IP Routing Features
Configuring DHCP Relay
Operating Notes
■ This implementation of DHCP relay with Option 82 complies with the
following RFCs:
• RFC 2131
• RFC 3046
■ Moving a client to a different port allows the client to continue
operating as long as the port is a member of the same VLAN as the
port through which the client received its IP address. However,
rebooting the client after it moves to a different port can alter the IP
addressing policy the client receives if the DHCP server is configured
to provide different policies to clients accessing the network through
different ports.
■ The IP address of the primary DHCP relay agent receiving a client
request packet is automatically added to the packet, and is identified
as the giaddr (gateway interface address). (That is, the giaddr is the
IP address of the VLAN on which the request packet was received
from the client.) For more information, refer to RFC 2131 and RFC
3046.
■ DHCP request packets from multiple DHCP clients on the same relay
agent port will be routed to the same DHCP server(s). Note that when
using 802.1X on a 5300xl switch running software release E.09.xx or
greater, a port's VLAN membership may be changed by a RADIUS
server responding to a client authentication request. In this case the
DHCP server(s) accessible from the port may change if the VLAN
assigned by the RADIUS server has different DHCP helper addresses
than the VLAN used by unauthenticated clients.
■ Where multiple DHCP servers are assigned to a VLAN, a DHCP client
request cannot be directed to a specific server. Thus, where a given
VLAN is configured for multiple DHCP servers, all of these servers
should be configured with the same IP addressing policy.
■ Where routing switch “A” is configured to insert its MAC address as
the Remote ID in the Option 82 fields appended to DHCP client
requests, and upstream DHCP servers use that MAC address as a
policy boundary for assigning an IP addressing policy, then replacing
switch “A” makes it necessary to reconfigure the upstream DHCP
server(s) to recognize the MAC address of the replacement switch.
This does not apply in the case where an upstream relay agent “B” is
configured with option 82 replace, which removes the Option 82 field
originally inserted by switch “A”.
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