Number of interface drops normal?

[GeneralDzur]

Setup is:

ISP-->modem-->(10baseT)Router(100baseT)-->switch


When I do a 'sho int fa0/0' I get this:


'Output queue 0/40, 0 drops; input queue 4/75, 168 drops'

This is from 3 days.

Just recently I changed the Queue method on the ISP-facing interface (e1/0) to Weighted Fair. Is the number of drops I'm seeing on FA0/0 normal? Would it help to alleviate this to enable WFQ and/or Weighted Early Random Detect?

- stephan

 

[jneiberger]

WFQ is an outgoing queueing mechanism and has no bearing on drops in the input queue. If your input queue is full or overflowing, that means that the interface is receiving more traffic than it can process.

 

[GeneralDzur]

Is there anything I can do to alleviate the problem? WRD maybe, or would that require Custom Queueing to be setup?

- stephan

 

[jneiberger]

There's not a lot that you can do. You're thinking in the wrong direction. The techniques you're mentioning would alleviate *outbound* congestion, not inbound.

Are you talking about the 10BaseT interface facing the ISP? If so, then you might try talking to them to see if they can rate limit their output so that it will never overwhelm your input interface.

I'd have to do some homework but I suppose it might also be possible that frames are backing up in the input queue because your router has no place to send them or is having problems sending them toward their destination. If that's the case then you might do a little investigation to make sure traffic is exiting your router correctly.

Still, my best guess is that the ISP is just sending more traffic than your input interface can handle.

 

[GeneralDzur]

The overloaded interface in this case is the LAN-facing Fast Ethernet (fa0/0).

I guess it's because it's taking a 100Mb link with over 250 users and trying to cram their data out a 10Mb-to-2Mb connection?

- stephan

 

[jneiberger]

Oh, that may be the case, then. I thought you were talking about the ISP-facing interface. If your users are connected to the switch in your diagram then I don't think there is much you can do.

You might be able to configure inbound Committed Access Rate but that doesn't really buy you anything. It just moves the "dropping" process further into the router by letting CAR do it instead of dropping the excess traffic at the input queue.

If your router is capable of CAR, it might be interesting to configure it to see if the input drops stop. I'm not sure that they will. I don't remember enough about the interface architecture and input queues. It would be an interesting experiment if you feel brave, though.

 

[GeneralDzur]

I feel brave, but the question is whether or not my boss feels brave enough to LET me be brave. The router in question is our production router, with no backups.

Thanks for your thoughts

- stephan