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BGP's finite state machine has six states: Idle, Connect, Active, OpenSent, OpenConfirm, Established. What each means, why Active is a problem state, and the diagnostic that maps each stuck state to a real cause.

Tagged and untagged frames are the half of every VLAN problem nobody explains clearly. The mental model, when each is correct, the native VLAN, and the cross-vendor (Cisco / Aruba / Juniper) cheat sheet.

VLAN hopping bypasses Layer 2 isolation. The two attacks (switch spoofing via DTP, double tagging via the native VLAN) and the four-line Cisco configuration that defeats both.

The 802.1Q VLAN tag is 4 bytes inserted into Ethernet frames to carry VLAN identity across trunks. TPID, TCI, PCP, DEI, VID, and the native VLAN explained byte by byte.

RSTP (802.1w) replaced classic 802.1D's 30-50 second convergence with sub-second failover. The five key changes that make it possible, and how to deploy Rapid PVST+ on Cisco IOS XE.

BGP and OSPF solve different problems and almost always run together in production. The technical differences, when each is the right answer, and the architecture pattern most enterprises end up with.