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Forum Replies

  1. Hi rené, i have another question: How can we use DHCP Relay when we have multiple subinterfaces for multiple VLANs on the fastethernet 0/0 ( ie: fastethernet 0/0.1, 0/0.2 and so on) of the middle router knowing that each VLAN has its own addressing scheme ??


  2. I just found this and i think it solves the problem: while configuring the DHCP router for multiple VLANs, we must specify the default-router (it’s the gateway adress of each VLAN) from which the request is comming (as shown below). This way the DHCP router assigns IPs from the particular pools of each VLAN.

    (config)# ip dhcp pool VLAN1
    (dhcp-config)# network
    (dhcp-config)# default-router
    (dhcp-config)# exit
  3. 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?

  4. 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!


  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


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