MTU not regrowing?

[Carlos Ferreira]

don't forget the header of the ping. the -s flag specifies the amount of
data that the ping carries. By specifying a size of 500, you are creating a
ping packet larger than the MTU.


On 24 June 2014 19:43, Andrea Venturoli <ml at netfence.it> wrote:

> Hello.
>
> Today I experienced something weird (at least for me) on a 8.4 system:
>
> _ the system had vlan3 interface, with default MTU (1500 bytes);
> _ "ping -D -s 1400 somehost" would work, but "ping -D -s 1500 somehost"
> would yield "frag needed and DF set" (forgive me if the message is not
> exact, I don't have it anymore);
>
> _ to make some tests I reduced MTU size with "ifconfig vlan3 mtu 500";
> _ now, of course, "ping -D -s 400 somehost" would work, but "ping -D -s
> 500 somehost" would yield "frag needed and DF set";
>
> _ then I raised MTU again with "ifconfig vlan3 mtu 1500" (notice ifconfig
> would actually report this as "mtu 1500" was shown);
> _ however the results were as before, i.e. "ping -D -s 400 somehost" would
> work, but "ping -D -s 500 somehost" would yield "frag needed and DF set";
>
> _ no way I could ping with a packet bigger than 500 bytes until I rebooted.
>
> Is this expected behaviour? Any way to get around this?
>
>  bye & Thanks
>         av.
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-- 

Carlos Miguel Ferreira
Researcher at Telecommunications Institute
Aveiro - Portugal
Work E-mail - cmf at av.it.pt
Skype & GTalk -> carlosmf.pt at gmail.com
LinkedIn -> http://www.linkedin.com/in/carlosmferreira