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Konstantin Dunaev

cisco 2621 and UDP traffic


we've got a strange problem with our c2621XM. It's connected to a 5 Mbit WAN (ethernet) and a normal TCP traffic (WWW, FTP and so on) doesn't produce any problem - we got full 5 Mbit.

But there is an UDP streaming application on the LAN side and if it starts to send an UDP traffic we get on the LAN interface many input  Ignored and Overrun errors, and CPU goes high upto 100%.

We've set the LAN interface on both sides to manually  10Mbit/Full, but without any result - still too many errors and CPU is too high.

Does c2600 process an UDP traffic differenttly? I can't really remember it and haven't see it before.

Peter Paluch
Hall of Fame Cisco Employee

Hello Konstantin,

I wonder - are there any services running on the C2600 that would need to perform additional tasks over the UDP streams, like:

  • NAT
  • ACL / IP Inspect / ZBFW
  • IP virtual reassembly
  • IP packet fragmentation (very likely! - note that TCP may decrease the window size, avoiding the fragmentation while UDP is incapable of doing that)

Would you also mind checking the show proc cpu sorted with the UDP stream on and checking what process is consuming most of the CPU time? Is it IP Input?

Best regards,


Hello Peter,

thank you for responce.

on c2600 there are no any NAT, ACL or FW or other "funny" features. It does nothing except routing from LAN to WAN.

Virtual reassambly is not configured as well.

MTU could be an issue (I've doublechecked  - it's not really an Ethernet,  it's a DSL ) but we've set MTU 1000 on the server directly.

I'll try to get the "show proc cpu sort" during the streaming.

Hi Peter,

thank you again for the advice, it's realy some kind of MTU issue. I've configured cache-flow on the LAN interface and found out, that all UDP traffic comes with a packet length 1536 bytes, which too big for sure. It seems that server admin did not realy changed the MTU size on the server.