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Forum Replies

  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-pr

    ... Continue reading in our forum

  4. Hi Rene,

    Can you explain the packet flow if the DHCP server is 3 routers away from the client network? In this scenario, what DHCP fields would change at every router?

  5. 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

    Rene

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