Networking Security
Routing & Switching
Ethernet, VLANs, network devices, configuration and verification, with Cisco switch and router commands.
- 72 notes
- 646 minutes of reading
- Ethernet, VLANs and Token Ring: logical topologies explainedHow Ethernet works (CSMA/CD, 10BASE-T, Fast and Gigabit Ethernet), why VLANs split broadcast domains and how they are configured, and what Token Ring was. 9 min · Intermediate
- Network devices: NIC, hub, switch, bridge, router, access point, modem, firewall and moreWhat each network device does and which OSI layer it works at: NIC, hub, switch, bridge, repeater, router, access point, modem, content filter, firewall, load balancer, packet shaper and VPN concentrator. 9 min · Beginner
- Configuring and verifying network connections: Windows settings, ping, traceroute, Telnet and SSHHow to set up a network adapter and IP settings in Windows 10 and 11, choose automatic or manual addressing, and verify connectivity with ping, traceroute, Telnet and SSH. 8 min · Beginner
- Cisco switch commands: VLANs, trunks and port securityThe important Cisco IOS switch commands for basic setup, secure management with SSH, VLANs and trunks, port security, DHCP snooping and dynamic ARP inspection, and verification. 8 min · Intermediate
- Cisco router commands: interfaces, routing, ACLs and NATThe important Cisco IOS router commands for interfaces, static and OSPF routing, inter-VLAN routing, standard and extended ACLs, NAT/PAT, DHCP, and management services such as NTP and syslog. 8 min · Intermediate
- How switches work: MAC address learning, flooding, forwarding and agingHow a Layer 2 switch builds its MAC address table from source addresses, when it forwards, floods or filters a frame, how aging works across several switches, and how MAC flooding attacks are stopped with port security. 8 min · Beginner
- How routers forward packets: the routing table, longest prefix match, AD and metricsA walk through what a router does with every packet, how to read a Cisco routing table, how longest prefix match picks the route, and how administrative distance, metrics and equal-cost paths decide what goes into the table. 9 min · Intermediate
- Static, default and floating static routes: configuration, tracking and pitfallsHow to configure static routes, default routes and floating static backup routes on Cisco IOS for IPv4 and IPv6, the difference between next-hop and exit-interface forms, tracking a route with IP SLA, and the mistakes that cause outages. 8 min · Beginner
- OSPF fundamentals: link-state routing, areas, cost and basic configurationHow OSPF works as a link-state protocol: LSAs, the link-state database and SPF, router IDs, cost and reference bandwidth, single and multi-area design, and a clean, secure basic configuration with verification commands. 10 min · Intermediate
- OSPF neighbours and DR/BDR: adjacency states, network types and troubleshootingHow OSPF routers become neighbours, the adjacency states from Down to Full, what must match for an adjacency to form, how the designated router is elected, OSPF network types, authentication, and a step-by-step troubleshooting method. 10 min · Advanced
- EIGRP fundamentals: DUAL, successors, feasible successors and the composite metricHow EIGRP works: neighbours and its three tables, the bandwidth-and-delay metric, feasible distance and reported distance, the feasibility condition that guarantees loop-free backup routes, unequal-cost load balancing and basic configuration. 9 min · Intermediate
- Inter-VLAN routing in practice: router-on-a-stick, SVIs and troubleshootingA practical guide to routing between VLANs: how a packet travels from one VLAN to another, configuring router-on-a-stick and Layer 3 switch SVIs on Cisco IOS, DHCP relay, controlling traffic between VLANs, and a checklist for when it does not work. 9 min · Intermediate
- Spanning tree in practice: port states, path cost, PortFast and BPDU GuardThe practical side of spanning tree: classic and rapid port states and roles, how path cost decides the root port, choosing the root bridge, and the protection features (PortFast, BPDU Guard, Root Guard, Loop Guard) every access and distribution switch should use. 9 min · Intermediate
- Switch interfaces: speed, duplex, error counters and err-disabled portsHow to set up and read switch interfaces: speed and duplex auto-negotiation, duplex mismatches, what CRC errors, runts, giants and output drops mean, why ports become err-disabled and how to recover them safely. 8 min · Beginner
