[cisco-nas] Fragmentation & MTU

[Aaron Leonard]

info at beprojects.com wrote:

> I am trying to figure out what the real difference is between 
> frame-relay fragmentation and setting the MTU on an interface.  If I 
> have a frame connection and I set fragmentation to 500, it will 
> fragment the packets.  If I set the MTU to 500, it will also fragment 
> the packets.  I'm guessing the difference is that Fragmentation is 
> layer2, whereas MTU is layer3.  Is that correct?  


Yep.

Fragmentation and reassembly at L2 - assuming it works
correctly - is better than L3 fragmentation.  Setting an
IP MTU of 500B will tend to adversely impact performance.

> Also, what about the affects on QoS?  If I setup LLQ outbound on the 
> frame interface, I know fragmentation takes place before the LLQ 
> queueing, but what about if I set the MTU.  Will that take place 
> before queueing or is it after and therefore somewhat worthless from a 
> QoS perspective?  Thanks.
>
> Peder


L3 fragmentation will occur prior to queueing.

Aaron