Configuring Bootp/Dhcp Forwarding Parameters - Dell PowerConnect B-RX Configuration Manual

Configuring BootP/DHCP forwarding parameters

Beginning with release 02.7.00, the DHCP relay will allow for IP address grants that do not match
the subnets configured on the interface that the DHCP request was received. A host on an IP
network can use BootP/DHCP to obtain its IP address from a BootP/DHCP server. To obtain the
address, the client sends a BootP/DHCP request. The request is a subnet directed broadcast and is
addressed to UDP port 67. A limited IP broadcast is addressed to IP address 255.255.255.255
and is not forwarded by the device or other IP routers.
When the BootP/DHCP client and server are on the same network, the server receives the
broadcast request and replies to the client. However, when the client and server are on different
networks, the server does not receive the client's request, because the device does not forward the
request.
You can configure the device to forward BootP/DHCP requests. To do so, configure a helper
address on the interface that receives the client requests, and specify the BootP/DHCP server's IP
address as the address you are helping the BootP/DHCP requests to reach. Instead of the server's
IP address, you can specify the subnet directed broadcast address of the IP subnet the server is in.
NOTE
The IP subnet configured on the port which is directly connected to the device sending a
BootP/DHCP request, does not have to match the subnet of the IP address given by the DHCP server.
BootP/DHCP forwarding parameters
The following parameters control the device's forwarding of BootP/DHCP requests:
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BigIron RX Series Configuration Guide
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Helper address – The BootP/DHCP server's IP address. You must configure the helper address
on the interface that receives the BootP/DHCP requests from the client. The device cannot
forward a request to the server unless you configure a helper address for the server.
Gateway address – The device places the IP address of the interface that received the
BootP/DHCP request in the request packet's Gateway Address field (sometimes called the
Router ID field). When the server responds to the request, the server sends the response as a
unicast packet to the IP address in the Gateway Address field. (If the client and server are
directly attached, the Gateway ID field is empty and the server replies to the client using a
unicast or broadcast packet, depending on the server.)
By default, the device uses the lowest-numbered IP address on the interface that receives the
request as the Gateway address. You can override the default by specifying the IP address you
want the device to use.
Hop Count – Each router that forwards a BootP/DHCP packet increments the hop count by 1.
Routers also discard a forwarded BootP/DHCP request instead of forwarding the request if the
hop count is greater than the maximum number of BootP/DHCP hops allows by the router. By
default, the device forwards a BootP/DHCP request if its hop count is four or less, but discards
the request if the hop count is greater than four. You can change the maximum number of hops
the device will allow to a value from 1 – 15.
NOTE
The BootP/DHCP hop count is not the TTL parameter.
Configuring forwarding parameters
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