Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

line protocol light

Hi all, what does this light mean, if I have a carrier detect light but no line protocol what does it mean, can the circuit be active if the lp is down? also say if its an isdn line, how would I know its active would the lp be off or on ?


Re: line protocol light

Carrier Detect is a signal that comes on when you have received signal from the remote end CSU or DCE device. It is used for a layer 1 detection of the remote side. You can have Carrier Detect on and not have IP up and running. Remember IP is a layer 3 protocol so you may have the interface shutdown on the router and the layer 1 is up showing Carrier Detect. In your case of the ISDN line if the call is up your BRI card will show the D and B channel green. If you have a PRI line then you cannot view anything physical (except the T-1 layer 1 stats : CD, ALM, Loop), you need to do some show commands (SHOW ISDN STAT, SHOW ISDN SERVICE, SHOW ISDN HISTORY, SHOW ISDN ACTIVE). These commands will show you the current state of your ISDN lines whether a BRI line or a PRI line.... Good Luck...