[c-nsp] jumbo frames

[Dave Temkin]

On Sat, 4 Mar 2006, Gert Doering wrote:

> Hi,
>
> On Sat, Mar 04, 2006 at 12:10:48AM -0600, Tim Winders wrote:
> > MTU on SVI's?
> >
> > Do you mean the VLAN interfaces on the 6500?
> >
> > And what *should* happen with them?
>
> They need to match the MTU of all hosts connected to the respective
> VLAN.
>
> Imagine a jumbo packet coming in "from the outside", and the router
> forwarding it to an end host that can only handle MTU 1500 packets - the
> packet will be lost.  If the SVI knows that the MTU in *this VLAN* is
> only 1500, the router can fragment the packet, or send an ICMP fragmentation
> required message back to the sender.
>
> Unfortunately, at least on some 35xx switches (I don't remember the specific
> model nor the IOS version - it was at a customer's site, and I no longer
> have access to the devices), this does not seem to be possible - there
> is only a global L3 MTU value, and it is automatically applied to ALL
> L3 interfaces.  Which sucks.
>
> gert
>

Correct, however on those platforms you just use the "ip mtu" command to
lower it down and then it's not a problem (the mtu interface command
doesn't work).

-Dave