DHCP Server IV

IP Address Reservation

DHCP supports IP address reservation for clients. The reserved IP addresses can be those in the address pool or not. If an address of the address pool is reserved, it is no longer assignable. Addresses are usually reserved for DNS servers.

Address Pool Lease

Different address pools can have different IP address leases, but addresses in one pool have the same lease.

Generally, there is a valid period for the IP address dynamically assigned to the client. The server withdraws the IP address after the valid period expires. If the client intends to continue to use this IP address, it needs to extend the IP address lease.

When obtaining an IP address, the DHCP client enters the binding state. Three timers are set on the client to control lease update, lease rebinding, and lease expiration. When assigning an IP address to the client, the DHCP server specifies values for the timers. If the server does not set the values for the timers, the client uses the default values. Table 1 lists the default values of the timers.

When the lease renewal timer expires, the DHCP client must renew its IP address lease. The DHCP client automatically sends a DHCP REQUEST packet to the DHCP server that assigns its currently-used IP address. If the IP address is valid, the server replies with a DHCP ACK packet to entitle the client a new lease, and then the client re-enters the binding state. If the client receives a DHCP NAK packet from the server, it enters the initializing state.

After the client sends a DHCP REQUEST packet for extending the lease, the client remains in the updating state and waits for a response. If the client does not receive a response from the server after the server rebinding timer expires, the client assumes that the original DHCP server is unavailable and starts to broadcast a DHCP REQUEST packet.

Any DHCP server on the network can reply to this request with a DHCP ACK or DHCP NAK packet.

If receiving a DHCP ACK packet, the client returns to the binding state and re-sets the lease renewal timer and server binding timer; if all the received packets are DHCP NAK packets, the client goes back to the initializing state. At this time, the client must stop using this IP address immediately, return to the initializing state, and request a new IP address.

If the client does not receive any response before the lease expiration timer expires, the client must stop using the current IP address immediately and return to the initializing state. Then, the client sends a DHCP DISCOVER packet to apply for a new IP address again (This is implemented based on RFC 2131.)

Hot Backup

For a S9300&S9300E with two MPUs, DHCP data on the two MPUs is backed up in real time. Therefore, after the master/slave switchover is performed, the slave MPU becomes the master MPU; therefore, the DHCP server can function and allocate IP addresses to clients normally.