Foundry Networks Switch and Router Installation And Configuration Manual page 516

Foundry Switch and Router Installation and Configuration Guide
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
limited 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.
NOTE: The BootP/DHCP hop count is not the TTL parameter.
Configuring an IP Helper Address
The procedure for configuring a helper address for BootP/DHCP requests is the same as the procedure for
configuring a helper address for other types of UDP broadcasts. See "Configuring an IP Helper Address" on
page 15-72.
Changing the IP Address Used for Stamping BootP/DHCP Requests
When the Layer 3 Switch forwards a BootP/DHCP request, the Layer 3 Switch "stamps" the Gateway Address
field. The default value the Layer 3 Switch uses to stamp the packet is the lowest-numbered IP address
configured on the interface that received the request. If you want the Layer 3 Switch to use a different IP address
to stamp requests received on the interface, use either of the following methods to specify the address.
The BootP/DHCP stamp address is an interface parameter. Change the parameter on the interface that is
connected to the BootP/DHCP client.
USING THE CLI
To change the IP address used for stamping BootP/DHCP requests received on interface 1/1, enter commands
such as the following:
BigIron(config)# int e 1/1
BigIron(config-if-1/1)# ip bootp-gateway 109.157.22.26
These commands change the CLI to the configuration level for port 1/1, then change the BootP/DHCP stamp
address for requests received on port 1/1 to 192.157.22.26. The Layer 3 Switch will place this IP address in the
15 - 74 December 2000