• 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.
• If the management VLAN IP address configuration changes after
mgmt-vlan
has been configured as the
remote ID suboption, the routing switch dynamically adjusts to the new IP addressing for all future DHCP
requests.
• The management VLAN and all other VLANs on the routing switch use the same MAC address.
Overview of DHCP
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.
The 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.)
Enabling DHCP relay operation
For the DHCP relay agent to work on the switch, you must complete the following steps:
Procedure
1.
Enable DHCP relay on the routing switch (the default setting.)
2.
Ensure that a DHCP server is servicing the routing switch.
3.
Enable IP routing on the routing switch.
4.
Ensure that there is a route from the DHCP server to the routing switch and back.
5.
Configure one or more IP helper addresses for specified VLANs to forward DHCP requests to DHCP servers
on other subnets.
Chapter 14 Dynamic Host Configuration Protocol
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