Troubleshooting Tips

1. Verify Switch Connections
a. Bad or Damaged Cables – many packet errors, or constant loss of link
b. Make sure Cat 5, 5e, or 6 / Fiber – ensure distance and port type
c. Verify Link Status
2. Verify Transceiver Module –
a. Use only Cisco SFP’s in a Cisco switch. Each has an internal EEPROM that is encoded with security information and provides Cisco the ability to identify and validate the module
b. If there is a Copper TAP in the link, verify power, connections, etc.
3. Use the “show interfaces” privileged EXEC command to verify the port or interface error-disabled.
4. Ping the end device
5. Speed, Duplex, Auto-Negotiation, and Cabling Problems – port statistics show a large amount of Alignment, CRC, FCS or late collisions
6. Switch Checklist
a. Run the show version command to verify IOS (latest version?)
b. Run the show interface command (verify all expected interfaces are up)
c. Check for errors on each interface (see#5)
d. Run the show cdp neighbor details command to verify all expected cisco neighboring devices are displayed
e. Run ping tests to verify connectivity to directly attached devices (ACL, Policy)
7. Frame Gozindas (Double Sided captures) for network proof and to eliminate any network magic
8. Look at TTL value in IP header for how many hops
9. Look at Fragmentation Flags fields in the IP header for Don’t Fragment bit / More Fragments
10. Quality of Service
11. Subnet / Routing / Mask problems - Look in trace file for ICMP messages (not pings) Redirects, Unreachable
12. With the trace: Measure Network Latency - 3 way handshake time, ARP Default Gateway Request / Response, DHCP DORA completion time, DNS Request / Response
13. Find Sync Packet – Look at TCP Maximum Segment Size (1460 most efficient) note other options in effect, SACK, Scaling Window, Timestamps, etc.
14. TCP Window size – efficiency of data transmission based on TCP flow control
15. TCP Retransmissions – Sequence Number – ACK number problems - Resets
16. Urgent Bit / Application using for notification
17. Application Thread processing time / Application filling the max segment size