4 Replies Latest reply: May 11, 2012 11:50 AM by Bryan RSS

    3750 vlan routing problem


      Hoping someone here can help me with this routing issue on a 3750. It should be pretty straight forward since everything is directly connected.


      I only have 2 subnets both on their own VLAN.  Originally we had another 191 subnet which is now gone.


      VLAN 193



      VLAN 194



      Last week our ISP changed our internal router gig 0/0 ip address from to


      I updated on the 3750 the default route from to and updated the default gateway from to


      Original port 1/0/1 on the 3750 that was connected the router is set like so.


      interface GigabitEthernet1/0/1

      description to Router


      We had no problems with routing with the switchport set the way it was above prior to the IP change, but now only one vlan can get out at one time.  Both cannot see outside and they can no longer talk to each other.


      If I leave that port the way it is 193 cannot get out, but 194 can.

      If I make port 1/0/1 an access port with VLAN 193.  193 can get out just fine, but of course 194 can not get out at that point.


      I tried making 1/0/1 a trunk for all VLANs and that did no good at all nothing gets out at that point.


      Here is the routing table


      Gateway of last resort is to network

  is subnetted, 2 subnets
      C is directly connected, Vlan193
      C is directly connected, Vlan194
      S* [1/0] via


      Thanks for any assistance

        • 1. Re: 3750 vlan routing problem

          Hi Bryan.


          Before the IP address change, it looks like the ISP had a subnet that was unique unto itself and did't conflict with your subnets.  So before the change your ISP was using the subnet  In order for your subnets, 193 and 194 to get to external addresses, they used 191.  Now after the change, your ISP is conflicting with your 193 address space.  So traffic that need to leave the 193 subnet can't get off of the VLAN because your ISP is using that space.


          So ask your ISP to change to something that doesn't conflict with what you already have in your network, change VLAN 193 to maybe VLAN 195 or something else, or maybe NAT is an option, but messy.

          • 2. Re: 3750 vlan routing problem

            Thanks for the reply Erick,  I get the concept, but the details remain murky.  For the time being I had to remove the 194 VLAN, I had enough addresses in 193 for now.  I will need to add it back soon though.  Once I removed the 194 VLAN everything worked great so that still doesn't make a lot of sense to me. The only thing that changed for now was the removal of the 194 VLAN.  193 has no problems communicating in and outside the network now.  IP routing is still on, gateway is still 193.1 and everything works, but as soon as I put VLAN 194 back in with the 194.1 IP address everything goes down the toilet.  Unfortunately I don't think changing the subnet the ISP is using is an option.  That's not my call.  I will look into NAT though.  Thanks again for the reply.

            • 3. Re: 3750 vlan routing problem

              The IP that the ISP has given you is part of VLAN 193.

              Please post the output show ip int brief for the switch, and show vlan.

              • 4. Re: 3750 vlan routing problem

                Just thought I post an update.  That this has been figured out.  The problem was that when our ISP changed the IP addresses on the router interfaces they forgot to update the static routes.  everything on our end was fine there were no problems.  the way it is setup above works fine.