Troubleshooting Direct Serial Connections

    August 31, 2006

A prime topic of your CCNA and CCNP CIT exams will be connecting Cisco routers directly via their Serial interfaces, and while the configuration is straightforward, there are some vital details and show commands you must know in order to pass the exams and configure this successfully in production and home lab networks.

Lets take a look at a sample configuration.

Connecting Cisco routers directly via their Serial interfaces works really well once you get it running – and getting such a connection up and running is easy enough. You can use show controller serial x to find out which endpoint is acting as the DCE, and its the DCE that must be configured with the clockrate command.

R3#show controller serial 1

HD unit 1, idb = 0x11B4DC, driver structure at 0x121868

buffer size 1524 HD unit 1, V.35 DCE cable

R3(config)#int serial1

R3(config-if)#ip address

R3(config-if)#clockrate 56000

R3(config-if)#no shut

Failure to configure the clockrate has some interesting effects regarding the physical and logical state of the interfaces. Lets remove the clockrate from R3 and see what happens.

R3(config)#int s1

R3(config-if)#no clockrate 56000


18:02:19: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state to down

The line protocol doesnt drop immediately, but it does drop. Lets run show interface serial1 to compare the physical and logical interface states.

R3#show int serial1

Serial1 is up, line protocol is down

Physically, the interface is fine, so the physical interface is up. Its only the logical part of the interface – the line protocol – that is down. Its the same situation on R1.

R1#show inter serial1

Serial1 is up, line protocol is down

While a router misconfiguration is the most likely cause of a serial connection issue, thats not the only reason for clocking issues. Ciscos website documentation mentions CSU/DSU misconfiguration, out-of-spec cables, bad patch panel connections, and connecting too many cables together as other reasons for clocking problems. Still, the number one reason for clocking problems in my experience is simply forgetting to configure the clockrate command!

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Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage (, home of FREE CCNA and CCNP tutorials and daily exam questions, as well as The Ultimate CCNA and CCNP Study Packages.

For a FREE copy of his latest e-books, “How To Pass The CCNA” or “How To Pass The CCNP”, and for free daily exam question, visit the website and download your copies!