A complicated network, but possible to create.

Discussion in 'Networking Issues' started by SAPo57, Sep 7, 2006.

  1. SAPo57

    SAPo57 Network Guru Member

    This has been a problem for quite sometime now, but I think I got a solution, so what I just need is an evaluation to see if what I did to my network is right.

    Well, let me put the facts down before anyone suggests something that I've tried doing before:

    First of all, using RIP or Dynamic routing is impossible to setup on this network right now (maybe later on with more advanced equipment).

    Second, I have a cable modem/router that can be bridged, but its' DHCP server cannot be disabled (it doesn't allow that option).

    And third, I need a way to use two gateways at the same time in a way that if one fails the other can keep clients connected to the internet (no EIGRP is being used nor any load-balancing router).

    Heres my current network setup:

    ====INTERNET====------------SBG cable modem/router (BRIDGED)------------

    ---WRT54GS (uses MAC clonning)----------DSL modem/router====INTERNET====

    Now as you can see the WRT54GS handles NAT operations automatically from the SBG modem/router. If the SBG modem fails to establish a connection with the ISP my WRT54GS can get a WAN IP from the DSL modem/router.

    How is this possible? Well, since my WRT54GS is using AUTOMATIC-DHCP it can receive a WAN IP address if there's an available gateway (DSL modem cannot be bridged, because I need to use PPPoE instead on the WRT54GS).

    The MAC address clone configuration together with the bridged SBG cable modem sets a higher priority, so the WRT54GS and the SBG router will connect first "IF" it has a connection to the ISP. If not, my DSL router will give me a WAN IP.

    NOTE: to receive WAN IP's from both gateways I had to use a switch to connect both gateways to and then used one network cable to connect to the WRT54GS's WAN port from the switch and another cable to one of the LAN ports on the router, so that I may connect to each gateway's interface.

    Also, on my WRT54GS's setup screen I left the gateway address as, so that it may choose any gateway automatically that is available. And instead of putting all the DNS IP addresses of each gateway under the DHCP setting I just put the IP address of each gateway and left the third DNS setting as, so that the router can automatically choose a primary DNS IP address (DEPENDING ON WHICH GATEWAY I'M CONNECTED TO) from the ISP as well.

  2. Guyfromhe

    Guyfromhe Network Guru Member

    That should in theory work, but not a great idea because your sending dhcp reuqests and not acking one of them...
    Thats sorta here nor there kinda thing though... It should grab an IP from the first responding DHCP server, however if one ISP fails it won't hop to the other one automatically... it will have to wait until the lease expires, which will likely be a couple days...
    you might be able to write a little shell script that checks if the link goes down and sends another DHCP request....
  3. SAPo57

    SAPo57 Network Guru Member


    I can't disable DHCP on the WRT54GS because if I do that then if the SBG modem fails or something clients will still be locked to the SBG router's DHCP server (same problem can happen with the DSL router) and If I reboot the WRT54GS a DHCP conflict can occur as there are two DHCP servers from the two gateways and the WRT54GS won't be able to choose one, right?

    But if you can show me how to write some of those scripts I would gladly appreciate it.
  4. Guyfromhe

    Guyfromhe Network Guru Member

    both the cable and dsl modem would be plugged into the WAN port, the DHCP server on the wrt only runs on the LAN ports.
    This is fine.
  5. SAPo57

    SAPo57 Network Guru Member

    So, what were you trying to say from your first reply about the DHCP requests and acking?
  6. Guyfromhe

    Guyfromhe Network Guru Member

    Your router will make a DHCP request to your cable modem and your DSL when it tries to get an IP...
    They will both supply an IP but only one will get a response from your router, thats not normal behavior... my only concern would be if it hopped from one to the other when your lease expired.
  7. SAPo57

    SAPo57 Network Guru Member

    Actually wouldn't it only matter for the DSL modem handing out an IP address from its' DHCP server, because as I said before the Cable modem is bridged and doesn't give an IP address from its' DHCP server to the WRT54GS rather a Public IP address from the ISP. The wrt54gs's WAN IP would be something like when connected to the SBG.
  8. Guyfromhe

    Guyfromhe Network Guru Member

    I'm talking about your cable ISPs DHCP server, not your cable modems.
    It's actually not so bad if you don't respond to your DSL modems DHCP server.
  9. SAPo57

    SAPo57 Network Guru Member

    You should of just said "the DHCP server of public IP addresses the ISP hands out". If you just say "DHCP server" you can be talking about any type of server that hands out either dynamic or static addresses which can confuse others.:thumbup:
  10. Guyfromhe

    Guyfromhe Network Guru Member

    Their all the same DHCP server just running at a different location.
  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice