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New Member

Serial line troubleshooting


I use a serial line to connect a site to another site within a distance of 6 km.

I use cisco 2600. Wen showing interface for this line i see in local interface 2% error, and in the other side 90% errors (crc,frame). This the output i see:

Serial0/0 is up, line protocol is up

Hardware is PowerQUICC Serial

Description: LS aeroport-siege

Internet address is

MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,

reliability 208/255, txload 1/255, rxload 1/255

Encapsulation HDLC, loopback not set

Keepalive set (10 sec)

Last input 00:00:00, output 00:00:00, output hang never

Last clearing of "show interface" counters never

Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 4

Queueing strategy: weighted fair

Output queue: 0/1000/64/0 (size/max total/threshold/drops)

Conversations 0/1/256 (active/max active/max total)

Reserved Conversations 0/0 (allocated/max allocated)

Available Bandwidth 1158 kilobits/sec

5 minute input rate 3000 bits/sec, 2 packets/sec

5 minute output rate 2000 bits/sec, 2 packets/sec

392 packets input, 84985 bytes, 0 no buffer

Received 18 broadcasts, 0 runts, 0 giants, 0 throttles

28609 input errors, 15195 CRC, 12053 frame, 0 overrun, 0 ignored, 1363 abor

Please,i need help.

Thank you.


Re: Serial line troubleshooting

New Member

Re: Serial line troubleshooting

I have already see this document, but if you look at the output, you can see that that imput errors is at 5000% !!!!. Its seems illogical?no?


New Member

Re: Serial line troubleshooting

HI Mourad,

90% of the time these type of issues are within the telcos network. First of all contact your telco and ask them to investigate.

On your side there are essentially three things you could try -

1. Change the cables between the router and the CSU.

2. Use another port on the routers.

3. Swap out the CSU (if its not maintained by the Telco).

You should use loop testing at each stage of your troubleshooting to isolate the fault.

Hope this helps



Re: Serial line troubleshooting

Before you start swapping out hardware , I would make the Lec loop up the smart jack and sress the line , if the see errors it is theier problem , Also find out if there is an extended demarc.

If the Lec is looping the smartjack and testing clean then have them loop the CSU and see what they get , If clean from the same place they are testing from have them loop the other smart jack. if clean have them loop the other CSU , loops are the best tool for isolating the source of errors in a network .

Carriers have a bad habit of testing from the closest DAX and saying the line is clean , in other words they just test a small portion of the circuit. More ofter than not there is no problem with your hardware , it is either a carrier problem or a misconfiguration on a CSU. either case swapping hardware is not the answer.


Re: Serial line troubleshooting

You are assuming the errors are counted as packets; they are not. Framing errors and crc errors are bad packets and are not counted as packets. Instead of calc'ing % calc the ratio of good to bad packets.

New Member

Re: Serial line troubleshooting

Dave - Did you read the first line of my post ???.

You make reference to smartjacks but you should bear in mind that smartjacks are common in N America but not elsewhere in the world. Did it occur to you that Mourad is outside of America ?.

IMHO - If it is not a telco problem (inc csu) then it is most probably a hardware issue.

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