Allied Telesis AT-S63 - Manual (Page 333)

AT-S63 Management Software Features Guide
Section VII: Routing 333
Overview
The AT-S63 Management Software comes with a BOOTP relay agent for
relaying BOOTP messages between clients and DHCP or BOOTP
servers.
When a client sends a BOOTP request to a DHCP or BOOTP server for an
IP configuration, it transmits 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 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 a lot of subnets. Another
solution is to use a BOOTP relay agent, which transfers client requests
across subnet boundaries.
The BOOTP 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 BOOTP relay 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 BOOTP 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 BOOTP 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 will be forwarded to a BOOTP 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.