IGMP Snooping
In Ethernet networks there are three types of frames — broadcast frames which are destined to all devices in the network, directed frames which are sent to one specific device, and multicast frames which are sent to a group of one or more devices. Some Ethernet protocols utilize multicast frames to send data to multiple devices at the same time. These protocols generally create a large amount of multicast traffic. Switches with IGMP (Internet Group Multicast Protocol) snooping can automatically send the multicast frames only to devices that have requested these frames. This keeps the multicast frames from flooding devices that have not requested these frames. Some devices may be unable to perform their normal activities when they receive a large amount of unwanted multicast traffic. This multicast filtering can be important in large EtherNet/IP networks.
Redundancy
Redundancy is a popular feature in managed Industrial Ethernet switches. Basically, they provide the ability to interconnect these switches in a manner such that if one interconnecting cable were to fail, another cable or set of cables would take over. The time in which this recovery takes place is called the recovery time. There are two popular IEEE redundancy standards, Spanning Tree Protocol (STP) and Rapid Spanning Tree Protocol (RSTP). STP (IEEE 802.1D) is the older, slower-to-recover protocol. RSTP (IEEE 802.1w) is the newer, faster-to-recover version of STP. STP generally can recover from a fault within 30–60 seconds. RSTP, generally, can recover in 1–2 seconds.
Because of these long recovery periods, many industrial switch vendors have created their own proprietary ring redundancy protocols. These can usually recover in less than 300 ms. In these networks the switches are connected in a ring (see figure below). These protocols generally select one switch-to-switch link in the network to be disabled. This is the backup link. When another switch-to-switch link fails, the backup link is enabled, thus repairing the network.
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STP/RSTP vs Proprietary Ring
Each redundancy method has its advantages and its disadvantages. STP and RSTP can be wired in a ring or in just about any configuration imaginable. There are several vendors selling STP- and RSTP-compatible switches. STP/RSTP networks, however, are almost always slower to recover than proprietary ring networks. Proprietary Ring protocols must be wired in a ring or in several rings and all switches in the ring must come from the same vendor. Also, STP/RSTP networks can provide faster recovery times if you utilize a mesh network. This requires three connections between switches, while the ring network only requires two. You should consult your switch vendor for their network recovery time.
SNMP
SNMP is the Single Network Management Protocol that allows network management applications to communicate with a switch in a standardized way and request status information and set configurations. Most switches also support a similar set of management data (MIBs). As most managed switches support this protocol and support similar data, one application can communicate with multiple switches. This allows one application to monitor multiple switches at the same time, thus providing a global view of the network. Network management applications provide many features such as the ability to map the network, the ability to receive error frames from the switches, etc. There are also a few vendors who sell applications which convert SNMP data into OPC data for use in HMI systems.
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