Infrastructure, Systems, and Connectivity
Modern enterprise routers must forward millions of packets per second, far beyond what a routing table lookup on every single packet could sustain, which is why Cisco Express Forwarding exists as the default forwarding architecture on virtually all Cisco devices. This article explains the performance limitations of older forwarding methods, walks through how CEF's FIB and adjacency table achieve high-speed forwarding, and covers the essential commands for verifying CEF operation.
EIGRP combines the simplicity of distance-vector routing with fast convergence properties that rival link-state protocols, using a distinctive algorithm that guarantees loop-free paths without requiring a full topology map. This article explains what makes EIGRP an "advanced" distance-vector protocol, covers the composite metric it uses to select paths, walks through the DUAL algorithm's feasible successor concept, and covers essential configuration and verification commands.
A single-area OSPF design, discussed earlier in this series, does not scale to large networks, since every router must process the full topology database of every other router. This article explains why OSPF areas exist, covers the different Link-State Advertisement types that carry information between areas, explains the role of Area Border Routers, and walks through configuring route summarization to keep large multi-area networks efficient.
Every interior routing protocol covered so far in this series operates within a single organization's network, but connecting separate organizations together across the internet requires an entirely different protocol built around policy rather than pure shortest-path calculation. This article explains what makes BGP a path-vector protocol, covers the distinction between eBGP and iBGP, walks through essential path attributes used for path selection, and covers basic BGP configuration and verification.
Real enterprise networks often run multiple routing protocols simultaneously, whether due to mergers, legacy equipment, or vendor requirements, and these protocols do not automatically share routes with each other. This article explains why redistribution becomes necessary, covers the critical metric mismatch problem between protocols, walks through configuring redistribution between OSPF and EIGRP, and covers the routing loop risks that make careful redistribution design essential.
Every host on a network relies on a single default gateway, and that gateway becoming a single point of failure would undermine the redundancy carefully built everywhere else in the network. This article explains why first hop redundancy matters, walks through HSRP's active/standby model, compares it against the open-standard VRRP, and covers GLBP's added ability to load-balance traffic across multiple routers simultaneously.
Sending the same video stream individually to a thousand viewers would waste enormous bandwidth, and multicast solves this by delivering a single stream efficiently to exactly the devices that actually want it. This article explains how multicast addressing differs from unicast and broadcast, covers IGMP as the protocol hosts use to join multicast groups, and walks through how PIM builds the distribution trees that carry multicast traffic efficiently through a network.
Every technology covered so far in this series -- VLANs, routing protocols, redundancy protocols -- needs an overall architectural framework to be deployed coherently rather than as an ad hoc collection of features. This article explains the classic three-tier hierarchical design model, covers the distinct role each layer plays, explains the simplified two-tier collapsed core alternative, and discusses how these models extend into modern data center design.
Traditional networking, covered throughout most of this series, distributes intelligence across every individual device, each making its own independent forwarding decisions. Software-Defined Networking fundamentally changes this by centralizing that intelligence, and this article explains the control plane and data plane separation underlying SDN, covers how SD-WAN applies these principles specifically to wide area network connectivity, and explains the practical benefits this architectural shift provides.
Basic EIGRP configuration, covered earlier in this series, only scratches the surface of what the protocol can do -- EIGRP can distribute traffic across paths with genuinely different costs, and can summarize routes at any arbitrary point in the network rather than only at fixed area boundaries. This article explains EIGRP's variance-based unequal-cost load balancing, covers flexible route summarization at any interface, and walks through EIGRP stub routing for hub-and-spoke topologies.
The basic BGP path selection process, covered earlier in this series, follows a fixed order of attributes, but real networks need to actively influence which path gets chosen rather than passively accepting the default outcome. This article explains how route maps filter and modify BGP routing information, covers Local Preference and MED as the two primary levers for influencing path selection, and introduces BGP communities as a flexible tagging mechanism for coordinating policy across an entire network.
Multi-area OSPF, covered earlier in this series, already reduces database size by separating a network into areas, but OSPF offers further specialized area types that reduce routing table size even more aggressively by filtering out unnecessary external routes entirely. This article explains the LSA types that must be suppressed to create each specialized area type, walks through configuring stub, totally stubby, and not-so-stubby areas, and covers the specific trade-offs each design choice involves.
