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  1. There is absolutely no better place on the internet to study CISCO related materials than here. I am in Ghana, how do I purchase your books.

  2. Hi Rene,

    Will the ip helper-address always be the address of the interface on the the DHCP server that connects to the DHCP relay?

  3. Hi Diana,

    A router becomes a DHCP relay when it “relays” DHCP broadcasts to another interface. We do this with the “IP helper-address” command.

    This command, however, does more than just forwarding DHCP broadcasts, here’s a list:

    • UDP 69 - TFTP
    • UDP 67 - BOOTP Client
    • UDP 68 - BOOTP Server
    • UDP 37 - Time Protocol
    • UDP 49 - TACACS
    • UDP 53 - DNS
    • UDP 137 - NetBios
    • UDP 138 - NetBios Datagram

    So by default, it will relay all broadcasts to these UDP destination ports. If you want you can also enable relaying for other destination ports. For example:

    ip forward-protocol udp 3000

    This would relay broadcast packets with destination UDP port 3000. You can also disable some of the default ports:

    no ip forward-protocol udp 67
    no ip forward-protocol udp 68
    no ip forward-protocol udp 137
    no ip forward-protocol udp 138

    Hope this helps!


  4. Hi Shree,

    Nothing will change. The router that is configured for DHCP relay will create these unicast packets. Other routers only have to route these packets to the DHCP server, that’s it. Here’s a packet capture of these unicast packets btw:

    DHCP Relay Unicast Packets


  5. Hello Juan

    When the DHCP server is in the same subnet, the following communications take place:

    • DHCPDISCOVER is broadcast on both layer 2 and layer 3 (MAC and IP)
    • DHCPOFFER as a response to the discover is unicast. It uses the MAC address of the original sender as the destination MAC and the proposed IP address as the destination IP (even though the DHCP client does not yet have an IP address assigned. This doesn’t matter since communication is happening at Layer 2 for now since we are on the same subnet)
    • DHCPREQUEST is also broadcast on both Layer 2 and Layer 3. Take a look at this sample wireshark capture of a DHCP Request. Notice the destination MAC and the destination IP are broadcast addresses:
    • DHCPACK from DHCP server to client is also unicast.

    Now in the case of a relay agent, refer to the diagram from the lesson. All traffic between the relay agent R1 and the DHCP client H1 remains the same as that described above. However, as stated in the lesson by Rene, the traffic between the R1 and the DHCP server that exists on another subnet is unicast.

    I hope this has been helpful!


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