Overview - Allied Telesis AT-8100L/8 User Manual

Chapter 29: DHCP Relay Overview

Overview

The switch has a DHCP relay agent to relay BOOTP messages between
clients and DHCP or BOOTP servers.
A client that transmits a request for an IP configuration to a DHCP or
BOOTP server has to send the request as a broadcast packet because it
does not know the IP address of the server. This can present a problem
when a client and DHCP or BOOTP server reside on different subnets,
because broadcast packets do not cross subnet boundaries. One possible
solution is to have a DHCP or BOOTP server on each subnet where there
are clients, though this could be problematic if there happen to be many
subnets. Another solution is to use a DHCP relay agent, which transfers
client requests across subnet boundaries.
The relay agent does more than simply forward BOOTP requests from
clients to servers. It modifies the requests so that, from the perspective of
the server, it becomes the originator of the request. The responses from
the servers are directed to the agent, which sends the messages on to the
clients as either broadcast or unicast packets, depending on the
requirements of the clients.
To implement the DHCP relay agent on the switch, you need to be familiar
with routing interfaces, which route packets between different local
subnets on the switch in the IPv4 packet routing feature. Each routing
interface functions as the DHCP relay agent for the clients in its subnet,
forwarding BOOTP requests from the clients and responses from the
servers.
If you will be using the IPv4 packet routing feature on all the local subnets,
then, by default, all of the clients will have access to a DHCP relay agent
because each subnet will have a routing interface. However, if IPv4 packet
routing will be limited to some but not all the local subnets of the switch,
then only those BOOTP requests from clients on a subnet with a routing
interface can be forwarded by a DHCP relay agent.
Here is an overview of the process. When a routing interface receives a
BOOTP request with a value of 0.0.0.0 in the gateway (giaddr) field in the
packet, it assumes the request originated from a client on its subnet. In
response, it replaces the value in the field with its IP address and forwards
the packet on to the server. If more than one IP address of DHCP or
BOOTP servers are specified on the switch, the interface sends the same
request to each server. If the client and server reside on the same subnet,
the routing interface does not forward the request.
If an interface receives a BOOTP request with a non-zero value in the
gateway field, it assumes the client who originated the request resides on
another subnet, and so routes the request as a unicast packet without any
change, other than incrementing the hop count.
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