MTU not regrowing?

[Andrea Venturoli]

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.