9. Switch Interfaces

Cisco CLI for Switches

Accessing Privileged EXEC Mode

To start configuring or viewing details on a Cisco switch:

SW1> enable

Displaying Interface Information

To view all interfaces on Switch 1:

SW1# show ip interface brief

• Purpose: Displays all interfaces with key information similar to the command on Cisco routers.
• Columns:
  • Interface: Lists all interfaces.
  • IP-Address: Shows the assigned IP addresses.
  • Method: Displays the IP address assignment method.
  • Status (Layer 2): Indicates whether the interface is “up” or “down.”
  • Protocol (Layer 1): Indicates the protocol status (also “up” or “down”).

Key Point: Unlike routers, switches do not default to ‘administratively down.’ Unconnected ports will show as “down/down,” indicating no physical connection.

Displaying Interface Status

To view the status of all interfaces on SW1:

SW1# show interfaces status

• Purpose: Lists various attributes of all interfaces on the switch.
• Columns:
  • Ports: Lists all ports on the switch.
  • Name: Displays the description of the interface (configurable by the user).
  • Status: Shows the current connection status.
  • Vlan: Indicates the VLAN associated with the interface (VLAN 1 is the default).
  • Duplex: Shows if the connection is in half or full duplex mode.
  • Speed: Displays the connection speed.
  • Type: Describes the interface type and capabilities.

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Interface Range Configuration

Deactivating Unused Interfaces

Unused interfaces can be a security risk, so it’s a good idea to deactivate them. You can apply changes to multiple interfaces at once.

1. Enter Global Configuration Mode:

   SW1# config t

2. Select a Range of Interfaces:

   SW1(config)# interface range f0/5 - 12

   • Purpose: Selects interfaces FastEthernet 0/5 to FastEthernet 0/12.

3. Apply Configurations:

   • Add a description:

     SW1(config-if-range)# description ## not in use ##

   • Shut down the interfaces:

     SW1(config-if-range)# shutdown

4. Verification:

   • Use show interface status in Privileged EXEC mode or do show interface status in Global Configuration mode to confirm the interfaces are administratively down.

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Full Duplex vs. Half Duplex

Half Duplex

• Operation: Devices cannot send and receive data simultaneously. If receiving data, the device must wait before sending.
• Use Case: Historically used in LAN hubs, now rare.
• Issue: Causes collisions when multiple packets are received by the hub.
• Collision Domain: All devices connected to a hub are in the same collision domain.

Full Duplex

• Operation: Devices can send and receive data simultaneously, with no need to wait.
• Use Case: Supported by most modern switches, reducing or eliminating collisions.

Comparison:

• Hubs: Layer 1 devices; collisions are common and handled by CSMA/CD.
• Switches: Layer 2 devices; collisions rarely occur.

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Speed and Duplex Autonegotiation

Autonegotiation

• Purpose: Interfaces that support multiple speeds (e.g., 10/100/1000 Mbps) default to autonegotiating both speed and duplex settings with connected devices.
• Process: Interfaces “advertise” their capabilities and negotiate the best possible speed and duplex mode.

If Autonegotiation is Disabled

• Speed: The switch will attempt to match the speed of the connected device. If unsuccessful, it defaults to the slowest supported speed (e.g., 10 Mbps).
• Duplex: If the speed is 10 or 100 Mbps, the switch defaults to half duplex. If the speed is 1000 Mbps or higher, the switch uses full duplex.

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Interface Counters and Errors

Monitoring Interface Errors

To monitor and troubleshoot interface errors:

SW1# show interfaces <interface name>

Error Metrics:

• Packets Received / Total Bytes Received
• Runts: Frames smaller than the minimum size (64 bytes).
• Giants: Frames larger than the maximum size (1518 bytes).
• CRC Errors: Frames that failed the Cyclic Redundancy Check (CRC).
• Frame Errors: Frames with incorrect formatting due to errors.
• Input Errors: Total of various input errors, including runts, giants, and CRC errors.
• Output Errors: Frames the switch tried to send but failed due to errors.

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