4.8 Given a scenario, troubleshoot and resolve common WAN issues - Paiet/Tech-Journal-for-Everything GitHub Wiki

4.8 Given a scenario, troubleshoot and resolve common WAN issues

  • Customer Premise Equipment (CPE)

    • Cisco Loopback Page
    • Smart jack/NIU (Google Images)
      • Installed between the demarcation point and the CSU/DSU
      • Provides signal conversion
        • Convert the framing protocol if needed
      • PRI/T1
    • Demarc
      • Demarcation point
      • This is the spot where the telco's responsibility ends and the customer's begins
      • Typically a device in a telco closet
        • Customer is responsible to connect the CPE to the demarc device
    • Loopback
      • Virtual interface for testing proper operation of the networking software
      • Loopback plugs are used to test physical operation of the interface's ability to transmit and receive
    • CSU/DSU
      • Channel Service Unit/Data Service Unit
      • Connects the serial line of the customer router to the telco network
      • Can be an external device
      • Can be a WIC in the router
    • Copper line drivers/repeaters
      • Amplifies or repeats the signal to extend the demarc up to an additional 5000 feet
      • Helps connect equipment across campuses, multiple floors of a building, or between office buildings

  • Loss of internet connectivity

    • Check customer equipment before reporting the outage to service provider
      • Check power to all devices including the smart jack, copper line drivers, and/or repeaters
        • Don't forget physical cable connections
      • Perform loopback tests to the telco equipment to establish that the problem is not with the CPE
        • Soft loopback
          • Use the router OS to perform loopback test
        • Hard loopback
          • Use a loopback plug to test
        • Telco-assisted loopback
          • After performing soft and hard loopback tests and eliminating CPE as the problem
          • You and the telco will start testing your way farther and farther into the telco cloud until the root of the problem is discovered

  • Interface errors

    • Link status

      • Is the link down?
      • Cable unplugged?
      • show interface FastEthernet 0/0

    • Interface is misconfigured

      • Speed and duplex settings

        • Mismatched speeds will not connect
        • Mismatched duplex will cause errors
        • Use autonegotiate or manually set the speed and duplex
        Router(config)# int ser1/0
Router(config-if)# speed (10, 100, 1000)
Router(config-if)# duplex (auto, full, half)


      - Input queue drops
    + Indicates that the router is being overwhelmed
        with input traffic
- Output queue drops
    + Indicates packet loss due to interface congestion
    + Heavily impacts VoIP systems
        - Probably need to adjust QoS
- Input errors
    + Cabling issues
    + Hardware failures
    + Duplex mismatch
    + `show controllers serial 1/0`
        - Look for `line state` and clocking
    + `show interface serial 1/0`
        - Look for `Serial1/0 is up, line protocol is down`
- Output errors
    + Happens when collisions occur
    + Could be interference on the ISP's line

  • Split horizon

    • Caused by multiple Permanent Virtual Circuits (PVCs) on the same interface when using Frame Relay
      • Split Horizon is a method of preventing routing loops that happen when routes are received from the same interface that they were advertised from
    • Make sure to use sub-interfaces
      • R1(config)# interface Serial0/0.102 point-to-point
      • R1(config)# interface Serial0/0.103 point-to-point

  • DNS issues

    • Problems with DNS can simulate a network outage
      • Users won't be able to resolve FQDNs leading them to believe that the network is down
      • DNS servers; local or ISP owned?
      • Check your servers for proper operation

  • Interference

  • Router configurations

    • Improper router configuration will prevent connectivity
      • Using the wrong routing protocol
      • Setting the wrong IP information
      • Setting the wrong protocol or encapsulation type
    • Check routing table and default route

  • Company security policy

    • Throttling
      • Can slow down certain services like social media and updating news feeds
    • Blocking
      • Policy may dictate that a specific service or port is to be blocked/dissallowed
        • This can simulate an network problem
    • Fair access policy/utilization limits
      • In place to preserve bandwidth

  • Satellite issues

    • Latency