User's Manual 144 Document #: LTRT-89730
Mediant 3000
13.2 Ethernet Interface Redundancy
The device supports an Ethernet redundancy scheme. At the beginning of the start-up
procedure, the device tests whether the ‘primary’ Ethernet interface is connected, by
checking the existence of the Ethernet link carrier. If it's connected, the start-up procedure
commences as usual. If not, the start-up application tries the ‘secondary’ Ethernet
interface. If this interface is connected, the whole start-up procedure is performed using it.
If both interfaces are not connected, the start-up procedure commences using the
parameters, tables, and software residing on the device's non-volatile memory. Note that
Ethernet switchover occurs only once during the start-up procedure (at the beginning). If
the Ethernet interface fails after the selection is made, the device does not switch over to
the second port.
After start-up is complete and the operational software is running, the device continues to
use the Ethernet port used for software upload. The device switches over from one
Ethernet port to the other each time an Ethernet link carrier-loss is detected on the active
Ethernet port, and if the Ethernet link of the other port is operational. Switchover occurs
only once per link loss (i.e., the ‘secondary’ interface stays the active one even if the
‘primary’ interface has returned to life). After start-up, the device generates a gratuitous
ARP message each time a switchover occurs.
For correct functionality of the redundancy mechanism, it's recommended to configure both
links to the same mode. It is essential that both link partners (primary and secondary) have
the same capabilities. This ensures that whenever a switchover occurs, the device is able
to provide at least the same Ethernet services as were provided prior to the switchover. In
addition, it's recommended to set the physical secondary link prior to resetting the device
(since the MAC configuration cannot be changed thereafter).
Note that since the two Ethernet ports use the same MAC address, the external switches
connected to the device can in some cases create a noticeable switchover delay due to
their internal switching logic, though at the device level, the switchover delay is minimal
(milliseconds).
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