[c-nsp] MTU on GigE

[Douglas E. Warner]

On Wednesday 19 October 2005 04:35, Vincent De Keyzer wrote:
> I was under the impression that the MTU on a GigE interface was higher than
> 1500 bytes. Is it true?

Not necessarily.  These typically default to 1500 to be backwards compatible 
with your standard 10/100 LAN.
Setting your MTU higher on this one gigabit interface will cause your boxes 
with 1500 byte MTUs drop the bigger packets.  Typically you will want to 
setup separate VLANs that has a router in between that will chop up the 
larger packets to deliver to the smaller-MTU'd interfaces.
The feature you're probably looking into is typically called "Jumbo Frames" in 
the switching world, and your switches will need to support them as well.  
These can range from 1500bytes to 9000bytes.  And the reason to use bigger 
frames when you're pushing lots of bandwidth is to save on interrupts.
Of course, if this is a point-to-point link you're setting up, you just need 
to make sure that both ends of the link have the same MTU.  The devices at 
either end will split up the packets when they leave the lower-MTU 
interfaces.  And actually - having a higher MTU on a point-to-point link 
between two devices the router isn't going to construct bigger packets, it 
will just pass the smaller packets along.
Sorry - enough rambling.  Hopefully there was something useful in there.

-Doug

-- 
Douglas E. Warner    <dwarner at ctinetworks.com>     Network Engineer
CTI Networks, Inc.   http://www.ctinetworks.com    +1 717 975 9000
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