Troubleshooting and Diagnostics
Layered troubleshooting and diagnostics
Troubleshoot bottom-up or follow the path: physical layer, then Layer 2, then Layer 3, then application. Check interface status, speed/duplex, CRC and input errors, PoE, and transceiver diagnostics. At Layer 2, verify VLAN membership, trunk allowed lists, MAC learning, STP state, LAG/LACP status, and VSX/VSF health. At Layer 3, check ARP, routing table, OSPF neighbors, and ACL counters. Use logs, NAE, mirroring, packet capture, and cable tests. Compare counters over time rather than reading a single snapshot.
Traps: blaming STP for a duplex mismatch; ignoring CRC errors that indicate cabling/SFP problems; assuming a LAG is up because one member is up; and forgetting that an ACL or port security violation can silently drop traffic. Also check asymmetric routing and return-path issues.
Worked example/rule of thumb: A port shows increasing CRC/alignment errors while other ports are clean. That points to the cable, connector, or optic, not to OSPF or STP. Rule: isolate by layer, change one variable at a time, and verify the return path as well as the forward path.