BigIron RX Series Configuration Guide 21153-1001810-01Configuring forwarding parameters 7Configuring BootP/DHCP forwarding parametersBeginning with release 02.7.00, the DHCP relay will allow for IP address grants that do not matchthe subnets configured on the interface that the DHCP request was received. A host on an IPnetwork can use BootP/DHCP to obtain its IP address from a BootP/DHCP server. To obtain theaddress, the client sends a BootP/DHCP request. The request is a subnet directed broadcast and isaddressed to UDP port 67. A limited IP broadcast is addressed to IP address 255.255.255.255and 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 thebroadcast request and replies to the client. However, when the client and server are on differentnetworks, the server does not receive the client’s request, because the device does not forward therequest.You can configure the device to forward BootP/DHCP requests. To do so, configure a helperaddress on the interface that receives the client requests, and specify the BootP/DHCP server’s IPaddress as the address you are helping the BootP/DHCP requests to reach. Instead of the server’sIP address, you can specify the subnet directed broadcast address of the IP subnet the server is in.NOTEThe IP subnet configured on the port which is directly connected to the device sending aBootP/DHCP request, does not have to match the subnet of the IP address given by the DHCP server.BootP/DHCP forwarding parametersThe following parameters control the device’s forwarding of BootP/DHCP requests:• Helper address – The BootP/DHCP server’s IP address. You must configure the helper addresson the interface that receives the BootP/DHCP requests from the client. The device cannotforward 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 theBootP/DHCP request in the request packet’s Gateway Address field (sometimes called theRouter ID field). When the server responds to the request, the server sends the response as aunicast packet to the IP address in the Gateway Address field. (If the client and server aredirectly attached, the Gateway ID field is empty and the server replies to the client using aunicast or broadcast packet, depending on the server.)By default, the device uses the lowest-numbered IP address on the interface that receives therequest as the Gateway address. You can override the default by specifying the IP address youwant 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 thehop count is greater than the maximum number of BootP/DHCP hops allows by the router. Bydefault, the device forwards a BootP/DHCP request if its hop count is four or less, but discardsthe request if the hop count is greater than four. You can change the maximum number of hopsthe device will allow to a value from 1 – 15.NOTEThe BootP/DHCP hop count is not the TTL parameter.
