Certain HP printers equipped with an Ethernet Network Interface Card
(NIC) use the Bootstrap Protocol ( BOOTP) to acquire IP addresses and relative
network information. BOOTP allows a diskless client to configure itself
dynamically at the time of booting. This includes discovering its own IP
address and boot information. Using BOOTP, a host can broadcast a request on
the network and get information required from a BOOTP server. All IP addresses
that a BOOTP server allocates are permanent.
By default, a router does not forward any broadcasts and hence the
broadcasts from the BOOTP clients do not reach the BOOTP server if they are
separated by a Cisco router. This document provides special precautions that
may be necessary when the BOOTP server and the HP printers or BOOTP clients are
separated by a router.
There are no specific prerequisites for this document.
This document is not restricted to specific software and hardware
The information presented in this document was created from devices in
a specific lab environment. All of the devices used in this document started
with a cleared (default) configuration. If you are working in a live network,
ensure that you understand the potential impact of any command before using
For more information on document conventions, see the
Cisco Technical Tips
The client broadcasts (MAC and IP) a BOOTP request. If a Cisco router
sees this, has a helper address configured, and is forwarding UDP port 67, it
does the following:
The router places the IP address of the interface which received the
broadcast in the "giaddr" field (meaning Gateway IP Address). This will be the
address to which the BOOTP server will send the BOOTP response.
The router forwards this packet as an IP unicast to the IP helper
The BOOTP server gets the packet, finds the client's MAC address in
its tables, and sends a reply with the client's IP address and boot file
The reply is unicast directly to the Cisco router's IP address
When the router receives the BOOTP response (UDP port 68), it
retrieves the original client's MAC address and IP address from the data
portion of the packet and sends the packet as a MAC and IP unicast out
whichever interface is directly connected to the client's IP subnet (unless the
"broadcast reply" bit was set by the client; in that case, it is a MAC-level
Some protocol analyzers do not understand the function of a gateway.
The bit that indicates that a gateway (router) is involved causes the packet to
be flagged as an invalid packet when it is really valid.
The IP address assigned by the BOOTP server might not be correct,
especially if the client has been moved. In that case, it may be sent out the
wrong interface, or dropped if the target subnet is not directly connected to
that router (the most common problem).
Only the first router encountered acts as a gateway. Between the
gateway and BOOTP server, the packets are routed normally.
There are a few other options mentioned in
Make sure of the following:
The helper-address is set to point at the correct BOOTP server. Use
interface configuration command on the
router interface receiving the client BOOTP broadcasts to forward them to the
The BOOTP server assigns a valid IP address.
The router is helping UDP 67 (on by default).
The debug udp gives very detailed information on what
the router is doing.