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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 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

  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 Lay
    ... Continue reading in our forum

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