Question about Serial Interface Output

Vask3n

Hi, I recently took the ICND2 exam and have a question about my experience with one of the questions, I will do my best to phrase this question in a way that does not give the question away since I know that is not OK.

Basically as an example look at this output which is from the offical cisco documentation found on this link
Troubleshooting Serial Line Problems  [T1/E1 & T3/E3] - Cisco Systems

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Router# show interfaces serial
Serial 0 is up, line protocol is up
Hardware is MCI Serial
Internet address is 150.136.190.203, subnet mask is 255.255.255.0
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 1/255
Encapsulation HDLC, loopback not set, keepalive set (10 sec)
Last input 0:00:07, output 0:00:00, output hang never
Output queue 0/40, 0 drops; input queue 0/75, 0 drops
Five minute input rate 0 bits/sec, 0 packets/sec
Five minute output rate 0 bits/sec, 0 packets/sec
16263 packets input, 1347238 bytes, 0 no buffer
Received 13983 broadcasts, 0 runts, 0 giants
2 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 2 abort
1 carrier transitions
22146 packets output, 2383680 bytes, 0 underruns
0 output errors, 0 collisions, 2 interface resets, 0 restarts

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So let's say we have this output from our show interface serial 0/0 right?

Well if we look at the 5 input averages for input and output, they are both 0. To think out loud, to me this seems to indicate that there is an encapsulation mismatch or lack of connection between the two ends of this serial link, but this is what I am asking for some help.

The encapsulation listed is HDLC, but that is only for THIS end of the serial link, so if we do not have the output from the other end we have no way of knowing if there is a mismatch, so what does an average of 0 indicate in this scenario, or at least what are some possibilities?

Vask3n

Just as a side note, I highly recommend these "Troubleshooting" documentations from Cisco, like Troubleshooting Serial Line problems or Troubleshooting VTP because they basically give you every detail you need to know about interpreting output and the protocols.

NetworkVeteran

Encapsulation HDLC, loopback not set, keepalive set (10 sec)

Since you have keepalives enabled, if there were an encapsulation mismatch I'd expect the line to go down within 30 seconds. If you can't wait that long, I'd use 'debug chdlc slarp' (ios-xr) or the equivalent IOS 'debug serial' command. If you'd like to see the behavior when one side has cHDLC and the other side PPP, I'd lab it up using GNS3 or real devices. I can't remember making or seeing that particular mistake in the recent past; there may be better indicators when it happens.

TheNewITGuy

well the line is up and protocol is up so there is no encapsulation issue.

The only indicator of any issues is the carrier reset and the interface resets..

Last input 0:00:07, output 0:00:00, output hang never < -- input but not sending traffic, so this means we're receiving data from the far end but not sending any..