Switch Interfaces | Day 9

• Topics covered


  • Interface speed and duplex
  • Speed and duplex autonegotiation
  • Interface status
  • Interface counters and errors
• Switches are used to connect end hosts to. This catalyst switch might have 48 PCs connected to the RJ45 ports, and then connect to a router with the SFP fiber optic ports.
• Router interfaces are in an administratively disabled state by default, meaning they have the 'shutdown' command applied. Switch interfaces, however, are different. They don't have the shutdown command applied, so if you connect them to another device they'll usually be in the up/up state with no configuration required.
• Router interfaces have the shutdown command applied by default, so they will be in the administratively down/down state by default. Switch interfaces do not have the shutdown command applied by default, so they will either be in the up/up state, if they are connected to another device, or the down/down state, if not connected.
• Speed


  The data rate in bits per second, such as 10, 100, or 1000 megabits per second
• Duplex


  Whether a device is able to both send AND receive data at the same time
• Autonegotiation allows two devices to negotiate speed and duplex settings without us having to manually configure them.
• Cisco devices keep various counters regarding the traffic that passes through them, such as how many errors have occurred, etc.
• The 'show interfaces status' command provides more detailed information about switch interfaces compared to 'show ip interface brief'.
• Half-duplex


  The device cannot send and receive data at the same time. If it is receiving a frame, it must wait before sending a frame.
• Full-duplex


  The device CAN send and receive data at the same time. It does not have to wait.
• In modern networks that use switches, all devices can use full duplex on their interfaces.
• CSMA/CD (Carrier Sense Multiple Access with Collision Detection)


  The mechanism used by Ethernet devices to avoid and deal with collisions in a half-duplex situation
• Switches operate at layer 2, using layer 2 addressing, MAC addresses, to send frames to specific hosts. They also won't try to send two frames to the same host at once.
• Half-duplex


  Device cannot send and receive data at the same time. If it is receiving a frame, it must wait before sending a frame.
• Full-duplex


  Device can send and receive data at the same time, it does not have to wait.
• Devices attached to a hub must operate in half-duplex
• Devices attached to a switch can operate in full-duplex
• Speed and duplex autonegotiation

  1. Interfaces advertise their capabilities to neighbors

  2. Negotiate the best speed and duplex settings they are both capable of
• If autonegotiation is disabled on a device

  Switch will use the slowest supported speed and half-duplex if speed is 10 or 100 Mbps, full-duplex if speed is 1000 Mbps or greater
• Autonegotiation should be used on all devices in the network
• Runts


  Frames smaller than the minimum Ethernet frame size of 64 bytes
• Giants


  Frames larger than the Ethernet maximum frame size of 1518 bytes
• CRC


  Frames that failed the cyclic redundancy check in the Ethernet frame
• Frame


  Frames with incorrect or illegal format
• Input errors


  Total of various error counters including runts, giants, CRC, and frame
• Output errors


  Frames the switch tried to send but failed due to an error
• CSMA/CD is used on half-duplex interfaces to detect and avoid collisions
• Duplex mismatch between interfaces with autonegotiation disabled


  Collisions will occur
• The 'show interfaces' command shows various counters of errors detected on an interface