Operating Notes - HP 2920 Series Multicast And Routing Manual

Table 8 DHCP operation for the topology in Figure 10
Client Remote ID
X 10.38.10.1
Y 10.38.10.1
Z 10.38.10.1
1
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.) This 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.

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.
Because routing is not allowed between the Management VLAN and other VLANs, a DHCP
server must be available in the management VLAN if clients in the management VLAN require
a DHCP server.
1
giaddr DHCP server
10.39.10.1 A only
10.29.10.1 B or C
10.15.10.1 B or C
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.
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.
Configuring DHCP relay 75