How to Check for a Down Link by Using the Ip-link Command?

Publication Date:  2012-07-17 Views:  188 Downloads:  0
Issue Description
Topology: USG5320 (port 1: 1.1.1.2)--------Layer-2 switch--------China Telecom device (1.1.1.3)
USG5320 (port 2: 2.1.1.2)--------Layer-2 switch--------CNC device (1.1.1.3)
The USG5320 is connected to the China Telecom device through a Layer-2 switch. You are required to check whether a switchover to the link between the USG5320 and the CNC device is performed when the link between the USG5320 and the China Telecom device is down.
Note: The previous addresses are for example only.
Alarm Information
None.
Handling Process
Run the ip-link command on the USG5320 to check a specified IP address by ICMP or ARP (by default, ICMP). If the command finds that the address is inaccessible, the USG5320 considers that the link to the China Telecom device is down.
Note: This command can check only addresses on the same network segment as the firewall port. Moreover, the address to be checked must be the gateway address.
On the USG5320, run the ip-link command to check whether the IP address, 1.1.1.3, of the China Telecom device is reachable. If no, the static routing regards that this link is down (two default routes are configured) and switches to the link to the CNC device.
ip-link check enable
ip-link  1 destination  1.1.1.3
Root Cause
Fault 1: The USG5320 can detect a connection failure between itself and the Layer-2 switch.
Fault 2: The USG5320 cannot detect a connection failure between the Layer-2 switch and the China Telecom device, because the link between the USG5320 and the Layer-2 switch is up.
Suggestions
If the switch between the USG5320 and the China Telecom device is a Layer-3 switch and the firewall interface address and the China Telecom device address are not on the same network segment, do not use the ip-link command to check the gateway address.

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