23.3.1 Stanby link capability

(1) Description

The standby link functionality replaces a faulty port with a standby port in the same channel group to maintain the number of active ports in the channel group. This functionality can prevent a reduction of available bandwidth if a fault occurs.

This functionality is available only when static link aggregation is used.

(2) How to select a standby link

The maximum number of active ports in a channel group is set in the configuration. The rest of the ports in the channel group are standby ports.

Standby ports are determined first by the port priority set in the configuration, then by the port number, and then by the highest selection priority. In other words, if the port priority is the same, the port number is used to determine the port priority. The criteria for determining the standby ports are listed below, in order of highest selection priority.

1. Port priority

   Selected as a listening port from a port with a higher priority.

2. Port number

   Selected as the standby port in descending order of port number.

The figure below shows an example that explains how the standby link functionality works. In this example, four ports belong to a channel group, and the maximum number of active ports is three.

Figure 23-3: Example of the standby link facility configuration

(3) Standby Link Mode

The standby link functionality has the following two modes:

• Link-down mode

  In link-down mode, the status of the standby links changes to link-down. Ports on the remote device that do not support the standby link functionality can also be used as standby ports.

• Link-not-down mode

  In link-not-down mode, sending from standby links stops, but the status of the standby links does not change to link-down. Because the standby links are in the link-up status, monitoring of faults can also be performed for these standby ports. Note that in this mode, standby ports do not send data, but can receive data. A device that does not support the standby link functionality does not detect the link-down status on the partner device, and can continue sending to the partner device. In link-not-down mode, connecting to the device is possible.

In link-down mode, if there is only one active port in a channel group and a fault occurs on that port, the channel group is temporarily shut down when the faulty port is replaced with a standby port. In link-not-down mode, the standby port replaces the faulty port without the channel group shutting down.

The status of a port in operation is one of the following:

• The maximum number of active ports is set to 1 by using the max-active-port configuration command.

(4) Considerations for Stacking

When the standby link functionality is used in the link-down mode and when a port on the backup switch side is selected as the standby port, if a failure occurs in the master switch or the stack link which in turn causes the backup switch to become a new master switch, the standby port remains down. In this case, use the activate operation command to activate the port.