Infrastructure, Systems, and Connectivity
Instead of choosing between redundancy and bandwidth, EtherChannel combines multiple physical links into a single logical connection that provides both simultaneously, without Spanning Tree blocking any of the links. This article explains how EtherChannel bundles ports together, compares the PAgP and LACP negotiation protocols used to form a bundle safely, and covers the essential configuration and verification commands.
VLANs isolate broadcast domains from each other at Layer 2, but real applications still need devices in different VLANs to communicate, which requires routing between them at Layer 3. This article explains the legacy router-on-a-stick approach using subinterfaces, the modern and more scalable Switch Virtual Interface approach on Layer 3 switches, and the essential configuration and verification commands for both.
Manually configuring an IP address on every device does not scale, and remembering numeric IP addresses for every service is impractical, which is why DHCP and DNS exist as essential supporting services in nearly every network. This article explains how DHCP automatically assigns IP addressing information, covers configuring a Cisco device as a DHCP server or relay agent, and explains how DNS resolves human-readable names into IP addresses.
Access Control Lists let a router or switch selectively permit or deny traffic based on source, destination, and protocol information, forming the foundation of basic network security and traffic filtering. This article explains how ACLs process traffic sequentially, covers the difference between standard and extended ACLs, walks through wildcard mask calculation, and explains critical placement rules that determine whether an ACL works as intended.
The limited supply of public IPv4 addresses made it impossible for every device worldwide to have its own globally unique address, and Network Address Translation solved this by letting many private devices share a small number of public addresses. This article explains the three main NAT types, walks through configuring static NAT, dynamic NAT, and PAT on a Cisco router, and covers the essential commands for verifying active translations.
OSPF is the most widely deployed interior routing protocol in enterprise networks, using a fundamentally different approach than simply exchanging routing tables between neighbors. This article explains what a link-state protocol actually is, how OSPF routers become neighbors and build a shared topology database, how the cost metric determines the best path, and the essential commands for configuring and verifying single-area OSPF.
IPv4's limited address space made a successor protocol inevitable, and IPv6 provides an address space so vast that address exhaustion is no longer a practical concern. This article explains the structure of an IPv6 address, the shorthand notation rules used to write it compactly, the different IPv6 address types, and the essential commands for configuring and verifying IPv6 on a Cisco device.
Before layering on advanced security features, every network device needs basic hardening to prevent unauthorized access and protect against common Layer 2 attacks. This article covers securing device management access with strong authentication, encrypting stored passwords, and configuring port security to restrict which devices can connect to a switch port.
The MAC-learning and ARP mechanisms that make Ethernet networks function are also fundamentally trusting, creating openings for attacks that redirect or intercept traffic without ever touching a firewall. This article explains how a rogue DHCP server or ARP spoofing attack works, and covers how DHCP Snooping and Dynamic ARP Inspection work together to close these Layer 2 vulnerabilities.
Not all network traffic is equally sensitive to delay, and treating a voice call the same as a large file download during periods of congestion produces a poor experience for both. This article explains why QoS matters, covers the classification and marking of traffic using CoS and DSCP, walks through queuing strategies that determine which traffic is serviced first, and covers the essential configuration for applying QoS policies on a Cisco device.
