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Cisco dhcp+secondary static IP?

Discussion in 'Networking, Telephony & Internet' started by tin, Oct 3, 2018.

  1. tin

    tin Member

    Joined:
    Jul 31, 2001
    Messages:
    6,420
    Location:
    Narrabri NSW
    I've got a Cisco router doing ADSL with zone based firewall running. I'm going to switch to NBN VDSL soon, and intend to stick a bridged modem on a spare FastEthernet port, put said port into the internet zone, set the interface to use DHCP (the ISP we're looking at uses DHCP) and all the usual magic.

    My problems come when I then want to monitor the modem. Basically I need to have DHCP for the internet, but need a second static address to route to the modem.

    Obvious first idea was "ip address dhcp" and "ip address <details> secondary" on the port I'm going to use. But Cisco apparently don't see the point of that, and disable the secondary address when you enable DHCP.
    Second idea was VLANs. But the modem doesn't support VLANs on the LAN side.

    Any ideas?
     
  2. evilasdeath

    evilasdeath Member

    Joined:
    Jul 24, 2004
    Messages:
    5,005
    depends on your modem and how it presents itself

    a) sometimes you can route the private ip of the WAN interface as a /32 with the interface as next hop (this works with cable but in this case the modem kinda routes)

    b) burn a LAN port on your switch and connect that back to the modem and assign the ip in your lan zone again this will depend on if your modem still advertises a private range on the lan ports while in bridge mode.

    c) put a managed switch in the middle and out the WAN through a vlan and add an SVI on the intermediate switch (also burns ports)
     
  3. OP
    OP
    tin

    tin Member

    Joined:
    Jul 31, 2001
    Messages:
    6,420
    Location:
    Narrabri NSW
    Finally got to trying some stuff... Turns out it was as simple as DHCP on the main fa0/1 interface, and then tell a subinterface to use VLAN 1 with a static IP for the modem.

    At least in theory, using a test setup. Only way to find out for sure is sign up for NBN :p
     

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