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The answers to some commonly asked questions can be found below:

   Configuring 10/100BaseT Equipment to Connect to SM-25XX Link Protector®s
 Spanning Tree/MAC Address Issues
"Soft" Failure Detection

The primary issue to keep in mind when connecting other equipment to a Shore Microsystems Ethernet Link Protector is the issue of link speed and duplex configuration.  Although some SM-series units (e.g. the SM-2501) will allow auto-negotiation of link speed and duplexing, it is usually a better solution to "hard code" the connected equipment to the speed and duplex settings desired for the link.  Presetting these parameters allows quicker link setup when a switchover event takes place.  For example, for the SM-2501 (a single port 100BaseT unit), it is best to hardcode all connected ports (protected, primary, and backup) to 100Mbps and set the duplex configuration to either full or half on all attached ports.  This easily accomplished on many systems with simple port configuration commands or NIC driver initialization commands.

Also see the Spanning Tree/MAC FAQ for additional information.

Shore Microsystems Link Protector®s monitor a primary and backup connection to the external network.  This usually translates to connections to primary and backup switches which operate on the MAC address of the protected (server) port.  In normal operation, the protected port is connected to the primary port and thus the primary switch will learn and advertise the presence of this MAC address to the rest of the network.  One advantage of the SM-2XXX series is that it is only possible for the protected port to be connected to either the primary or backup switches at any one time (never both) and thus prevent routing loops.  However, in the event that a primary port fails, it is necessary for the backup switch to relearn the MAC address of the server port to which it is now attached.  

To guarantee quick relearning of the MAC address on the backup switch port, it is advisable to run some type of process on the protected port device (typically a server) that will regularly launch Ethernet frames toward the SM-2XXX unit.  For example, a continuous "ping" process is simple and effective for this function.  The pings should be launched toward a IP address located somewhere on or beyond the primary or backup switches.

To further speed the MAC relearning process, several switch vendors provide improved methods for propagating MAC address information through the network.  For example, Cisco Systems products implement the "Portfast" method where the normal spanning tree requirements are curtailed and a port can immediately transition from a "down" state to the "forwarding" state.  With Portfast, it is possible to switch between primary and backup ports without losing frames.  Other switch vendors provide similar capabilities.

Soft failures are defined as those sorts of failures in which the low-level link is operational but some higher level function is not operable.  For example, if a port card in a switch has a firmware crash, the link may still appear to be functioning normally but no useful data transfer can occur.  This is possible since the hardware physical (PHY) chip or chips may still be powered up and "happy" even though the firmware crash prevents the overall port card from operating correctly.  

In this case, the SM-2XXX units will not respond to this type of failure since no link failure has occurred.  This sort of problem, although less likely than a cable/wiring problem (which will cause the SM-2XXX to automatically respond to the failure), can be handled by external management functionality.  One way to implement this sort of functionality is for a network manager application to routinely poll the protected device and in the event that the polling is unsuccessful, use the SNMP management features to force a switchover event to the backup port.  This capability can also be implemented with a simple shell script.

SM-27XX Series Link Protector®s offer the additional capability of idle packet switching.  With this feature, a programmable threshold of received frames can trigger a switchover event.  For example, if a frame is no received in 2 seconds, the link can be considered as "failed" even though the link is still "up" in the Layer 1 sense.