paths into a standby (blocked) state. If one networksegment in the Spanning-Tree Protocol becomesunreachable, or if Spanning-Tree Protocol costs change,the spanning-tree algorithm reconfigures the spanning-treetopology and re-establishes the link by activating thestandby path.Spanning-Tree Protocol operation is necessary if thefrequency converters are running in a ring/redundant linetopology.How to Configure MCA 121 EtherNet/IP16 MG.90.J3.02 - VLT® is a registered Danfoss trademark44