David

For serial interfaces generally the first indicator reflects basic physical interface connectivity. It looks at the interface and checks pins that have voltage on them. If it finds a set of pins with voltage that correspond to a valid connector type then it declares that the interface is up. The second indicator reflects the ability to transmit on the interface. This is based on successful transmission and receipt of keepalives. If the interface is able to transmit and receive keepalives then it declares that the line protocol is up. And if keepalives are not successful then it declares that the line protocol is down.

So if the line shows as down down then there is a problem with physical connectivity (and it has no ability to transmit and receive). If the line shows up down then we believe that we have physical connectivity but no ability to transmit and receive on the interface. And if the interface shows up up then we have physical connectivity and the ability to transmit and receive on the interface.

So down down indicates a problem with the connection. And up down indicates that we believe that we have a valid connection but no ability to transmit and receive.

HTH

Rick