What could be possible reason of Line Protocol Down?

[Ken Lee]

Hi Experts,

What will be the possible reason of "Line Protocol Down" when your telco has tested end to end layer 1 is good ?

As Line protocol is more on layer 2, The telco tells me that is Router to Router Connection that giving the problem but I have ensure that the IP address /30 WAN IP are configured correctly. Not sure any other things I need to do. The  Sh int and Sh run int s4/0 as below:

"Serial4/0 is up, line protocol is down
  Hardware is M4T

MTU 1500 bytes, BW 128 Kbit, DLY 20000 usec,
    reliability 255/255, txload 1/255, rxload 1/255
Encapsulation HDLC, crc 16, loopback not set
Keepalive set (10 sec)
Restart-Delay is 0 secs
Last input never, output 00:00:00, output hang never
Last clearing of "show interface" counters 4d23h
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
    0 packets input, 0 bytes, 0 no buffer
    Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
    0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
    16 packets output, 1254 bytes, 0 underruns
    0 output errors, 0 collisions, 4 interface resets
    0 unknown protocol drops
    0 output buffer failures, 0 output buffers swapped out
    4 carrier transitions     DCD=up  DSR=up  DTR=up  RTS=up  CTS=up

......

Router A:

interface Serial4/0

bandwidth 128
ip address 172.50.10.9 255.255.255.252
no ip redirects
no ip unreachables
no ip proxy-arp
ip nat inside
ip virtual-reassembly
ntp disable
no fair-queue
serial restart-delay 0
no cdp enable
end

Router B:

interface Serial2/0

bandwidth 128
ip address 172.50.10.10 255.255.255.252
no ip redirects
no ip unreachables
no ip proxy-arp
ip nat inside
ip virtual-reassembly
ntp disable
no fair-queue
serial restart-delay 0
no cdp enable
end

Thanks!

KA

[sean_evershed · Accepted solution]

Hi, See below a troubleshooting guide for serial lines:

http://www.cisco.com/en/US/docs/internetworking/troubleshooting/guide/tr1915.html

As it suggests a couple of things you can try are:

- Configure a clock rate.

- Checking the cabling.

- Check the output from the show controllers command.

Please remember to rate all posts that are helpful.

[paolo bevilacqua]

Your circuit is faulty. Telco can claim anything they want, but is faulty. Escalate and do not pay until is fixed.

[Latchum Naidu · Accepted solution]

Hi,

There are many reason for the line protocol down.

1. The cable connected from the router may be Bad.
2. The CU is not sending the clock. Check whether all the pins such as DCD, DSR, DTR,RTS,CTS are UP.
3. There should be same config for Keepalive at both the ends. Either no keepalive or keepalive set
4. The encapsulation type should be same at both the ends.
5. The cable is not connected at the remote end.

Also make sure the the interfaces settings are same at your end and the provider end interfaces.

Please rate the helpfull posts.
Regards,
Naidu.

[ciscoben2009]

i would say it is your ISP if you do a show controllers it should confirm if you are getting clocking from the ISP if you are not then that should help prove to them that the problem is with them

[Ken Lee]

Thanks all for your input!!

The correct answer is the Serial Cable problem. Once changed, it worked like a charm!!

Have rated the answers for Sean and Naidu. Thanks guys!

Regards,

Ken

[sean_evershed]

Thanks for the rating Ken. Good to see that it was an easy problem to fix.

Cheers

Sean