A Device as a BOOTP Relay Agent
If BOOTP request requires multiple hops before reaching the BOOTP server,the
BOOTP relay agent routes the packet via IP to all BOOTP servers that it knows
about. If any other router receives this packet while it is being routed via IP,it will
examine the packet to determine whether it is a BOOTP packet and route that
packet toward the BOOTP servers that it knows about. The following describes the
BOOTP relay agent forwarding process:
1. A device acting as the local BOOTP relay agent, receives the BOOTPrequest
packet from the BOOTP client, modifies the checksum, places an IP header on
the packet with the relay agent’s IP address copied into the body of the BOOTP
request, and routes the packet to all BOOTP servers.
2. The BOOTP servers receive the request and look up the client’s MAC address
in their database. If a server finds the client’s address, it formats a BOOTP reply
containing the client’s IP address, the location of its boot file, and the boot
server name. The reply is then sent to the BOOTP relay agent.
3. The BOOTP relay agent receives the reply,makes an entry in its ARP table for
the client, and then forwards the reply to the BOOTP client.
4. The client then continues to boot using the information that is contained in the
BOOTP reply packet to initiate a TFTP request to the boot server.
Enabling/Disabling BOOTP Forwarding
Toenable or disable BOOTP forwarding on the router, enter the following
appropriate command at the IP configuration prompt:
IP Config> enable bootp
IP Config> disable bootp
When enabling BOOTP,you are prompted for the following values:
vMaximum number of application hops you want the BOOTP request to go.
This is the maximum number of BOOTP relay agents that can forward the
packet. This is
not
the maximum number of IP hops to the BOOTP server.A
typical value for this parameter is 4.
vNumber of seconds you want the client to retry before you forward the BOOTP
request.

This parameter is not commonly used.

A typical value for this parameter
is 0.
After accepting a BOOTP request, the router forwards the BOOTP request to each
BOOTP server. If there are multiple servers configured for BOOTP,the transmitting
server replicates the packet.
Configuring a BOOTP Server
The BOOTP server is either an AIX or UNIX host with a

bootpd

daemon, or a DOS
host (running software available from FTP Software). The BOOTP server contains a
file (maintained by the network administrator) that lists all the BOOTP clients that
this server is responsible for, and their associated IP addresses, boot file locations,
and boot server names.
When the BOOTP server receives a BOOTP request, it matches the MAC address
of the client with the MAC address in its BOOTP file. If a match occurs, the server
Using the Boot CONFIG Process
Chapter7. The Boot CONFIG Process 85