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Overruns counter in the show interfaces command output is increasing

Core Issue

If the capacity of the interface is exceeded, the frame that is currently being  received is dropped and the overrun counter is incremented.

Each network interface on Cisco routers consists of a chipset for converting signals received in the media into bits of information, and a small packet buffer into which this information is stored before being copied into the I/O memory. On some interface types, this chipset and packet buffer cannot handle a long burst of frames. Such interfaces are meant to provide connectivity to a certain network type, and not to switch packets at line rate. The line rate of these interfaces is often higher than the switching capacity of the router. Therefore, building an interface that receives more traffic than the router can handle only increases the cost, without adding any real value to the router architecture.

Some examples of connectivity interfaces include PA-GE on c7200 routers, GEIP on c7500 routers, and FastEthernet interfaces on c3620 and c3640 routers.

Resolution

In a small number of cases, the overrun counter may be incremented because of a software defect. However, in the majority of cases, it indicates that the receiving capability of the interface was exceeded. Nothing can be done on the router that reports overruns. If possible, the rate that frames are coming should be controlled at the remote end of the connection. Otherwise, if the number of overruns is high, the hardware should be upgraded. 

Note: If the router drops packets because of performance problems of the main CPU (and not because of the capacity of the interface itself), the ignored counter is increased, and not the overruns counter.

Version history
Revision #:
1 of 1
Last update:
‎06-22-2009 03:35 PM
Updated by:
 
Comments
Cisco Employee

This document applies to the times when "Process Switching" was the primary means of switching.

 

CEF replaced process switching almost 14 years ago (since early 2000). When viewed in current scenario where CEF is the default switching mechanism, the contents will lead to mus-understanding.

 

Examples cited belong to hw that are way past "End of Life".
 

 

 

 

Please do not come to any conclusion for handling inputErrors (ignores and overruns kind).

I found this issue on 7609 IOS version 12.2(33)SRD1, how should I do? For increase queue, I think it’s not a root cause.

 

7609s-1#sho int te1/1 | inc cou|ove|dro
  Last clearing of "show interface" counters 00:05:00
  Input queue: 0/2000/70893255/0 (size/max/drops/flushes); Total output drops: 0
     34 input errors, 0 CRC, 0 frame, 23947205 overrun, 0 ignored

 

After I configured for increase interface queue:

 

7609s-1(config-if)#hold-queue ?
  <0-4096>  Queue length

 

7609s-1(config-if)#hold-queue 4096 in

 

7609s-1#sho int te1/1 | inc cou|ove|dro
  Last clearing of "show interface" counters 00:06:16
  Input queue: 0/4096/4439847/0 (size/max/drops/flushes); Total output drops: 0
     41 input errors, 0 CRC, 0 frame, 4439806 overrun, 0 ignored

 

7609s-1#sho mod
Mod Ports Card Type                              Model              Serial No.
--- ----- -------------------------------------- ------------------ -----------
  1    4  CEF720 4 port 10-Gigabit Ethernet      WS-X6704-10GE      xyz

Mod  Sub-Module                  Model              Serial       Hw     Status
---- --------------------------- ------------------ ----------- ------- -------
  1  Centralized Forwarding Card WS-F6700-CFC       xyz  4.1    Ok

 

Thank you very much.