How to prevent the Output drops in cisco router serial interface

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Jan 12th, 2010
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Hi,


I am very new to the support community, I am facing one issue, in my router serial interface output drops are more,  link speed is 2 Mbps, even in off peak hours also output drops I can see, Can some one give me a solution to the same apart from increasing the bandwidth.


Thanks in advace.


Ani John

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Reza Sharifi Tue, 01/12/2010 - 08:25
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Hello Ani,


You may have a bad cable or problem with your CSU/DSU.



If input errors are accumulating on both ends of the connection, clocking of the CSU is the most likely problem.

If only one end is experiencing input errors, there is probably a DSU clocking or cabling problem.


Please refer to this document for more info on how to troubleshot serial interfaces:


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


HTH

Reza

paolo bevilacqua Tue, 01/12/2010 - 15:26
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You may have a bad cable or problem with your CSU/DSU.


Incorrect, oputput queue drops are only and exclusively cause by congested interface.

Giuseppe Larosa Tue, 01/12/2010 - 10:23
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Hello Ani,

from a telnet session you can copy the text and you can paste it in a text file making reading easy even if you use OS shell you can do this.


about your problem you may need to add outbound shaping  because you can have burst of traffic that fills output queue and that causes the output  drops.


Hope to help

Giuseppe

Phillip Remaker Tue, 01/12/2010 - 10:35
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Drops are a normal reaction to link overloading - are you sure you don't have any traffic running around the clock that is naturally bursty (e.g. Bittorrent)?


It might be interesting to enable Netflow and analyze the traffic patterns.

rigoel Tue, 01/12/2010 - 12:08
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Hey Ani,


Output drops are caused by a congested interface. For example, the traffic rate on the outgoing interface cannot accept all packets that should be sent
out. The ultimate solution to resolve the problem is to increase the line speed. However, there are ways
to prevent, decrease, or control output drops when you do not want to increase the line speed.
You can prevent output drops only if output drops are a consequence of short bursts of data.
If output drops are caused by a constant high-rate flow, you cannot prevent the drops.


Here is a reference link for the same:


Troubleshooting Input Queue Drops and Output Queue Drops
http://www.cisco.com/en/US/products/hw/routers/ps133/products_tech_note09186a0080094791.shtml


However, please check that you are not getting Layer 1 errors like Input Errors, CRCs, aborts on the Serial Interface itself.

If we are getting that, it would indeed cause the drops to accelerate.

In case, if the Layer 1 errors are coming up , try the following:


#Swapping the Cable with a good known one

#Swapping off the WIC card in question.


Hope that helps!

Rishika Goel

paolo bevilacqua Tue, 01/12/2010 - 15:25
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However, please check that you are not getting Layer 1 errors like Input Errors, CRCs, aborts on the Serial Interface itself.

OP included show interface, there are no layer 1 errors.


If we are getting that, it would indeed cause the drops to accelerate.

Incorrect, layer 1 errors DO NOT cause queu ouput drops


In case, if the Layer 1 errors are coming up , try the following:

#Swapping the Cable with a good known one

#Swapping off the WIC card in question.

Incorrect, the most common cause of layer 1 errors is faulty circuits, not router cards.

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