topology: DHCP server---S5700---PC
1. when PC1 unplug from S5700 and plug in PC2 that PC2 can't obtain ip address.
1. Check the configuration and found that there are L2 DHCP snooping configuration.
2. check the dhcp snooping user-bind entry and found that it will be deleted after around 30s.
3. And try to plug PC2 to the swtich in around 50s, it can obain IP address.
when PC1 connect to S5700, it will create a dhcp snooping user-bind item, and it will still exsit for a while around 30s even PC1 has already been unpluged from the switch. And PC2 will not obtain ip address when plug fast to the switch in that case.