iptables settings lag for an hour

[pollux0]

after configuring linux as a firewall/router, it took over an hour for the settings to kick in. For instance, I set up snat which immediatly worked but port forwarding did not. An hour later port forwarding worked. Can anyone explain why? I am not sure if it is the linux box itself or mabey old entries in the switch or router's ARP tables outside the firewall. I know a little about cisco systems and i think that ARP tables expire in minutes, not hour(s).

thanks