Core: Layer 3 Control Plane
Requirement / business driver Minimize traffic loss on failure to meet application SLAs (voice/video, storage, trading). The convergence budget - attack each phase Detect -> propagate -> compute (SPF/DUAL) -> update FIB. You shrink total loss by improving every phase, not just one. * Detection: carrier-delay/debounce tuning, link-down
Core: Layer 3 Control Plane
In one line The things that slow or break convergence beyond raw timers - so a design that looks fast on paper can still lose traffic. The factors * Recursion - a route whose next hop must itself be resolved (e.g. BGP next-hop via the IGP) adds a dependency; next-hop
Core: Layer 3 Control Plane
In one line Steer traffic by manipulating routing metrics/attributes - the cheapest TE lever, before you reach for tunnels (SR-TE / RSVP-TE). The levers * IGP - cost/metric tuning: OSPF reference-bandwidth and per-link cost, IS-IS wide metrics, EIGRP delay/K-values; used to set primary vs backup
Core: Layer 3 Control Plane
Requirement / business driver Deliver one-to-many (or many-to-many) efficiently with the right tree type and a resilient rendezvous-point (RP) design. PIM modes ModeTree / RPBest forNote PIM-SSMSource-specific, no RPKnown sources (IPTV, market data)Simplest; needs IGMPv3 / MLDv2 PIM-SMShared tree via RP, then switch to
Core: Layer 3 Control Plane
Requirement / business driver IPv6 is driven by public-IPv4 scarcity, regulatory or customer mandates, reaching IPv6-only services, or future-proofing. The hard design question is rarely "how does IPv6 work" - it's where to start and how to coexist with IPv4 without breaking applications. The phased
Core: Data Plane Transport
In one line The transport layer decides reliability, ordering, congestion behaviour, and how visible traffic is to the network - and that shapes buffering, load-balancing, and security design. The three you must reason about * TCP - connection-oriented, reliable, ordered, congestion-controlled. The workhorse for web/APIs, storage, and bulk transfer;
Core: Data Plane Transport
In one line TCP throughput is governed by window / RTT / loss, so the network's latency and drop behaviour directly cap application speed. The mechanics that matter * Windowing + bandwidth-delay product (BDP): max in-flight = window; throughput ~= window / RTT. Long-fat networks (high BW x high RTT) need window
Core: Data Plane Transport
Requirement / business driver Move full-size packets end to end without silent black-holing - the classic, hard-to-diagnose failure when MTU and overlays collide. The mechanics * IPv4 can fragment in transit (DF bit aside); IPv6 does NOT fragment in transit - only the source may, guided by PMTUD. * PMTUD relies
Core: Data Plane Transport
Requirement / business driver Absorb transient bursts and signal congestion without either dropping latency-sensitive traffic or adding bufferbloat - a core data-centre and WAN-edge decision. The forces * Microbursts: sub-second spikes that overrun interface buffers even when average utilisation is low (see Factors affecting convergence) - they cause drops that
Core: Data Plane Transport
Requirement / business driver Spread traffic across parallel paths/links evenly and without reordering - the basis of CLOS/spine-leaf and LAG scaling. How it works * ECMP / LAG hash a flow (typically the 5-tuple) to one path, keeping a flow's packets in order. Per-packet balancing spreads better
Core: Data Plane Transport
Requirement / business driver Secure data in transit while preserving the visibility and control the network needs - a tension that grows as encryption becomes pervasive. The shift * TLS over TCP, DTLS over UDP, and QUIC's mandatory TLS 1.3 mean most traffic is now encrypted by default. * QUIC rides
Core: Network Virtualization
In one line Label-switched forwarding decoupled from the IP lookup - the substrate that makes scalable VPNs, traffic engineering, and fast reroute possible. Forwarding plane Label switch routers (LSRs) swap labels hop-by-hop along a Label Switched Path (LSP); the ingress PE pushes a label, the egress pops it,