what does a router do with excess traffic on an interface

[carl_townshend]

Hi all

I have a router with 4 meg a second input, this then connects to a 2 meg e1 interface for the output to the wan, would the router try to buffer any of the traffic by default, or would the output queue drop it ?

cheers

[Peter Paluch]

Carl,

That depends very much on the configuration of the egress interface. Can you post it? There are different defaults for various interfaces and their speeds, and they strongly influence the resulting queueing behavior.

In general, however, packets are always queued into the hardware FIFO queue of the interface (also called the Tx queue or Tx ring). If this ring gets full, software queueing may ensue in which the packets are stored into a software queue in the router's RAM, and get dequeued according to the active queueing algorithm. If this software queue is also full, subsequent packets will be dropped.

Best regards,

Peter

[Joseph W. Doherty]

Disclaimer

The   Author of this posting offers the information contained within this   posting without consideration and with the reader's understanding that   there's no implied or expressed suitability or fitness for any purpose.   Information provided is for informational purposes only and should not   be construed as rendering professional advice of any kind. Usage of  this  posting's information is solely at reader's own risk.

Liability Disclaimer

In   no event shall Author be liable for any damages whatsoever (including,   without limitation, damages for loss of use, data or profit) arising  out  of the use or inability to use the posting's information even if  Author  has been advised of the possibility of such damage.

Posting

As Peter explains, when dropping will commence, and what will be dropped, can vary.  However, consistently oversubscribing the egress bandwidth by 2x will usually result in dropping half the packets very quickly.  On the other hand, if the input just occasionally bursts to 2x egress bandwidth you might see many drops or none - "it depends".

[CSCO11460466]

Hello dear Carl

For your question you it deppends of  your configuration.

If you don't have any QoS method the interface will be saturated.

A normal value on an interface cold be like this

reliability 255/255, txload 1/255, rxload 1/255

If the interface is saturated can be like this

reliability 255/255, txload 240/255, rxload 236/255

The router don't buffer the traffic by default, because this can cause very high CPU.

By default serial interfaces use "weighted fair" queue , so this means that the interface is gonna give more priority to small packets, like telnet, ssh,icmp.

The previous explanations was for a device that don't have any QoS configuration.

If you want to prevent that your router be saturated, you can implement QoS configuration by selecting your interesting traffic by an ACL for example an then use it in a "class-map"

The class-map only is for selectiong interesting traffic.

Then you have to configure a policy-map in order to tell the route how to handle this traffic. Maybe you can give some bandwidth, more priority,etc.

The last step is to declare in the interface. It can be Input or Output way.

Example:

ip access-list extended "interesting traffic"

permit tcp 192.168.2.0 0.0.0.255 any

class-map test

match

access-group interesting traffic

!

policy-map traffic

class test

bandwidth percert 40

!

interface s0/0

policy-map traffic output

!

I hope this can help you!!

Best regards