3Com Switch 8807, Configuration Manual

Page: 683 / 883

Some Concepts about DHCP 683
IP address carried in the packet, and broadcasts a DHCP_Request packet to
each DHCP server. The packet contains the IP address carried by the
DHCP_Offer packet.
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Acknowledgement. Upon receiving the DHCP_Request packet, the DHCP
server that owns the IP address the DHCP_Request packet carries sends a
DHCP_ACK packet to the DHCP client. And then the DHCP client binds TCP/IP
protocol components to its network adapter.
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IP addresses offered by other DHCP servers (if any) through DHCP_Offer
packets but not selected by the DHCP client are still available for other clients.
2 Second round registration
A second round registration goes through the following steps:
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After going through the first round registration successfully and logging out,
when the DHCP client logs on to the network again, it directly broadcasts a
DHCP_Request packet that contains the IP address assigned to it in the first
round registration instead of a DHCP_Discover packet. .
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Upon receiving the DHCP_Request packet, if the IP address carried in the
packet is still available, the DHCP server owning the IP address answers with a
DHCP_ACK packet to enable the DHCP client to use the IP address again.
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If the IP address is not available (for example, it is occupied by other DHCP
client), the DHCP server answers with a DHCP_NAK packet, which enables the
DHCP client to go through steps in the first round registration.
3 Prolonging the lease time of IP address
An IP address assigned dynamically is valid for a specified lease time and will be
reclaimed by the DHCP server when the time expires. So the DHCP client must
update the lease to prolong the lease time if it is to use the IP address for a longer
time.
By default, a DHCP client updates its IP address lease automatically by sending a
DHCP_Request packet to the DHCP server when half of the lease time elapses. The
DHCP server, in turn, answers with a DHCP_ACK packet to notify the DHCP client
of the new lease.
BOOTP Relay Agent Bootstrap protocol (BOOTP) relay agent is an Internet host or router that
transports DHCP messages between the DHCP server and DHCP clients. BOOTP is
designed for remote boot, mainly to notify the connected client about the location
of the boot file.
DHCP is an extension of the BOOTP mechanism. This feature enables an existing
BOOTP client to interoperate with the DHCP server without changing the installed
software. RFC 1542 describes in detail the interactions among BOOTP, DHCP client
and DHCP server.
DHCP and BOOTP Relay
Agent
Like BOOTP, DHCP also works in the Client/Server mode. This protocol enables a
DHCP client to request dynamically the DHCP server for the configuration
information, including important parameters such as the allocated IP address,
subnet mast, and default gateway, and the DHCP server can configure these
parameters for the client conveniently.