Configuring Bootp/Dhcp Forwarding Parameters - Foundry Networks FESX Manual

Configuring an IP Helper Address
To forward a client's broadcast request for a UDP application when the client and server are on different networks,
you must configure a helper address on the interface connected to the client. Specify the server's IP address or
the subnet directed broadcast address of the IP sub-net the server is in as the helper address.
You can configure up to 16 helper addresses on each interface. You can configure a helper address on an
Ethernet port or a virtual interface.
To configure a helper address on interface 2 on chassis module 1, enter the following commands:
FastIron SuperX Router(config)# interface e 1/2
FastIron SuperX Router(config-if-1/2)# ip helper-address 1 207.95.7.6
The commands in this example change the CLI to the configuration level for port 1/2, then add a helper address
for server 207.95.7.6 to the port. If the port receives a client request for any of the applications that the Layer 3
Switch is enabled to forward, the Layer 3 Switch forwards the client's request to the server.
Syntax: ip helper-address <num> <ip-addr>
The <num> parameter specifies the helper address number and can be from 1 – 16.
The <ip-addr> command specifies the server's IP address or the subnet directed broadcast address of the IP sub-
net the server is in.

Configuring BootP/DHCP Forwarding Parameters

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
Foundry Layer 3 Switch 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 Layer 3 Switch does not forward the request.
You can configure the Layer 3 Switch 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 sub-net the server is in.
BootP/DHCP Forwarding Parameters
The following parameters control the Layer 3 Switch's forwarding of BootP/DHCP requests:
• 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 Layer 3 Switch cannot forward a
request to the server unless you configure a helper address for the server.
• Gateway address – The Layer 3 Switch 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 Layer 3 Switch 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 Layer 3
Switch 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, a Foundry Layer 3 Switch
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 Layer 3 Switch will allow to a value from
1 – 15.
December 2005 © Foundry Networks, Inc.
Configuring IP
16 - 49