[c-nsp] MTU size, fragmentation and drops

[Nathan Ward]

> 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