Table 31: DHCP operation for the topology in Figure DHCP Option 82 when using the
management VLAN as the remote ID suboption
Client Remote ID
giaddr
DHCP server
X
10.38.10.1
10.39.10.1
A only
If a DHCP client is in the management VLAN, its
DHCP requests can go only to a DHCP server that is
also in the management VLAN. Routing to other
VLANs is not allowed.
Y
10.38.10.1
10.29.10.1
B or C
Clients outside of the management VLAN can send
DHCP requests only to DHCP servers outside of the
management VLAN. Routing to the management
VLAN is not allowed.
Z
10.38.10.1
10.15.10.1
B or C
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, see 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 servers. When using 802.1X on a switch, a port's VLAN membership may be changed by a RADIUS
server responding to a client authentication request. In this case the DHCP servers 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 servers to recognize the MAC address of the replacement switch. This does not apply in the
case where an upstream relay agent "A" 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 forward client requests and server responses without any effect on Option 82
fields in the packets.
• If the routing switch cannot add an Option 82 field to a client's DHCP request because the message size
exceeds the MTU size, the request is forwarded to the DHCP server without Option 82 data and an error
message is logged in the switch's Event Log.
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Aruba 3810 / 5400R Multicast and Routing Guide for ArubaOS-
Switch 16.08
