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  • Network loop control

    IF I have a server with 2 network cards, each connecting to different network. I have a router which defines and routes the trafic between these network. what are the chances of having a loop because of this server having 2 network cards?

  • #2
    Re: Network loop control

    Its possible. You actually have 2 routers:

    -the 'multihomed' server (with 2 nic's) is a router
    -so is the (I'm assuming) 'hardware' router

    You would really have to sit down and examine your topology- logical and physical- to see if a 'loop' is possible.

    Do you suspect there is one? Some strange behaviour?

    regards,

    theterranaut

    Comment


    • #3
      Re: Network loop control

      You could run a routing protocol on the server and the router. That way, they could communicate with each other and decide the best route, creating a "loop free" network.

      Although, for such a small network the trouble might not be worth the benefits. But that it up to you to decide.

      Thanks for your post!

      David
      David Davis - Petri Forums Moderator & Video Training Author
      Train Signal - The Global Leader in IT Video Training
      TrainSignalTraining.com - Free IT Training Products
      Personal Websites: HappyRouter.com & VMwareVideos.com

      Comment


      • #4
        Re: Network loop control

        Wouldn't RRAS have to be configured on the server to make a routing loop possible? Has the OP set the server up as a router? All the OP said was that he had a multihomed server...

        Just asking.
        Regards,
        Jeremy

        Network Consultant/Engineer
        Baltimore - Washington area and beyond
        www.gma-cpa.com

        Comment


        • #5
          Re: Network loop control

          hi Jeremy,
          yes, I see what you mean. But by default, unless you put packet filters
          on its interfaces, a W2K/2K3 server with a NIC in 2 separate networks
          will happily route between these networks. It seems to be something thats
          built into the OS. You can confirm this by shutting down a single NIC server,
          adding in another NIC, powering up, adding an IP on a different network
          for the new NIC, and issuing 'route print' at the command prompt. Hey presto,
          the routes appear! Just like connected routes in the Cisco world.

          Adding RRAS does add a lot more 'functionality', NAT'ing and such like, but
          the old "2 NICs, 1PC, 1 Copy of Windows Server 2003" makes a fairly functional
          (though restricted to Ethernet traffic) router.

          Cheers!

          theterranaut
          Last edited by theterranaut; 29th November 2006, 16:29.

          Comment


          • #6
            Re: Network loop control

            Thanks TT. Is this functionality limited to the server OS? I did a little test and was unsuccessful.
            Here's the configs:

            Computer1 OS=Win 2k Pro Subnet=172.18.21.0
            Code:
            ===========================================================================
            Interface List
            0x1 ........................... MS TCP Loopback interface
            0x1000003 ...00 06 5b 90 48 34 ...... 3Com EtherLink PCI
            ===========================================================================
            ===========================================================================
            Active Routes:
            Network Destination        Netmask          Gateway       Interface  Metric
                      0.0.0.0          0.0.0.0     172.18.21.65   172.18.21.153       1
                    127.0.0.0        255.0.0.0        127.0.0.1       127.0.0.1       1
                  172.18.21.0    255.255.255.0    172.18.21.153   172.18.21.153       1
                172.18.21.153  255.255.255.255        127.0.0.1       127.0.0.1       1
               172.18.255.255  255.255.255.255    172.18.21.153   172.18.21.153       1
                  172.19.12.0    255.255.255.0    172.18.21.130   172.18.21.153       1
                    224.0.0.0        224.0.0.0    172.18.21.153   172.18.21.153       1
              255.255.255.255  255.255.255.255    172.18.21.153   172.18.21.153       1
            Default Gateway:      172.18.21.65
            ===========================================================================
            Computer2 (multihomed) OS=Win 2k Pro Subnet=both
            Code:
            ===========================================================================
            Interface List
            0x1 ........................... MS TCP Loopback interface
            0x1000003 ...00 03 6d 00 83 92 ...... Linksys LNE100TX Fast Ethernet Adapter (LN
            E100TX v4) NDIS5 Driver
            0x1000004 ...00 06 5b dc 3b c6 ...... 3Com EtherLink PCI
            ===========================================================================
            ===========================================================================
            Active Routes:
            Network Destination        Netmask          Gateway       Interface  Metric
                      0.0.0.0          0.0.0.0     172.18.21.65   172.18.21.130       1
                      0.0.0.0          0.0.0.0    172.19.12.129    172.19.12.62       1
                    127.0.0.0        255.0.0.0        127.0.0.1       127.0.0.1       1
                  172.18.21.0    255.255.255.0    172.18.21.130   172.18.21.130       1
                172.18.21.130  255.255.255.255        127.0.0.1       127.0.0.1       1
               172.18.255.255  255.255.255.255    172.18.21.130   172.18.21.130       1
                  172.19.12.0    255.255.255.0     172.19.12.62    172.19.12.62       1
                 172.19.12.62  255.255.255.255        127.0.0.1       127.0.0.1       1
               172.19.255.255  255.255.255.255     172.19.12.62    172.19.12.62       1
                    224.0.0.0        224.0.0.0    172.18.21.130   172.18.21.130       1
                    224.0.0.0        224.0.0.0     172.19.12.62    172.19.12.62       1
              255.255.255.255  255.255.255.255    172.18.21.130   172.18.21.130       1
            Default Gateway:      172.18.21.65
            ===========================================================================
            Computer3 OS=Win98se Subnet=172.19.12.0
            Code:
            Active Routes:
            
