[c-nsp] Large MTU on catalyst switch

[Gert Doering]

Hi,

On Mon, May 28, 2007 at 05:30:16PM +0200, Mark Pace Balzan wrote:
> Given that I only need some interfaces at 1504 and the rest at 1500, Im
> kind of stuck.   What are the ill effects of having an MTU at 1504 on
> the switchport, if the L3 interface of the router/host attached to that
> particular port is set at 1500 (I assume no pmtud/fragmentation related
> issues, since the MTU change is on the L2 switch port, not on the L3 IP
> interface), and yes the switch is intended to do plain L2 aggregation.

The switch MTU is basically just a maximum value for what can be handled
on a given Port/VLAN/globally.

If all end systems agree to use a smaller MTU, the switch wouldn't mind
(wouldn't even notice).

What you need to avoid is having one machine send large packets across
the fabric towards a machine that cannot handle >1500 byte packets.  But
that's quite independent of the switch settings in between (except that
the switch might drop the packet, silently, if the output switch port 
is set to "mtu 1500").


> Recommendations for which switch to go with,  anyone ?

We really like the 2950 and 2960s.  

2950s can go to a MTU of 1530, dunno about 2960s (didn't have the need yet).

gert

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Gert Doering - Munich, Germany                             gert at greenie.muc.de
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