Chapter 3 – Switch Port Configuration

Ethernet Concepts

  • How does half-duplex operation work?
    • By the nature of ethernet collision can happen. A technology called CSMA/CD (carrier sense multiple access collision detect) trying to resolve this by backing off for a random period of time when a collision occurs. After that the station can retransmit. The more crowded an Ethernet segment becomes the more collision will happen.
  • How does full-duplex work?
    • If we implement switches in an ethernet network the collision domains are broken. All switch port is a separate collision domain. This way the stations can transmit and receive simultaneously without the possibility of collision (so prevention is not needed either).
  • What are the specifications of FastEthernet?
    • maximum speed: 100Mbps
    • full- / half-duplex
    • UTP / fiber
    • UTP max length ~100 meter
    • Fast EtherChannel (FEC) up to 8 links
  • What are the specifications of GigabitEthernet?
    • maximum speed: 1Gbps
    • full- / half-duplex
    • UTP / fiber
    • support legacy ethernet speeds 10/100/1000
    • Gigabit EtherChannel (GEC) up to 8 links
  • What are the specifications of Ten-GigabitEthernet?
    • maximum speed: 10Gbps
    • only full duplex
    • Physical Media Dependent (PMD) interfaces:
      • LAN PHY: Interconnects switches in a campus network, predominantly in the core layer
      • WAN PHY: Interfaces with existing synchronous optical network (SONET) or synchronous digital hierarchy (SDH) network that were typically found in metropolitan-area networks (MAN)
  • What other scales exist in Ethernet?
    • 40Gbps and 100Gbps Ethernet. Both are bonds of multiple Ten-GigabitEthernet.
  • How does speed and duplex negotiation work?
    • speed is determined by electrical signaling so that either end of a link can determine what speed the other end is trying to use
    • Duplex is negotiated: If you want to use autonegotiation both side must be configured to autonegotiate, or else one side will NEVER get information about the other end’s duplexity.
      If autonegotiation fails a switch port always falls back to half-duplex because it offers collision detection.Speed and duplex mode can be configured or negotiated only on switch ports that support twisted-pair cabling.
  • What is “switchport auto negotiation” command used for?
    • The same as duplex and speed coomand. When autonegotiation is enabled, the port automatically detects the speed or pause method, and duplex of incoming signals based on the link partner. You can also detect link up conditions using the autonegotiation feature.
  • What is the numbering convention of a Cisco interface (ie.: Fa1/0/14) ?
    • interface type member/module/number
    • In the example: the 14th FastEthernet port on the first switch in the stack
  • What are the 2 ways to configure multiple interfaces at the same time?
    • interface range GigabitEthernet1/0/2, GigabitEthernet1/2/1
    • interface range GigabitEthernet1/0/1 – 1/0/10
  • How do you define interface macro?
    • define interface-range MyGroup Gig1/0/1, Gig2/0/10, Gig3/0/1 – 3/0/5
      interface range macro MyGroup
  • How do you configure port speed?
    • interface INTERFACE-NAME
        speed 10/100/1000
  • How do you configure port duplex mode?
    • interface INTERFACE-NAME
        duplex half/full
  • What is an error condition?
    • When an error condition is detected the switch port is put into errdisable  state and is disabled. This can be tuned so it is only triggered by specific errors.
    • Some options:
      all : Detects every possible cause
      arp-inspection : Detects errors with dynamic ARP inspection
      bpduguard : Detects when a spanning-tree bridge protocol data unit (BPDU) is received on a port configured for STP PortFast
      dhcp-rate-limit : Detects an error with DHCP snooping
      ■ dtp-flap : Detects when trunking encapsulation is changing from one type to another
      gbic-invalid : Detects the presence of an invalid GBIC or SFP module
      inline-power : Detects an error with offering PoE inline power
      l2ptguard : Detects an error with Layer 2 Protocol Tunneling
      ■ link-flap : Detects when the port link state is “flapping” between the up and down states
      loopback : Detects when an interface has been looped back
      pagp-flap : Detects when an EtherChannel bundle’s ports no longer have consistent configurations
      pppoe-ia-rate-limit : Detects errors with PPPoE Intermediate Agent rate limiting
      psecure-violation : Detects conditions that trigger port security configured on a port
      psp : Detects an error related to protocol storm protection
      security-violation : Detects errors related to 802.1X security
      sfp-config-mismatch : Detects errors related to SFP configuration mismatches
      small-frame : Detects errors when VLAN-tagged packets are too small and arrive above a certain rate
      storm-control : Detects when a storm control theshhold has been exceeded on a port
      udld: Detects when a link is seen to be unidirectional (data passing in only one direction)
  • How can you check the reason of being error disabled?
    • show interface status err-disabled
  • How can be a port recovered from Error Condition?
    • Manually: shutdown then a no shutdown
    • Automatically:
        errdisable recovery cause ERROR-CONDITION
        errdisable recocvery interval TIME-IN-SEC
  • How can you check which type of ERROR-CONDITIONs are going to be recovered automatically?
    • show errdisable recovery
  • What is the point of reenabling an errdisable port automatically?
    • When it is more important to keep a link up until the problem can be resolved.
  • How can you check port status effectively?
    • show interface
    • show interface status


  • What does the protocol and status means in a show int desc output?
    • protocol: physical or datalink layer
    • status: Layer 2
  • How can you notice a duplex mismatch?
    • Runts” and “Input errors” are visible in the show interface output.
  • What are the characteristics of Cisco Discovery Protocol?
    • it only work in one direction; advertisements are sent out periodically toward any listening device but nothing expected in return
      So if you want to fill your CDP table faster, you need to configure the timer of the neighbor devices.
    • works in Layer 2
    • by default CDP advertisements are sent out every 60 seconds
    • advertisements sent to a multicast address: 01:00:0C:CC:CC:CC
    • CDP is enabled by default
  • How do you turn off/on CDP?
    • It can be configured globally or under a specific interface
  • What is Layer Link Discovery Protocol (LLDP)?
    • Same as CDP but based on IEEE; work in multivendor network.
  • What are the characteristics of LLDP?
    • disabled by default (on Catalyst switches)
      You can check with: show lldp
  • How do you turn on/off LLDP?
    • globally: lldp run
    • globally: no lldp run


PoE (Power over Ethernet)


  • What type of PoE methods exist?
    • ILP (Cisco Inline Power) – 7W
    • PoE (IEEE 802.2af) – 15.4W
    • PoE+ (IEEE 802.3at) – 25.5W
    • UPoE (Cisco Universal PoE) – 60W
  • How can a PoE device detect a peer?
    • When a switch port is down, there power is also disabled ALTHOUGH the switch tries to detect whther a powered device is connected.
      It provides small voltage and measures the resistance wether the currect is being drawn by any other device.
  • How much power is sent?
    • By default the switch offer 15.4W / port. The peer can ask for more using CDP or LLDP advertisements and requests (it can ask up to 30W – 802.3at).
  • How do you configure PoE?
    • power inline auto/static max MILIWATTS
      • auto: the peer asks for power (CDP / LLDP)
      • static: you configure the exact power amount
      • max: you can configure the maximal power that could be asked by the peer (ie: 4000 = 4W)
  • What power classes exist?
    • 0 – 15.4W (default)
    • 1 – 4W
    • 2 – 7W
    • 3 – 15.4W
    • 4 – up to 30W (802.3at)
  • How do you turn off PoE?
    • power inline never
  • How do you verify PoE?
    • show power inline

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