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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 Dynamic Host Configuration Protocol (DHCP)
                     9  DHCP Client/Server Implementation, Features and Issues

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DHCP Client General Implementation and Management Issues
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DHCP Autoconfiguration / Automatic Private IP Addressing (APIPA)
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DHCP Message Relaying and BOOTP Relay Agents
(Page 2 of 2)

DHCP Relaying Process

Since DHCP was designed specifically to support BOOTP relay agents, the agents behave in DHCP much as they do in BOOTP. Of course DHCP has much more complex message exchanges, but as we've already seen, they are all still designed around the notion of a client request and server response. There are just more requests and responses. The BOOTP agent looks for broadcasts sent by the client and then forwards them to the server just as described in the topic on BOOTP relay agent behavior, and then returns replies from the server. The additional information in the DHCP protocol is implemented using additions to the BOOTP message format in the form of DHCP options, which the relay agent doesn't look at. It just treats them as it does BOOTP requests and replies.

So, in summary, when a relay agent is used, here's what the various client requests and server replies in the DHCP operation section become:

  • Client Request: When a client broadcasts a request, the relay agent intercepts it on UDP port 67. It checks the Hops field, and discards the request if the value is greater than 16; otherwise it increments the field. The agent puts its own address into the GIAddr field unless another relay agent has already put its address in the field. It then forwards the client request to a DHCP server, either unicast or broadcast on another network.

  • Server Reply: The server sees a non-zero value in GIAddr and sends the reply to the relay agent whose IP address is in that field. The relay agent then sends the reply back to the client, using either unicast or broadcast as discussed in the topic on DHCP addressing.

One difference between BOOTP and DHCP is that certain communications from the client to the server are unicast. The most noticeable instance of this is when a client tries to renew its lease with a specific DHCP server. Since it sends this request unicast, it can go to a DHCP server on a different network using conventional IP routing, and the relay agent does not need to be involved.

Key Concept: To permit DHCP clients and DHCP servers to reside on different physical networks, an intermediary device is required to facilitate message exchange between networks. DHCP uses the same mechanism for this as BOOTP: the deployment of BOOTP relay agents. The relay agent captures client requests, forwards them to the server, and then returns the server’s responses back to the client.



Previous Topic/Section
DHCP Client General Implementation and Management Issues
Previous Page
Pages in Current Topic/Section
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2
Next Page
DHCP Autoconfiguration / Automatic Private IP Addressing (APIPA)
Next Topic/Section

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