topic Re: Two routers connected with a serial line, why they can ping each other without clock rate setting? in Routing and SD-WAN

Two routers connected with a serial line, why they can ping each other without clock rate setting?

ew6DjDBNs48JaZYu3aiwaCrxvYfm2X — Mon, 08 Feb 2021 01:20:55 GMT

I'm pretty new to networking and still learning CCNA stuff. I'm reading a book and as I understand, when you are connecting two routers using a serial line, one router is considered DCE and the other is DTE. The DCE side should set clock rate in order to make them communicate successfully. Is my understanding correct? I did some test in gns3 and it seems my understanding is wrong. Below is the topology I have.

This is the configuration for R1.

R1#sh run | s interface Serial2/0
interface Serial2/0
ip address 192.168.1.1 255.255.255.0
serial restart-delay 0
R1#

This is the configuration for R2.

R2#sh run | s interface Serial2/0
interface Serial2/0
ip address 192.168.1.2 255.255.255.0
serial restart-delay 0
R2#

As you can see, I haven't set clock rate for both of them. But I can successfully ping from both ends.

R1#ping 192.168.1.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/17/36 ms
R1#

R2#ping 192.168.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/13/32 ms
R2#p

Is there anything wrong with my understanding or my testing?

Re: Two routers connected with a serial line, why they can ping each other without clock rate setting?

Giuseppe Larosa — Mon, 08 Feb 2021 08:04:58 GMT

Hello @ew6DjDBNs48JaZYu3aiwaCrxvYfm2X ,

your understanding is correct in real world with real cables , the DCE side of the cable can and must provide the clock rate.

This can be checked usually with show controller serial x/y to see the cable type connected to the router interface.

What you see in GNS3 is a known limitation of the emulation software that makes life a little easier then in real world.

Hope to help

Giuseppe

Re: Two routers connected with a serial line, why they can ping each other without clock rate setting?

ew6DjDBNs48JaZYu3aiwaCrxvYfm2X — Mon, 08 Feb 2021 09:08:58 GMT

Thanks for the clarification. In real world with real cables, if I remove clock rate on the current DCE end and enable it on the current DTE end, will the connection still work as expected?

Re: Two routers connected with a serial line, why they can ping each other without clock rate setting?

Giuseppe Larosa — Mon, 08 Feb 2021 09:43:50 GMT

Hello @ew6DjDBNs48JaZYu3aiwaCrxvYfm2X ,

no in real world the clock rate command is accepted only on the DCE side, if you try to give it on the DTE side you should get an error message.

This is the reason why we need to check what type of cable is actually connected to the interface.

Hope to help

Giuseppe

Re: Two routers connected with a serial line, why they can ping each other without clock rate setting?

Georg Pauwen — Mon, 08 Feb 2021 09:54:53 GMT

Hello,

it won't work. The clock has to be configured on the DCE end. I don't think you can even use the clock rate command on a DTE end, it will generate an error message.