Lecture Notes: Network Troubleshooting and Configuration Issues

Half-Duplex vs. Full-Duplex Ethernet

• Half-Duplex: Legacy devices, potential collisions
  • Collisions: Occur when two devices send data simultaneously
  • Collision Handling: Devices recognize collision, wait randomly, retransmit
  • Performance Issues: More collisions with more devices
• Full-Duplex: Modern networks, no collisions
  • Configuration Check: Verify devices are not mismatched (full vs. half)
  • Hardware Issues: Check NICs and drivers

Monitoring Network Performance on Cisco Switches

• Show Interfaces Command: Check runts, giants, input errors, CRCs, collisions, late collisions
• Interface Resets: Check configurations on switch and connected devices

IPv4 and Broadcast Domains

• Broadcasts: Normal for certain protocols
  • VLAN Limitation: Broadcasts limited to devices in the same VLAN
  • Performance Issues: Too many broadcasts can degrade network performance
  • Packet Capture: Identifies broadcast source and frequency
• Mitigation: Segment network into smaller subnets

Duplicate MAC and IP Addresses

• MAC Addresses: Unique, issues can indicate on-path attacks or manufacturing errors
  • Packet Capture: Detect duplicate MACs through ARP
  • ARP Cache: Check device MAC addresses
• IP Addresses: Duplicate IPs more common, especially with static assignments
  • DHCP Issues: Multiple servers or static IP conflict
  • Troubleshooting: Ping network, check ARP table, verify switch interfaces
  • DHCP Capture: Detect multiple DHCP server responses

Multicast Flooding

• Multicast Traffic: Sent to all switch ports by default
  • IGMP Snooping: Intelligent multicast traffic forwarding

Asymmetric Routing

• Definition: Different paths for outbound and inbound traffic
  • Firewall Issues: State-based devices may drop traffic
  • Traceroute: Identifies asymmetric routes

Switching Loops

• Definition: Network loop causing continuous traffic circulation
  • Prevention: Spanning Tree Protocol (STP)
  • Layer 3 Networks: Routing loops with incorrect next hop
  • Traceroute Detection: Identifies looping routes

Missing Routes

• Definition: Traffic sent to router with no forwarding path
  • ICMP Notification: Host unreachable message
  • Troubleshooting: Check routing tables on all routers for proper ingress and egress paths