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PowerConnect B-Series FCX Configuration Guide 839
53-1002266-01
Configuring IP parameters – Layer 3 Switches
26
Syntax: ip helper-address <num> <ip-addr>
The <num> parameter specifies the helper address number and can be from 1 through 16.
The <ip-addr> command specifies the server IP address or the subnet directed broadcast address
of the IP subnet the server is in.
Configuring BootP/DHCP relay 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 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 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 IP address as the address you are helping the BootP/DHCP requests to reach. Instead of
the server IP address, you can specify the subnet directed broadcast address of the IP subnet the
server is in.
BootP/DHCP relay parameters
The following parameters control the Layer 3 Switch forwarding of BootP/DHCP requests:
• Helper address – The BootP/DHCP server 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 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 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 through 15.
NOTE
The BootP/DHCP hop count is not the TTL parameter.