Cisco Networking: Switch Interface Configuration

Your new Cisco switch mostly already configured. You can just connect it to your network after removing it from the box. All the interfaces or ports on your switch are enabled by default, unless you disable them. To get the most out of your switch, you need to do a bit of work, though none of it is too scary.

When you enter the configuration of the Cisco IOS for a switch, the physical ports are logical interfaces, so you specify an interface by describing the physical location of a port. To configure the switch so that you can connect an appropriate device to a port and have it work, follow these steps:

  1. Connect to your switch and get into Configuration mode using a set of commands like the following:

    Switch1>enable
    Password:
    Switch1#configure terminal
    Enter configuration commands, one per line. End with CNTL/Z.
  2. Choose the interface by number.

    You are able to choose from Ethernet, FastEthernet, or Gigabit Ethernet type interfaces.

  3. Specify the interface number.

    All ports in current IOS versions are numbered, starting at the motherboard in one of these formats: network-module-slot/port, 0/interface-card-slot/port, or network-module-slot/interface-card-slot/port. To specify an interface, you specify the location of the port that represents the interface. Effectively, all modules are connected to the motherboard slot on the switch, which is always slot 0 and the first zero in the interface name.

    If you have a switch installed in a router HWIC or working with a chassis-based system, specify the slot that the card is installed into and then the interface in the format of 0/0/10 for the motherboard, 0 for the first HWIC slot, 1 for the first port on the network module.

    In my case, the switch does not have an expansion module, so I specify 0 for the motherboard and 1 for the first port on the switch.

    Switch1(config)#interface fastEthernet 0/1

    When working with switches, the first interface is numbered one, whereas when you work with most other Cisco devices, you find the first interface is zero.

  4. Set the specifics of the network connection or use the Auto settings for medium dependent interface crossover (MDIX), Duplex, and Speed settings.

    MDIX modes include Auto or Off, Duplex modes include Full Duplex or Half Duplex, while speeds will typically be from 10 Mbps up to the speed of the interface. (I cover MDIX in Book III, Chapter 1.)

    Switch1(config-if)#mdix auto
    Switch1(config-if)#duplex auto
    Switch1(config-if)#speed auto
  5. Add a description to the interface.

    Switch1(config-if)#description Firewall Connection

    It is a good idea, at least for important interfaces, to add a description. But because most ports will have computers belonging to ordinary network users, you may not want to add descriptions like “Bob’s office” for these ports. Important ports might include access points, routers, uplinks, or firewalls.

    Giving yourself a description may prevent you from changing the configuration on the wrong interface. These descriptions do not assist with the configuration; they just help prevent human error.

If you disable MDIX, be sure that you use the correct cable. If you have a computing device such as a computer, printer, or router, use a straight-through cable; if you want to connect two switches, you need a crossover cable.

If you are concerned about security in respect to how many computers a user connects, disable the MDIX setting so that the user will not be able to connect a personal switch or access point to their office network jack. Of course, the user could use a crossover cable if they are in the know, in which case public flogging of that user should be performed.

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