- Securing routing: protocol authentication, route filtering, uRPF and control plane protectionHow to protect routers and routing protocols: authenticating OSPF, EIGRP and BGP neighbours, passive interfaces, filtering routes with prefix lists, anti-spoofing with uRPF and ingress filtering, control plane policing, and RPKI for internet routing. 10 min · Advanced
- Cisco IOS CLI and basic device configuration: modes, help, switch management IP and router as a hostHow to move around the Cisco IOS command line, use help and shortcuts, save your work, give a Layer 2 switch a management IP address, apply a basic router configuration and use a router as a simple host or server. 9 min · Beginner
- Inside the switch: static MAC entries, CEF, SDM templates and unicast floodingWhat happens inside a Cisco switch after it learns MAC addresses: static MAC entries, Cisco Express Forwarding (CEF) with its FIB and adjacency table, SDM templates that share out hardware memory, and why asymmetric routing causes unicast flooding. 9 min · Intermediate
- 802.1Q trunking in depth: tags, native VLAN, DTP, troubleshooting and Q-in-QHow 802.1Q tags frames on a trunk, how to configure and restrict trunks, why the native VLAN matters, how Dynamic Trunking Protocol negotiates (and why to turn it off), how to troubleshoot VLANs and trunks, and how 802.1Q tunnelling (Q-in-Q) stacks two tags. 10 min · Intermediate
- Voice VLANs and Power over Ethernet: configuring ports for IP phonesHow a single switch port carries both a PC and an IP phone using a voice VLAN, how the phone learns its VLAN through CDP or LLDP-MED, how Power over Ethernet standards and classes work, and how to configure and check PoE on Cisco switches. 8 min · Beginner
- VTP in depth: modes, revision numbers, pruning and VTP version 3How the Cisco VLAN Trunking Protocol copies VLANs between switches, what the configuration revision number does, why an old switch can wipe out every VLAN, how VTP pruning works, and how VTP version 3 makes the protocol safer. 8 min · Intermediate
- Private VLANs, protected ports and VLAN ACLs: isolating hosts inside one subnetHow to stop devices in the same VLAN from talking to each other: private VLANs with promiscuous, isolated and community ports, the simpler protected port feature, and VLAN access maps (VACLs) that filter traffic inside a VLAN. 9 min · Advanced
- 802.1X and MAC Authentication Bypass: controlling who can use a switch portHow IEEE 802.1X port-based access control works with a supplicant, an authenticator switch and a RADIUS server, how MAC Authentication Bypass handles printers and cameras, a basic Cisco configuration, and how to roll it out without locking everyone out. 9 min · Intermediate
- SPAN, RSPAN and RMON: capturing and measuring switch trafficHow to copy traffic from switch ports to a packet analyser with local SPAN, carry it to another switch with RSPAN or across a routed network with ERSPAN, and collect built-in traffic statistics and threshold alarms with RMON. 8 min · Intermediate
- EtherChannel in practice: Layer 3 bundles, load balancing and troubleshootingBuilding on the basics of LACP and PAgP: how to configure Layer 2 and Layer 3 EtherChannels on Catalyst switches, how traffic is shared out with hashing, and how to read show etherchannel summary to fix bundles that will not form. 9 min · Intermediate
- Switch virtualisation: StackWise, VSS, StackWise Virtual and multichassis EtherChannelHow several physical Cisco switches can act as one: stacking with StackWise at the access layer, VSS and StackWise Virtual for pairs of distribution or core switches, multichassis EtherChannel, and the split-brain problem of dual-active detection. 8 min · Intermediate
- Spanning tree under the hood: BPDUs, timers, topology changes and reconvergenceWhat is inside a BPDU, how the hello, max age and forward delay timers work, how classic 802.1D spanning tree announces topology changes with TCN BPDUs, how long reconvergence takes after direct and indirect failures, and what UplinkFast and BackboneFast did about it. 9 min · Advanced
- PVST+, root bridge placement and Rapid Spanning Tree (RSTP) configurationHow Per-VLAN Spanning Tree runs a separate tree per VLAN, how to place root bridges to share load across uplinks, and how Rapid Spanning Tree converges in about a second with port roles, edge ports, link types and the proposal and agreement handshake. 10 min · Intermediate
