[CALUG] Gigabit Ethernet

Derek Rogillio dereklist at rogillio.net
Wed Oct 19 19:16:32 CDT 2005

On Wed, Oct 19, 2005 at 06:31:51PM -0400, Ray Lischner wrote:
> On Wednesday 19 October 2005 05:54 pm, Tim Thomas wrote:
> > The problem is that non-jumbo equipment will see a jumbo frame
> > as a "jabber" and will effectively lop off anything above the
> > standard data size of 1500 bytes.
> So why doesn't PMTU discovery work in this case? Why can't the jumbo 
> frame device discover the path MTU is only 1500 (or whatever) and send 
> frames accordingly when it needs to talk to that device?
> I've never delved down to this level of network stuff, so I'm still 
> learning.
> Anyway, I decided to order a jumbo-capable switch. I'm still not sure 
> how easy it will be to setup, but at least I'll have gigabit ethernet 
> to keep me happy while I experiment with jumbo frames.


Only the local broadcast domain is required to support jumbo frames.  Once it hits a layer 3 boundary, PMTU kicks in as normal.  It is entirely possible to use jumbo frames on a LAN but not on the WAN.  In fact, this is normal.

The reason you need jumbo-capable equipment everywhere across the broadcast domain is exactly as Tim describes above.  PMTU is only useful when going across a routed link.

Derek A. Rogillio
dereklist (at) rogillio (dot) net
JNCIE #42, CCIE #6146

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