To set redundancy between two 10BASE-T Modules, connect two links between the modules. The redundant link must be connected to a port on the same 10BASE-T Module as the primary link. Use a crossover adapter between each link unless you choose to uncross port 8 on one of the modules to make the connection. Refer to Chapter 1 for information on using crossover mode. Then use the SET PORT <slot.port> MODE REDUNDANT <slot.port> command to specify which port is the primary link and which is the backup link.
Note: If the 10BASE-T Modules are powered down, and then brought back up without the Ethernet Management Module present, a network loop could occur. To prevent a potential failure, 3Com advises that you disable the Port Enable dip switch setting for the backup port on one of the 10BASE-T Modules.
Once redundancy is configured, a switchover will occur under three conditions: a link failure; port partition; or a jabber condition. The switchover occurs when the primary link fails. (Note that in the unlikely event of a partial break in the link, a switchover may not occur. In this situation, use Network Management to manually switch over the ports.)
Once the switchover occurs, and the primary link becomes operational, a switch-over back to the primary link happens automatically if the cause of the original switchover was a link failure. If a jabber condition causes the switchover, the link will not automatically switch back to the primary once the problem is resolved. In this case, use Network Management to manually switch back to the primary link.
Refer to the ONline Ethernet Network Management Module Installation and Operation Guide for information on setting redundancy between 10BASE-T Module ports.
Designing and Expanding the Network 2 - 13