[c-nsp] MTU size, fragmentation and drops

CiscoNSP List CiscoNSP_list at hotmail.com
Thu Apr 14 22:13:46 EDT 2016

Thanks for the quick reply Nathan!   (Apologies for not replying inline, using hotmail, on phone)

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? 


From: Nathan Ward <cisco-nsp at daork.net>
Sent: Friday, 15 April 2016 12:06 PM
To: CiscoNSP List
Cc: cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] MTU size, fragmentation and drops

> On 15/04/2016, at 14:02, CiscoNSP List <CiscoNSP_list at hotmail.com> wrote:
> Hi Everyone,
> Generic question on mtu/fragmentation/drops that Im hoping someone can explain.
> Our PE cust Interface "had" MTU set to 9100
> Cust CE WAN Interface has MTU set to 1500
> If we pinged with anything above 1500bytes (No DF-bit set), packets were dropped.
> If we set our PE's Int to 1500, and pinged above 1500bytes, packets would pass (But obviously be fragmented).
> My question is why?  i.e. Why would reducing our PE's Ints MTU size to 1500 "allow" packets above 1500bytes to pass fragmented, but at 9100, they were dropped?

Hi “CiscoNSP List”,

What were you pinging from/to?

CE pinging the PE?

If so, the reply from the PE would have been larger than 1500B - remember the reply would contain the original payload from the request, and would not be fragmented.

Nathan Ward

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