Blog / Routing and Switching

Blog

What is Wrong, Unclear, Misleading? Let us know!

We are getting complaints about the questions in our practice tests and this is kind of normal, okay. Recently we got a feedback regarding one particular question and we decided to start this discussion where we are going to analyse those problematic for you questions. We are not going to give you the answers just like that. Instead, we will simulate the case if it is possible or will give you a link to the theory that explains this particular topic.

So, let's bite the bullet and here is the first feedback from our visitor:

"Really poorly formulated questions.
Q: How to enable serial interface?
A: no shutdown; clockrate, plus some more. Should be both, not just one of them."

Do you agree with that?

Luckily we are able quickly to connect two routers R1 and R2 with serial cable and to see the difference in both commands. It is important to mention that R1 is our DCE while R2 is our DTE equipment.

DCE to DTE Connection Example

Both routers are connected and clock rate was set to 64000 on both ends:

R1#show run interface Serial0/0
...
!
interface Serial0/0
 no ip address
 shutdown
 clock rate 64000
end

R2#show run interface Serial0/0
...
!
interface Serial0/0
 no ip address
 shutdown
 clock rate 64000
end

However, as you can imagine our interfaces are still down:

R1#show ip int brie
Interface                  IP-Address      OK? Method Status                Protocol
Serial0/0                  unassigned      YES unset  administratively down down

R2#show ip int brie
Interface                  IP-Address      OK? Method Status                Protocol
Serial0/0                  unassigned      YES unset  administratively down down

Now, let's bring them up with the no shutdown command and let's check the status:

R1(config)#int se 0/0
R1(config-if)#no shu
R1(config-if)#
*Mar  1 10:16:33.579: %LINK-3-UPDOWN: Interface Serial0/0, changed state to up
*Mar  1 10:16:34.583: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0, changed state to up

R2(config)#int se 0/0
R2(config-if)#no shu
*Mar  1 10:16:37.115: %LINK-3-UPDOWN: Interface Serial0/0, changed state to up
*Mar  1 10:16:38.119: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0, changed state to up

Bingo, we are up, up!

There is something important which is not mentioned in the lesson for the serial interfaces. The clockrate command was added as a permanent in the newer IOS versions and it cannot be removed manually. However it will disappear if you connect a DTE cable to the serial interface. Because the serial interfaces cannot be configured with command to work as DCE or DTE (it is Layer 1) this thing is only dependent by the cable. The clockrate command simply sets the clock rate to be used if a DCE-type cable is connected. Otherwise, that command is completely ignored. So, in our example, we can remove the clockrate configuration on R2 (DTE) and the serial interface should be still up, up.

R2#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R2(config)#int se 0/0
R2(config-if)#no clock rate 64000
R2(config-if)#end
R2#
*Mar  1 10:31:45.043: %SYS-5-CONFIG_I: Configured from console by console
R2#sh ip int brie
Interface                  IP-Address      OK? Method Status                Protocol
Serial0/0                  unassigned      YES unset  up                    up

To summarize it - the question has only 1 correct answer and that should be the no shutdown command. Yes, in the older IOS versions there was a problem if you forget to configure the clock rate on the DCE side but then only the protocol was down, not the interface. So even in this case the only command that we need to enable an interface is no shutdown.

Use the comments below to ask about other difficult for you questions.