Cisco CCNA Certification Exam: Direct Serial Connections

By: Chris Bryant

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. Let's 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 it's 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 172.12.13.3 255.255.255.0


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.  Let's remove the clockrate from R3 and see what happens.

R3(config)#int s1


R3(config-if)#no clockrate 56000


R3(config-if)#


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

The line protocol doesn't drop immediately, but it does drop.  Let's 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.  It's only the logical part of the interface - the line protocol - that is down.  It's 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, that's not the only reason for clocking issues.  Cisco's 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.  StillFree Web Content, the number one reason for clocking problems in my experience is simply forgetting to configure the clockrate command!

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