FastEthernet transmit error

[Thomas Vådahl]

We have installed a Cisco 2621 with to FastEthernet interfaces on a 50Mbit internet connection. Immediately after installing the router, the traffic was about 20Mbit/s, and the router displayed errors on one of the FastEthernet interfaces:

%AMDP2_FE-3-UNDERFLO: FastEthernet0/1 transmit error

The interface just went up and down, showing this error over and over. We tried setting full duplex on both the switch and the router, we tried changing cables and we tried other switchports. Nothing improved our situation.

A quick look at the switch interface showed that the number of "Runts" increased by 1 mill per second. Yes - we double checked. Not that I know what "Runts" are, but I know I don't want a million of them every second.

The CPU usage was about 50-60%, and "ARP Input" was the process that used most of the CPU-time. "IP Input" had just about 5% of the CPU time. The router is configured to route IP and nothing else, so I would think that "Ip Input" was the most CPU intensive process.

As you might guess, I have a couple of questions:

* Is the 2621 good enough for pushing 20Mbit+ ?

* What is "Runts"?

* Why would the "ARP Input" process use most CPU-time?

Hope you can help!

Best regards,

Thomas Vådahl

[ivillegas · Accepted solution]

Is the 2621 good enough for pushing 20Mbit+ ? You have to select the Router model based on the Packet forwarding performance i.e PPS. And Cisco 2621 has Up to 30Kpps. Based on this you have to check for the performance. And its ideal for your requirement. So at 30Kpps and the packet size is 64 bytes equals - 30x64x8 = 15360 Kbits

What is "Runts"?

Runts - These are frames smaller than 64 bytes with a bad FCS value.

http://www.cisco.com/warp/public/473/53.shtml#datalink

Why would the "ARP Input" process use most CPU-time?

Clearing the ARP cache uses up a lot of resources. You might want to check the following URL for more info on high CPU situations:

http://www.cisco.com/warp/public/63/highcpu.html