Product guide

IP Routing Features
Configuring DHCP Relay
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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”.
Relay agents without Option 82 can exist in the path between Option 82
relay agents and an Option 82 server. The agents without Option 82 will
forward client requests and server responses without any effect on Option
82 fields in the packets.
If the routing switch is not able to add an Option 82 field to a client’s DHCP
request due to the message size exceeding the MTU (Maximum Transmis-
sion Unit) size, then the request is forwarded to the DHCP server without
Option 82 information and an error message is logged in the switch’s Event
Log.
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