- Multiple Spanning Tree (MST): regions, instances and interoperating with PVST+How IEEE 802.1s Multiple Spanning Tree maps many VLANs onto a few spanning tree instances, what makes switches part of the same region, how to configure MST on Cisco switches, and what to watch when an MST region meets switches running PVST+. 9 min · Advanced
- The spanning tree toolkit: BPDU Filter, Loop Guard, UDLD, Bridge Assurance and PortFast behaviourThe less-known spanning tree protection features and where to use them: how PortFast really behaves, BPDU Filter in its global and interface forms, Loop Guard and UDLD against unidirectional links, the RSTP dispute mechanism and Bridge Assurance. 9 min · Advanced
- Troubleshooting spanning tree, its limitations and the alternatives (including FlexLinks)A practical method for spanning tree faults such as broadcast storms, the wrong root and flapping MAC addresses, the commands that reveal them, the built-in limitations of spanning tree, and the designs that replace it: EtherChannel, virtual switch pairs, routed access, fabrics and the legacy FlexLinks feature. 9 min · Advanced
- Classful vs classless routing, ip classless, ICMP redirects, directed broadcasts and IP unnumberedWhy classful routing protocols failed with modern subnetting, what auto-summary and discontiguous networks are, what the ip classless command changes in forwarding, and three router features you will meet in configurations: ICMP redirects, IP directed broadcasts and IP unnumbered interfaces. 9 min · Intermediate
- MTU, TCP MSS and PPPoE: fixing "some websites do not load" problemsWhat the maximum transmission unit and the TCP maximum segment size are, how path MTU discovery works and why it breaks, why PPPoE and tunnels reduce the MTU, and how to configure a PPPoE client and a simple PPPoE server on Cisco routers. 8 min · Intermediate
- Routing policy tools: wildcard bits, prefix lists, distribute lists, offset lists and route-mapsThe tools Cisco routers use to select and change routes: wildcard bits in ACLs, prefix lists with ge and le, distribute lists to filter routing updates, offset lists to adjust metrics, and route-maps that combine matching with set actions. 10 min · Advanced
- Policy-based routing (PBR) and reliable PBR with IP SLA trackingHow policy-based routing sends selected traffic by a different path than the routing table would choose, how to configure it with an ACL and a route-map, how to make it fail over with IP SLA and object tracking, and when to use something else. 8 min · Intermediate
- Route redistribution: seed metrics, route tagging and avoiding loops between protocolsHow routes move between routing protocols such as EIGRP, OSPF and RIP, why every protocol needs a seed metric, how to configure redistribution in both directions, and how route tags, filtering and administrative distance prevent loops and suboptimal paths. 10 min · Advanced
- GRE tunnels on Cisco IOS: configuration, recursive routing, tunnel keys and MTUHow Generic Routing Encapsulation builds a virtual point-to-point link across another network, how to configure a GRE tunnel and run a routing protocol over it, why recursive routing shuts tunnels down, what the tunnel key is for, and how to handle MTU and security. 9 min · Intermediate
- Network virtualisation on routers: VRF-lite, Cisco EVN and LISPHow virtual routing and forwarding (VRF) gives one router several separate routing tables, how to configure VRF-lite with static and dynamic routes, how Cisco Easy Virtual Network simplified carrying VRFs between devices, and how LISP separates a device's identity from its location. 9 min · Advanced
- Fast convergence: BFD, loop-free alternate fast reroute and CEF polarizationHow routers detect failures in milliseconds with Bidirectional Forwarding Detection, how OSPF and EIGRP loop-free alternates and remote LFA pre-install backup paths, and why equal-cost load sharing can become polarized and how to avoid it. 9 min · Advanced
- Optimized Edge Routing and Performance Routing (OER/PfR): legacy performance-based WAN routingWhat Cisco Optimized Edge Routing and its successor Performance Routing did: master controllers and border routers, the profile, measure, policy, control and verify phases, policy priorities and variance, timers, and why SD-WAN has replaced them. 8 min · Advanced
- Binary and hexadecimal for network engineers, and building your own subnetting cheat sheetHow binary and hexadecimal numbers work, how to convert between them and decimal quickly, why subnet masks and IPv6 and MAC addresses depend on them, and how to write a one-page subnetting cheat sheet from memory before an exam or a design task. 9 min · Beginner
