Alexander Gall gall at switch.ch
Mon Aug 28 14:39:01 CEST 2006

On Mon, 28 Aug 2006 12:04:53 +0200, Iljitsch van Beijnum <iljitsch at muada.com> said:

> On 28-aug-2006, at 9:58, Stig Venaas wrote:
>> A common problem with tunneling is also that the two ends of the  
>> tunnel are configured with different MTUs, which cause problems  
>> even if PMTUD might work. I've seen this happen many times.

> Actually I don't remember this being a problem in my tests because  
> tunnel implementations tend to be able to receive packets that are  
> larger than their MTU. I.e., when there is a tunnel between a Linux  
> system (usually 1480 byte tunnel MTU) and a BSD system (usually 1280  
> byte tunnel MTU) and the Linux system sends a 1480 byte packet, the  
> BSD system accepts the packet.

Cisco doesn't (and I think neither does Juniper).  Such packets are
considered to be "giants" and are dropped silently.  Like Stig I've
seen this many times in the wild.  Tunnels should always be configured
with an explicit MTU that is agreed upon by both endpoints.


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