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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
      9  TCP/IP Network Configuration and Management Protocols (BOOTP, DHCP, SNMP and RMON)
           9  Host Configuration and TCP/IP Host Configuration Protocols (BOOTP and DHCP)
                9  TCP/IP Bootstrap Protocol (BOOTP)

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BOOTP Vendor-Specific Area and Vendor Information Extensions
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3
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TCP/IP Dynamic Host Configuration Protocol (DHCP)
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BOOTP Relay Agents (Forwarding Agents)
(Page 3 of 4)

Normal BOOTP Operation Using a Relay Agent

The following shows, in simplified form, a revised set of BOOTP operation steps when a relay agent is involved. To keep the size of this topic manageable I have omitted the details of the basic request/reply process to focus on the relaying functionality, which you can also see graphically in Figure 258:


Figure 258: BOOTP Operation Using A Relay Agent

In this example, Device A is trying to access a BOOTP server, but the only one is on a different network; the two are connected by a workgroup router that is configured to act as a BOOTP relay agent. Device A broadcasts its request, which the router receives. It relays the request to the BOOTP server, Device D, and puts its own IP address (IPR) into the BOOTP GIAddr field. The BOOTP server sends the reply back to the router using address IPR. The router then broadcasts it on Device A’s local network so that A can receive it.

 


  1. Client Creates Request: The client machine creates its request normally. The existence of a relay agent is totally transparent to the client.

  2. Client Sends Request: The client broadcasts the BOOTREQUEST message by transmitting it to address 255.255.255.255. (Note that in the case where a client already knows both its own address and the address of a BOOTP server, we don't need the relay agent at all—both the request and reply can be sent unicast over an arbitrary internetwork.)

  3. Relay Agent Receives Request and Processes It: The BOOTP relay agent on the physical network where the client is located is listening on UDP port 67 on the server's behalf. It processes the request as follows:
    • It checks the value of the Hops field. If the value is less than or equal to 16, it increments it. If the value is greater than 16, it discards the request and does nothing further.

    • It examines the contents of the GIAddr field. If this field is all zeroes, it knows it is the first relay agent to handle the request and puts its own IP address into this field. (If the agent is a router it has more than one IP address, so it chooses the one of the interface upon which it received the request.)


  4. Relay Agent Relays Request: The relay agent sends the BOOTP request to the BOOTP server. If the relay agent knows the server's IP address it will send it unicast directly to the server. Otherwise, if the agent is a router, it may choose to broadcast the request on a different interface from the one on which it received the request. In the latter case, it is possible that multiple relay agents may be required to convey the request to the server. See below for more on this.

  5. Server Receives Request and Processes It: The BOOTP server receives the relayed request from the BOOTP relay agent. It processes it as normal.

  6. Server Creates Reply: The server creates a reply message as normal.

  7. Server Sends Reply: Seeing that the GIAddr field in the request was non-zero, the server knows the request was relayed. Instead of trying to send its reply back to the client that sent the request, it transmits the reply unicast back to the relay agent specified in GIAddr.

  8. Relay Agent Relays Reply: The BOOTP relay agent transmits the BOOTREPLY message back to the client. It does this either unicast or broadcast, depending on the value of the CIAddr field and the B (Broadcast) flag, just like a server does in the non-relay case.

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TCP/IP Dynamic Host Configuration Protocol (DHCP)
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