C
HAPTER
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| IP Services
Dynamic Host Configuration Protocol
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These fields identify the requesting device by indicating the interface
through which the relay agent received the request.
If DHCP relay is enabled, and this switch sees a DHCP client request, it
inserts its own IP address into the request so that the DHCP server will
know the subnet where the client is located. Depending on the selected
frame format set for the remote-id, this information may specify the MAC
address, IP address, or an arbitrary string for the requesting device (that
is, the relay agent in this context).
By default, the relay agent also fills in the Option 82 circuit-id field with
information indicating the local interface over which the switch received the
DHCP client request, including the VLAN ID, stack unit, and port. This
allows DHCP client-server exchange messages to be forwarded between
the server and client without having to flood them onto the entire VLAN.
The switch then forwards the packet to the DHCP server. When the server
receives the DHCP request, it allocates a free IP address for the DHCP
client from its defined scope for the DHCP client’s subnet, and sends a
DHCP response back to the DHCP relay agent (i.e., this switch). This switch
then passes the DHCP response received from the server to the client.
Figure 352: Layer 2 DHCP Relay Service
CLI REFERENCES
◆ "DHCP Relay Option 82" on page 1389
COMMAND USAGE
◆ You must specify the IP address for at least one active DHCP server.
Otherwise, the switch’s DHCP relay agent will not be able to forward
client requests to a DHCP server. Up to five DHCP servers can be
specified in order of preference.
If any of the specified DHCP server addresses are not located in the
same network segment with this switch, specify the default router
through which this switch can reach other IP subnetworks (see
“Configuring the IPv4 Default Gateway” or “Configuring the IPv6
Default Gateway”).
◆ DHCP Snooping Information Option 82 (see page 410) and DHCP Relay
Information Option 82 cannot both be enabled at the same time.
◆ DHCP request packets received by the switch are handled as follows:
■
If a DHCP relay server has been set on the switch, when the switch
receives a DHCP request packet without option 82 information from
Provides IP address
compatible with switch
segment to which client
is attached
DHCP
Server
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