Cisco CCNA, CCENT, And CCNP Lab Exercise:

Troubleshooting Point-To-Point Connections

By Chris Bryant, CCIE #12933

Sometimes the simplest configuration can have the most potential problems. That's particularly true of a direct connection between two Cisco routers, like the one we see here.

After configuring the IP addresses as shown, we tried to send some pings from R1 to R3, and they didn't go through. Running show interface serial1 yielded this output:

R1#show interface serial 1
Serial1 is administratively down, line protocol is down
  Hardware is HD64570
  Internet address is 172.12.13.1/24
  MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation HDLC, loopback not set

What's the problem? And what do we need to do to resolve the problem?

This fix is a basic one, but it's one we all forget on occasion - the phrase administratively down indicates an interface that needs to be opened!

The command no shutdown will take care of that - we'll run it here on both the serial interface on R1 and R3, just in case they're both shut down!

R1(config)#int s1
R1(config-if)#no shutdown

R3(config)#int s1
R3(config-if)#no shutdown
R3(config-if)#^Z
R3#
2d05h: %SYS-5-CONFIG_I: Configured from console by console
R3#
2d05h: %LINK-3-UPDOWN: Interface Serial1, changed state to up
R3#
2d05h: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state to up

Everything looks great after we opened the interfaces - I'm only showing the console output on R3, but we would have seen both the physical and the logical (the "line protocol") start of the interface go to up. So we can go to lunch now, right?

Well.... maybe. I'm always telling students that you should always stick around for a minute or so when you see the line protocol go up, because it's not always going to stay up. And as we see less than a minute later, this is one of those times!

2d05h: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state to down

Solving one problem - the "administratively down" interface - has led to a new problem, and this one's not quite so simple.

What exactly is the problem, and how can we solve it?

What command will we need to run on at least one of the routers to know where to start? Find out after this brief advertisement for the world's most effective and comprehensive CCNA and CCENT study tool - The Ultimate CCNA Study Package!

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The DTE, DCE, And The clockrate Command

A Cisco router serves as a DTE by default, but when they're directly connected via their Serial interfaces as they are here, one of them must serve as the DCE - and it's the DCE that must supply a clockrate to the DTE.

Fair enough! But how do we find out which interface is the DCE? We've got a DTE/DCE cable connecting the routers, and one end of the cable should be clearly marked "DTE" and the other "DCE" - but in some crowded router cabinets, it's hard to see that. Besides, the CCNA and CCENT exams probably won't give you that option!

The command show controller will give us the information we need, and a lot more! I'll run this command on both R1 and R3, editing the output on R3. The information we need is near the top of the output, so don't panic when you see how much information this command returns.

Here's everything you get with this command....

R1#show controller serial 1
HD unit 1, idb = 0x1DBFEC, driver structure at 0x1E35D0
buffer size 1524 HD unit 1, V.35 DTE cable
cpb = 0xE3, eda = 0x2940, cda = 0x2800
RX ring with 16 entries at 0xE32800
00 bd_ptr=0x2800 pak=0x1E4E90 ds=0xE36108 status=80 pak_size=0
01 bd_ptr=0x2814 pak=0x1E515C ds=0xE367C4 status=80 pak_size=0
02 bd_ptr=0x2828 pak=0x1E5428 ds=0xE36E80 status=80 pak_size=0
03 bd_ptr=0x283C pak=0x1E56F4 ds=0xE3753C status=80 pak_size=0
04 bd_ptr=0x2850 pak=0x1E59C0 ds=0xE37BF8 status=80 pak_size=0
05 bd_ptr=0x2864 pak=0x1E5C8C ds=0xE382B4 status=80 pak_size=0
06 bd_ptr=0x2878 pak=0x1E5F58 ds=0xE38970 status=80 pak_size=0
07 bd_ptr=0x288C pak=0x1E6224 ds=0xE3902C status=80 pak_size=0
08 bd_ptr=0x28A0 pak=0x1E64F0 ds=0xE396E8 status=80 pak_size=0
09 bd_ptr=0x28B4 pak=0x1E67BC ds=0xE39DA4 status=80 pak_size=0
10 bd_ptr=0x28C8 pak=0x1E6A88 ds=0xE3A460 status=80 pak_size=0
11 bd_ptr=0x28DC pak=0x1E6D54 ds=0xE3AB1C status=80 pak_size=0
12 bd_ptr=0x28F0 pak=0x1E7020 ds=0xE3B1D8 status=80 pak_size=0
13 bd_ptr=0x2904 pak=0x1E72EC ds=0xE3B894 status=80 pak_size=0
14 bd_ptr=0x2918 pak=0x1E75B8 ds=0xE3BF50 status=80 pak_size=0
15 bd_ptr=0x292C pak=0x1E7884 ds=0xE3C60C status=80 pak_size=0
16 bd_ptr=0x2940 pak=0x1E7B50 ds=0xE3CCC8 status=80 pak_size=0
cpb = 0xE3, eda = 0x3000, cda = 0x3000
TX ring with 1 entries at 0xE33000
00 bd_ptr=0x3000 pak=0x000000 ds=0x000000 status=80 pak_size=0
01 bd_ptr=0x3014 pak=0x000000 ds=0x000000 status=80 pak_size=0
0 missed datagrams, 0 overruns
0 bad datagram encapsulations, 0 memory errors
0 transmitter underruns
0 residual bit errors

... and here's everything you need for this particular situation:

R3#show controller serial 1
HD unit 1, idb = 0x101150, driver structure at 0x1065E8
buffer size 1524 HD unit 1, V.35 DCE cable

Now that we know R3's serial1 interface is the DCE in this connection, we can apply the clockrate command and watch the line protocol come up!

R3(config)#int serial1
R3(config-if)#clockrate 56000

01:00:51: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state
to up

We'll run show interface on both routers to verify the line protocol status, and then run pings to verify connectivity - and we're all done!

R1#show interface serial 1
Serial1 is up, line protocol is up
Hardware is HD64570
Internet address is 172.12.13.1/24

R3#show interface serial 1
Serial1 is up, line protocol is up
Hardware is HD64570
Internet address is 172.12.13.3/24

R1#ping 172.12.13.3

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.12.13.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 32/35/36 ms

R3#ping 172.12.13.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.12.13.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 32/35/36 ms

It's a simple config, but there are a lot of details when you're working with directly connected Cisco routers. Use your show controller and clockrate commands correctly, and you'll have that line protocol up and running in no time at all!

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