Topology: R2630 access the ATM network through CBX500. There was only one upstream link of 2M. However, with the growth of services, another ATM upstream link of 155M is added, and the one of 2M is set to active, and the one of 155M to slave.
Failures: Due to the movement of ER, only the link of 2M is connected, and the one of 155M is left. After a while, the one of 2M does not work well, and connect the one of 155M, but it cannot go up.
Undo the Standby sentence, and ATM2/0 goes UP at once.
Check the configurations of R2630, and it is found the interface of 155M (ATM2/0) is set to slave in 2M (Serial 1/0:0), and the sentence isstandby interface ATM2/0. If such a sentence is configured, the ATM2/0 as slave can only go up when the backup interface (Serial1/0:0) is down when both are up. When the backup interface is down, and the physical layer of backup interface is also down, the backup interface cannot work well even though it is connected.