3Com Switch 8800 Advanced Software V5 Configuration Guide

718 CHAPTER 49: DHCP OVERVIEW
Dynamic IP Address
Allocation Procedure
For dynamic allocation, a DHCP client obtains an IP address from a DHCP server via
four steps:
1 The client broadcasts a DHCP-DISCOVER message to locate a DHCP server.
2 A DHCP server offers configuration parameters such as an IP address to the client
in a DHCP-OFFER message.
3 If several DHCP servers send offers to the client, the client accepts the first received
offer, and broadcasts it in a DHCP-REQUEST message to formally request the IP
address.
4 All DHCP servers receive the DHCP-REQUEST message, but only the server to
which the client sent a formal request for the offered IP address returns a
DHCP-ACK message to the client, confirming that the IP address has been
allocated to the client, or returns a DHCP-NAK unicast message, denying the IP
address allocation.
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■ After the client receives the DHCP-ACK message, it will probe whether the IP
address assigned by the server is in use by broadcasting gratuitous ARP. If the
client receives no response within specified time, the client can use this IP
address. Otherwise, the client sends a DHCP-DECLINE message to the server to
request an IP address again.
■ If there are multiple DHCP servers, IP addresses offered by other DHCP servers
are assignable to other clients.
IP Address Lease
Extension
The IP address dynamically allocated by a DHCP server to a client has a lease. After
the lease duration elapses, the IP address will be reclaimed by the DHCP server. If
the client wants to use the IP address again, it has to extend the lease duration.
After the half lease duration elapses, the DHCP client will send the DHCP server a
DHCP-REQUEST unicast message to extend the lease duration. Upon availability of
the IP address, the DHCP server returns a DHCP-ACK unicast confirming that the
client’s lease duration has been extended, or a DHCP-NAK unicast denying the
request.
If the client receives the DHCP-NAK message, it will broadcast another
DHCP-REQUEST message for lease extension after 7/8 lease duration elapses. The
DHCP server will handle the request as above mentioned.
DHCP Message Format Figure 214 gives the DHCP message format, which is based on the BOOTP
message format and involves eight types. These types of messages have the same
format except that some fields have different values. The numbers in parentheses
indicate the size of each field in octets.