Dhcp Packet Forwarding; Unicast Forwarding; Broadcast Forwarding - HP ProCurve 6200yl Multicast And Routing Manual

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.

DHCP Packet Forwarding

The DHCP relay agent on the routing switch forwards DHCP client packets to
all DHCP servers that are configured in the table administrated for each VLAN.

Unicast Forwarding

The packets are forwarded using unicast forwarding if the IP address of the
DHCP server is a specific host address. The DHCP relay agent sets the
destination IP address of the packet to the IP address of the DHCP server and
forwards the message.

Broadcast Forwarding

The packets are forwarded using broadcast forwarding if the IP address of the
DHCP server is a subnet address or IP broadcast address (255.255.255.255).
The DHCP relay agent sets the DHCP server IP address to broadcast IP address
and will be forwarded to all VLANs with configured IP interfaces (except the
source VLAN).
IP Routing Features
Configuring DHCP Relay
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