DHCP and mystery of allowed(or not) connections

[raresz]

Hi. 
It's in educational purposes.
How does ISP can allow connections from DHCP-learned addresses?

I moved to other house, finally i have public IP, so i terminated ISP link with my Cisco router and tried to get static IP address. So first i got it via DHCP then when i knew the configuration parameters i have set everything statically(address, netmask, default route) - everything was like copied from DHCP configuration. And finally address which was set statically - doesn't let me to connect to Internet.

I know that and that's obvious ISP has protected himself against that type of movements. I'm not fighting with that i don't even need static IP but i'm learning networking so i'm quite curious which type of mechanism was used for that?

I was searching about that and only what i found is around DHCP snooping and IP source guard. Would be so easy to understand BUT:

DHCP snooping doesn't let you to connect until there is entry in DHCP snooping table or you configure it static entry. But i just before connected via DHCP so it should be entry in snooping table when i change configuration to static(using same IP address like with DHCP) - so i have still valid IP to MAC binding comparing to DHCP snooping table. 

Somebody maybe want to enlighten me how is it done?

[Richard Burts]

I am not authoritative about this but my best guess is that when you reconfigure the interface from dhcp to manually configured that it brings the interface down momentarily so that it can reset parameters and that when the interface drops that the entry is removed from the binding table.

[raresz]

Ok seems to be right way of thinking. Does client sends DHCPRELEASE when configuring from dhcp to static? I mean ad You said it could bring interface down but is it connected with any DHCP message? That message could remove entry from table as far as i remember.