3Com 5500 SI - Switch - Stackable, Configuration Manual

Page: 285 / 686

DHCP Configuration 285
Mechanism of option 82 supporting on DHCP relay
The procedure for a DHCP client to obtain an IP address from a DHCP server through
a DHCP relay is exactly the same as that for the client to obtain an IP address from a
DHCP server directly. The following are the mechanism of option 82 supporting on
DHCP relay.
1 A DHCP client broadcasts a request packet when it initiates.
2 If a DHCP server exists in the local network, it assigns an IP address to the DHCP client
directly. Otherwise, the DHCP relay on this network receives and processes the
request packet. The DHCP relay checks whether the packet contains option 82 and
processes the packet accordingly.
3 If the packet contains option 82, the DHCP relay processes the packet depending on
the configured policy (that is, discards the packet, replaces the original option 82 in
the packet with its own, or leaves the original option 82 unchanged in the packet),
and forwards the packet (if not discarded) to the DHCP server.
4 If the packet does not contain option 82, the DHCP relay adds option 82 to the
packet and forwards the packet to the DHCP server. The forwarded packet contains
the MAC address of the switch port to which the DHCP client is connected, the VLAN
to which the DHCP client belongs, and the MAC address of the DHCP relay.
5 Upon receiving the DHCP request packet forwarded by the DHCP relay, the DHCP
server stores the information contained in the option field and sends a packet that
contains DHCP configuration information and option 82 to the DHCP relay.
6 Upon receiving the packet returned from the DHCP server, the DHCP relay strips
option 82 from the packet and forwards the packet with the DHCP configuration
information to the DHCP client.
Request packets sent by a DHCP client fall into two categories: DHCP-DISCOVER
packets and DHCP-REQUEST packets. As DHCP servers coming from different
manufacturers process DHCP request packets in different ways (that is, some DHCP
servers process option 82 in DHCP-DISCOVER packets, whereas the rest process
option 82 in DHCP-REQUEST packets), a DHCP relay adds option 82 to both types of
packets to accommodate to DHCP servers of different manufacturers.
DHCP Client
Configuration
DHCP client configuration is described in the following section.
Configuring a VLAN Interface to Obtain an IP Address Using DHCP
Perform the following configuration in VLAN Interface View.
By default, the Switch attempts to obtain an IP address by DHCP on VLAN 1.
If you are attempting to stop the Switch from transmitting packets, you need to
disable all features which may generate packets. By default these are:
■
DHCP
■
Resilient ARP
■
Spanning Tree
Table 279 Configuring a VLAN Interface to Obtain an IP Address Using DHCP
Operation Command
Configure VLAN interface to obtain IP address using
DHCP
ip address dhcp-alloc
Remove the configuration
undo ip address dhcp-alloc