Configuring Dhcp Relay; Overview; Dhcp Packet Forwarding; Unicast Forwarding - HP 2920 Series Multicast And Routing Manual

Example 37 Output for show ip irdp
HP Switch# show ip irdp
Status and Counters - ICMP Router Discovery Protocol
Global Status : Disabled
VLAN Name
------------- -------- ----------- ------- ------- -------- ----------
DEFAULT_VLAN
VLAN20
VLAN30

Configuring DHCP relay

Overview

The Dynamic Host Configuration Protocol (DHCP) is used for configuring hosts with IP address and
other configuration parameters without user intervention. The protocol is composed of three
components:
DHCP client
DHCP server
DHCP relay agent
The DHCP client sends broadcast request packets to the network; the DHCP servers respond with
broadcast packets that offer IP parameters, such as an IP address for the client. After the client
chooses the IP parameters, communication between the client and server is by unicast packets.
HP routing switches provide the DHCP relay agent to enable communication from a DHCP server
to DHCP clients on subnets other than the one the server resides on. The DHCP relay agent transfers
DHCP messages from DHCP clients located on a subnet without a DHCP server to other subnets.
It also relays answers from DHCP servers to DHCP clients.
The DHCP relay agent is transparent to both the client and the server. Neither side is aware of the
communications that pass through the DHCP relay agent. As DHCP clients broadcast requests, the
DHCP relay agent receives the packets and forwards them to the DHCP server. During this process,
the DHCP relay agent increases the hop count by one before forwarding the DHCP message to
the server. A DHCP server includes the hop count from the DHCP request that it receives in the
response that it returns to the client.

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 is forwarded to all VLANs with configured IP interfaces
(except the source VLAN.)
Status Advertising Min int Max int Holdtime Preference
Address
Enabled multicast
Enabled multicast
Enabled multicast
(sec) (sec) (sec)
450 600 1800
450 600 1800
450 600 1800
0
0
0
Configuring DHCP relay 63