When Directly Connected Serial Interfaces Don't Function

By Galen Bass

Ccna exam success depends for the most part on noticing the details, and this is in particular genuine of configurations involving directly connected serial interfaces.

And naturally, it's not sufficient to observe these details - you've got to acknowledge what to do in regards to them! A cisco router is a dte by default, but directly connecting two dtes with a dce/dte cable is not sufficient. In the next example, r1 and r3 are directly connected at their serial1 interfaces. The line goes up briefly after being opened, but the line protocol goes down after in regards to 30 seconds. R3(config-whether or not)#int s1 R3(config-whether or not)#ip address 172. 12.

13. 3 255. 255. 255. 0 R3(config-whether or not)#no shutdown 2d18h: %link-3-updown: interface serial1, changed state to up 2d18h: %lineproto-5-updown: line protocol on interface serial1, changed state to up R3(config-whether or not)# 2d18h: %lineproto-5-updown: line protocol on interface serial1, changed state to down The problem is that one of the routers needs to act as the dce in order for the line protocol to come up and remain up.

Whether or not this were your ccna / ccnp home lab, you could just go over and consider the dte/dce cable to see which router had the dce end of the cable attached. In this example, although, we don't have physical admittance to the routers. How may we tell which router has the dce end of the cable attached? R3#show controller serial 1 Hd unit 1, idb = 0x1c44e8, driver structure at 0x1cbac8 Buffer size 1524 hd unit 1, v. 35 dce cable The show controller command gives us this info. (there's many more output that this with this command, but it's insignificant for our intentions.

) the router with the dce end of the cable needs to supply a clock rate to the dte, and we'll do just that with the interface-level clockrate command. R3#conf t Enter configuration commands, one per line. End with cntl/z. R3(config)#int serial1 R3(config-whether or not)#clockrate 56000 2d18h: %lineproto-5-updown: line protocol on interface serial1, changed state to up In just a couple of seconds, the line protocol goes up and stays up. When troubleshooting a connection, at all times run show interface initial.

Whether or not you see the combining shown beneath, the connection is physically fine but logically down. That's in general the effect of a necessitated keepalive not being present. With frame relay, it's in all probability an lmi issue, but with directly connected serial interfaces the issue is probably the dce end of the connection not offering clockrate. R3#show interface serial 1 Serial1 is up, line protocol is down Troubleshooting is a huge portion of the occupation, and it's a huge portion of the cisco ccna and ccnp programs also. Acknowledge your show and debug commands and you're on your way to passing the ccna!.

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