WAN Technologies — From Leased Lines to SD-WAN

Topologies: point-to-point (simple, costly), hub-and-spoke (cheap, hub = SPOF), full/partial mesh. Legacy still in syllabus as concepts: circuit-switched (PSTN/ISDN) and packet-switched (Frame Relay, ATM) — know the idea, not the config. Leased line: dedicated, constant bandwidth, encapsulation HDLC (Cisco default) or PPP (standard, CHAP/PAP, multilink). Ethernet WAN: Metro-Ethernet — E-Line (P2P), E-LAN (multipoint); feels like […]

MPLS — Labels, LDP & Layer 3 VPN

MPLS sits between L2 and L3 (“2.5”): forward by label, not longest-prefix. Ingress PE pushes, core P routers swap, egress pops. Labels are locally significant — each hop may use its own value for the same prefix. 32-bit header: 20-bit label, TC (QoS), bottom-of-stack, TTL. Groups of packets treated alike = FEC. Control plane: IGP […]