[c-nsp] MTU size, fragmentation and drops

CiscoNSP List CiscoNSP_list at hotmail.com
Fri Apr 15 03:03:01 EDT 2016

Cheers for confirming / explaining guys - Much appreciated.

MTU has been changed to match at both ends.

From: Peter Rathlev <peter at rathlev.dk>
Sent: Friday, 15 April 2016 5:00 PM
To: CiscoNSP List
Cc: cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] MTU size, fragmentation and drops

On Fri, 2016-04-15 at 02:13 +0000, CiscoNSP List wrote:
> > Our PE cust Interface "had" MTU set to 9100
> > Cust CE WAN Interface has MTU set to 1500
> Pinging from CE->PE
> So, CE would send fragmented packet at (for example) 1550....PE would
> receive, reassemble, and try to send at the original size, and
> therefore be dropped?

Yes, the PE reassembles the echo request from CE and replies with a
packet the same size (1550 bytes) that does not need to be fragmented
because the MTU says it can send 9100 bytes in one go.

And then the CE drops the packet on arrival.

MTU should always match between the ends of the same link.


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