Serial Interface Sync

There serial interface can be in a state where it is up but looped. This means that the circuit has a loop on the other side, but not connectivity to any other point.

In order to correctly read the status of the interface, use the option:

interface Serial0/0
down-when-looped

This will bring the status to:

Serial0/0 is up, line protocol is down (looped)
Hardware is PowerQUICC Serial

When the V.35 Layer 1 connection is actually estabilhed, the HDLC will also sync with the logs:


Serial0: HDLC myseq 3354903, mineseen 3354903*, yourseen 25701, line up
Serial0: HDLC myseq 3354904, mineseen 3354904*, yourseen 25702, line up
Serial0: HDLC myseq 3354905, mineseen 3354905*, yourseen 25703, line up
Serial0: HDLC myseq 3354906, mineseen 3354905, yourseen 25703, line up

May 28 14:12:54: %IP_SNMP-4-NOTRAPIP: SNMP trap source FastEthernet0/0 has no ip address
May 28 14:14:03: %IP_SNMP-4-NOTRAPIP: SNMP trap source FastEthernet0/0 has no ip address
6d03h: Serial0/0: HDLC myseq 53150, mineseen 53149*, yourseen 53150, line up (looped)
6d03h: Serial0/0: HDLC myseq 53151, mineseen 53150*, yourseen 53151, line up (looped)
6d03h: Serial0/0: attempting to restart
6d03h: PowerQUICC(0/0): DCD is up.

After this log, the serial will be in the state:

Serial0/0 is up, line protocol is up