              Network Address          Netmask  Gateway Address        Interface  Metric
                      0.0.0.0          0.0.0.0    172.19.12.129    172.19.12.145       1
                    127.0.0.0        255.0.0.0        127.0.0.1        127.0.0.1       1
                  172.18.21.0    255.255.255.0     172.19.12.62    172.19.12.145       1
                  172.19.12.0    255.255.255.0    172.19.12.145    172.19.12.145       1
                172.19.12.145  255.255.255.255        127.0.0.1        127.0.0.1       1
               172.19.255.255  255.255.255.255    172.19.12.145    172.19.12.145       1
                    224.0.0.0        224.0.0.0    172.19.12.145    172.19.12.145       1
              255.255.255.255  255.255.255.255    172.19.12.145    172.19.12.145       1
            This KB alludes to the fact that you need to enable IP routing but the only place I've seen that setting is in RRAS. http://support.microsoft.com/kb/140859
            Last edited by JeremyW; 29th November 2006, 23:54. Reason: Added color (ooo... ahhh)
            Regards,
            Jeremy

            Network Consultant/Engineer
            Baltimore - Washington area and beyond
            www.gma-cpa.com

            Comment


            • #7
              Re: Network loop control

              Thanks Jeremy,

              I'm away from the lab just now and cant check that I actually had to do anything additional... damn, you've got me thinking now!

              I do remember it being incredibly sensitive to daft things like the order in which the nics get assigned IP's and whether or not the respective interfaces had gateways or not. It basically creates the routes as the IPs get setup. I now distinctly remember having to play with the gateways and ridding myself of one of the default routes..I'll have to get back to you on this!

              Actually, can you check those gateways on the workstations for me:

              Computer#1: (172.18.21.0 network side) has an interface of 172.18.21.153, gateway of .65
              Computer#3: (172.19.12.0 network side) has an interface of .145, gateway of 172.19.12.129 yes?

              But looking at the multihomed box, I see:
              Interface: 172.18.21.130
              Interface: 172.19.12.62

              As I say, I'm riffing a bit here and may be misreading this (got roped into helping out with some imaging tonight <groans>) but I think you'll find that the gateways of 1 and 3 need to correspond to the nics on either side of the multihomed machine, ie, .130 and .62.

              (I'm gently preparing myself for the embarrassment of seeing that I have, in fact, stuck RRAS on that box after all...)



              theterranaut
              Last edited by theterranaut; 29th November 2006, 23:55.

              Comment


              • #8
                Re: Network loop control

                I'll give changing the gateways a shot but you'll notice I did configure static routes to the respective subnets. (Bolded above and now I'll make them red as well)
                Regards,
                Jeremy

                Network Consultant/Engineer
                Baltimore - Washington area and beyond
                www.gma-cpa.com

                Comment


                • #9
                  Re: Network loop control

                  I changed the default gateway on both the Win 2k and 98se to 172.18.21.130 and 172.19.12.62 respectively. Still nothing (echo requests) was routed to the other subnet.

                  Let me know what you find TT.

                  Thanks.

                  EDIT - just read "I now distinctly remember having to play with the gateways and ridding myself of one of the default routes". I missed that before. I'll try that and see what happens.
                  Last edited by JeremyW; 30th November 2006, 14:24.
                  Regards,
                  Jeremy

                  Network Consultant/Engineer
                  Baltimore - Washington area and beyond
                  www.gma-cpa.com

                  Comment


                  • #10
                    Re: Network loop control

                    Guys,
                    Thank you very much for your input. I am novice in this field, so I am trying my best to have my network stable.

                    I would apprciate if you can provide me links to tools for network monitoring, so that I can identify where problems are so that I can be proactive in my network administration activities.

                    I also would like to know what settings are critical to be enable in every switch in a network?

                    Thanking you.

                    Comment


                    • #11
                      Re: Network loop control

                      Originally posted by mariamislam View Post
                      I would apprciate if you can provide me links to tools for network monitoring, so that I can identify where problems are so that I can be proactive in my network administration activities.
                      Check out Wireshark http://www.wireshark.org/

                      I also would like to know what settings are critical to be enable in every switch in a network?
                      Others would be able to give you advice on this.

                      TT, I had a few extra minutes and I took one of the gateways out on the multihomed computer. Here's what happened:

                      Computer1 = 172.18.21.153 gateway 172.18.21.130
                      Computer2 = 172.18.21.130 gateway 172.18.21.65 (IF 1)
                      Computer2 = 172.19.12.146 gateway none (IF 2)
                      Computer3 = 172.19.12.145 gateway 172.19.12.146

                      Computer1
                      -ping 172.18.21.130 successful
                      -ping 172.19.12.146 successful
                      -ping 172.19.12.145 unsuccessful

                      Computer2
                      -ping 172.18.21.153 successful
                      -ping 172.19.12.145 successful

                      Computer3
                      -ping 172.19.12.146 successful
                      -ping 172.18.21.130 successful
                      -ping 172.18.21.153 unsuccessful
                      Regards,
                      Jeremy

                      Network Consultant/Engineer
                      Baltimore - Washington area and beyond
                      www.gma-cpa.com

                      Comment


                      • #12
                        Re: Network loop control

                        Hi Jeremy,
                        sorry I've not been back to get into this, I've been away from my lab for a few days o cant really test/check configs.

                        Thats interesting- it looks as though its not being returned from the respective gateway on the other side. I 'll get back onto this one as soon as I can.

                        Sorry guys-

                        theterranaut

                        Comment


                        • #13
                          Re: Network loop control

                          Originally posted by theterranaut View Post
                          Hi Jeremy,
                          sorry I've not been back to get into this, I've been away from my lab for a few days o cant really test/check configs.

                          Thats interesting- it looks as though its not being returned from the respective gateway on the other side. I 'll get back onto this one as soon as I can.

                          Sorry guys-

                          theterranaut
                          Hey no obligations here. Take your time.
                          Regards,
                          Jeremy

                          Network Consultant/Engineer
                          Baltimore - Washington area and beyond
                          www.gma-cpa.com

                          Comment

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