6.3.1. Switching and Routing Problems

STP Issues:

• Broadcast storm: STP not working, loops present—network overwhelmed
• Root bridge selection: Wrong switch is root, traffic takes suboptimal paths
• Port in wrong state: Stuck blocking, not transitioning to forwarding

VLAN Issues:

• Wrong VLAN assignment: Device in wrong VLAN, can't reach intended resources
• Trunk misconfiguration: Inter-switch link left as an access port (only one VLAN crosses, untagged), VLANs not allowed on the trunk, or native VLAN mismatch

Interface mismatch vs. loop: A duplex/speed mismatch slows only the one link it is on (late collisions, CRC errors—see 6.2.2), whereas a switching loop floods every port and drives switch CPU up.

Routing Issues:

• Missing route: No path to destination in routing table
• Wrong next-hop: Packets sent to wrong router
• AD/metric issues: Suboptimal path selected
• Routing loop: Routers keep forwarding a packet between each other until its TTL expires; traceroute shows the same hops repeating
• Asymmetric routing: Traffic returns by a different path than it went. A missing return route—or a stateful firewall that sees only one direction—breaks the flow even though the forward route exists. When traceroute dies partway, check routing at that hop and the return path
• Neighbors not forming: OSPF routers must share a subnet (and mask), area ID, hello/dead timers and authentication to become neighbors; the OSPF process ID is locally significant and need not match