Configuring Serial Ports in Cisco 2500-series Routers

So, I was trying to connect my 2507 and 2501 routers (DCE is connected to 2507) but something interesting came up today.
I tried to connect the two routers through serial ports (DCE/DTE) but ran into a (trivial) problem.

First, as we know when we want to bring up an interface, then we issue “no shutdown” command in the interface mode configuration. We also know that
show int s0 will show us the information
about the serial port 0.

But the result came up the following: Serial0 is down, line protocol is down.
Several things came into my mind:

1. This problem had anything to do with the cable problem. (I checked the cable! everything was good).
2. The other router was off. (I checked it again and it was on. Again everything was good).
3. Different IOS version, perhaps?? But this is too silly. I know my routers are old, but even then I don’t think those Cisco people are that dumb.
Just because two routers have different IOS, then they cannot connect?! That would be ridiculous. Mmmhh… maybe that’s why Juniper are bragging about
their single version of IOS.

Then I tried to shutdown the interface in my 2507 by issuing shutdown and issued show int s0 to check the interface. And I got: Serial0 is administratively down, line protocol is down.

I had no idea what administratively down really meant.
I said to myself just be patient and keep reading the book (I am using Lammle’s btw). And… Aha! It’s there.

Here are some good points from the book regarding connecting routers through serial ports:

– If we find:

Router#sh int s0
Serial0 is up, line protocol is down

then:
1. It is considered as a Data Link layer problem
2. Synchronize the routers. Check the “keepalive” and/or “encapsulation” of the routers. Make sure they match.
3. Make sure that the clock rate is set (you have to set the clock rate from the router that is connected to the DCE).

– If we find both routers:

Router#sh int s0
Serial0 is down, line protocol is down

then:
1. It is considered as a Physical layer problem
2. Check the DTE/DCE cable, make sure it is connected properly
3. Have we issued no shutdown?

– If we find one router:

Router#sh int s0
Serial0 is administratively down, line protocol is down

and the other router:

Router#sh int s0
Serial0 is down, line protocol is down

then:
1. Synchronize the routers. Check the “keepalive” and/or “encapsulation” of the routers. Make sure they match.
2. Make sure that the clock rate is set (you have to set the clock rate from the router that is connected to the DCE).

That’s it!

One thought on “Configuring Serial Ports in Cisco 2500-series Routers

  1. i ve connected three 2500 series routers R1 pinging is successes to R2 but R2 to R3 is not successes.i have connected R2 to R3 through s1 to s0 with the following networks s1 192.168.10.5 255.255.255.252 and s0 192.168.10.6 255.255.255.252.i have also configure clockrate to this interface because its a DCE side.So how can i troubleshoot this problem,

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