Bad Subnet Configuration on my Cisco 3750?

Posted on 2007-10-15
Last Modified: 2011-10-03
I have a 24-port Cisco 3750 switch that has been configured by a 3rd-party consultant who is no longer available to me, so I need help understanding and correcting this problem...

First, I will describe the current configuration to the best of my ability:  The switch is configured with one VLAN (the default).  The switch is on IP address, and it is configured as the default gateway for the subnet (let's call it "Subnet A") on my in-house network.  Port 3 on the switch is configured as the router for "Subnet B" (, also in-house.  Clients in Subnet B (10.0) are configured to use (port 3) as their default gateway.  The only reason we created Subnet B was to add available IP addresses; at the time, we did not have a need to isolate traffic between the subnets.

The PROBLEM is that all Subnet B clients are required to uplink to the 3750 switch via Port 3.   For some reason, I cannot connect Subnet B clients to any other port on the switch and have communication with Subnet A.  Is this supposed to be the case?  What can I do to allow the two subnets to communicate with each other from any port on the switch?

It seems to me that the consultant should have created VLANS instead of setting one port to be a router for one of the subnets, but this is NOT my area of expertise.

I am fast approaching the need to isolate traffic from other subnets, so this problem is critical to me.  Thanks for any help you can provide!
Question by:isdirect
    LVL 50

    Accepted Solution

    It sounds like the switch is set up with layer 3 ports. Whether or not this is the best design would require additional information as to the topology of the network.

    It could be that the 3750 is the distribution switch and you have all the users connected to access switches with no need for VLANs to extend beyond the access layer switches. In that case, this configuration would be appropriate.

    That's just one of the catches with design... Any way that works can be defended as the "right" way.
    LVL 28

    Expert Comment

    by:Jan Springer
    You can have multiple ports in the same vlan.

    int f0/x
      switchport mode access vlan  50
      ! or whatever subnet B's vlan should be
      ! repeat the config for each port

    int vlan 50 ! or whatever B's vlan should be
     ip address

    If you can't get into the switch because you don't know the passwords, do password recovery:

    Author Comment

    Obviously, the accepted solution isn't really a "solution" so much as it's an "explanation."  There doesn't seem to be a way to do what I want to do.

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