can I start a serial interface?

johnifanx98

CISCO_2513(config)#int s0
CISCO_2513(config-if)#no shut
CISCO_2513(config-if)#end
CISCO_2513#show int s0
Serial0 is down, line protocol is down

Why can I not start the serial 0? This s0 is not connecting to anywhere, so it's expected to see its line protocol is down. But should it show "Serial0 is up" ?

sratakhin

No, you have to connect the serial cable first.

johnifanx98

sratakhin wrote: »

No, you have to connect the serial cable first.

It looks like so. But why I can make e0 up even it does not have a cable connected?

CISCO_2513>show int e0
Ethernet0 is up, line protocol is down

mapletune

For switches, disconnected cables means: "down down"

so... you need to have layer 1 connectivity to get a "up down" at least. And layer 2 connectivity for "up up"

However, I'm not so sure why routers show ethernet interfaces as "up down" when it's only administratively up, but not physically connected.

For serial interfaces though, it's the same as switches. You need to have Layer 1 connectivity at least.

johnifanx98

mapletune wrote: »

For switches, disconnected cables means: "down down"

so... you need to have layer 1 connectivity to get a "up down" at least. And layer 2 connectivity for "up up"

However, I'm not so sure why routers show ethernet interfaces as "up down" when it's only administratively up, but not physically connected.

For serial interfaces though, it's the same as switches. You need to have Layer 1 connectivity at least.

Thank you! At least now I know my router is not going crazy.