- IPv6 prefixes and summarisation: finding the prefix and combining routesHow to find the IPv6 prefix of any address for a given prefix length, why nibble boundaries make it easier, how to handle lengths such as /61 that split a hex digit, and how to summarise several IPv6 prefixes into one route. 8 min · Intermediate
- OSPF packets, neighbour discovery and LSDB floodingThe five OSPF packet types, how two routers move from Down to Full, and how link-state advertisements are flooded, acknowledged, aged and refreshed so every router keeps the same database. 8 min · Intermediate
- OSPF LSA types and how to read the OSPF databaseWhat each OSPFv2 LSA type describes, which router creates it and how far it is flooded, plus a practical guide to reading show ip ospf database output to find where a route came from. 9 min · Intermediate
- OSPF network types: broadcast, point-to-point, NBMA, point-to-multipoint and loopbackHow each OSPF network type behaves: DR election, neighbour discovery, default timers and next-hop handling, with guidance on choosing the right type for Ethernet, WAN links, DMVPN and loopbacks. 9 min · Intermediate
- OSPF router ID, hello and dead timers, passive interfaces and cost tuningHow to configure and verify the OSPF router ID, change hello and dead intervals safely, use passive interfaces to protect LANs, and set reference bandwidth and interface cost so OSPF picks the paths you expect. 8 min · Intermediate
- Multi-area OSPF: configuration, ABRs, path selection and virtual linksHow to configure OSPF across several areas, what an ABR does, how OSPF chooses between intra-area, inter-area and external routes, and when (and when not) to use a virtual link. 9 min · Intermediate
- OSPF authentication: plain text, MD5, HMAC-SHA and TTL securityWhy OSPF neighbours should authenticate each other, how plain text, MD5 and HMAC-SHA authentication work and are configured on Cisco IOS, how to rotate keys, and how TTL security adds protection. 8 min · Intermediate
- OSPF default routes, summarisation and route filteringHow to advertise a default route into OSPF, summarise inter-area and external routes on ABRs and ASBRs, and filter routes with filter-lists, distribute-lists and summary options, including the traps to avoid. 9 min · Advanced
- OSPF stub, totally stubby and NSSA areas explained with configurationWhat OSPF stub, totally stubby, not-so-stubby (NSSA) and totally NSSA areas block and allow, how to configure each on Cisco IOS, how type 7 LSAs are translated and what the P-bit does. 9 min · Advanced
- Advanced OSPF: throttling, stub router, graceful restart, fast reroute and OSPFv3An overview of the OSPF features used in larger networks: SPF and LSA throttling, incremental SPF, prefix suppression, stub router, graceful shutdown and restart, loop-free alternate fast reroute, OSPFv3 address families, and how to troubleshoot missing routes. 10 min · Advanced
- EIGRP packets, neighbour adjacency, timers and the neighbour and topology tablesThe EIGRP packet types and reliable transport, what must match for two routers to become neighbours, hello and hold timers, static neighbours, passive interfaces, the router ID, and how to read the neighbour and topology tables when troubleshooting. 9 min · Intermediate
- EIGRP metric in depth: K values, the formula, wide metrics and unequal-cost load balancingHow the EIGRP composite metric is calculated from K values, a worked example, why load and reliability are left off, how wide metrics fix fast links, and how variance enables unequal-cost load balancing safely. 9 min · Advanced
- EIGRP named mode and authentication: MD5, HMAC-SHA-256 and per-interface keysHow EIGRP named mode is structured and why it is preferred, and how to secure EIGRP neighbours with MD5 or HMAC-SHA-256 authentication, using key chains and different keys per interface. 8 min · Intermediate
- EIGRP summarisation, stub routing, stuck in active and default routesHow EIGRP manual and automatic summarisation work, how leak maps let specific routes through, how stub routing and summaries limit the query scope that causes stuck-in-active, and the ways to advertise a default route. 9 min · Advanced
- EIGRP route filtering and advanced features: distribute-lists, route-maps, add-path, OTP and troubleshootingHow to filter EIGRP routes with distribute-lists using ACLs, prefix-lists and route-maps, plus a tour of advanced features: add-path, LFA, Over the Top, graceful shutdown, external route handling, legacy Frame Relay behaviour and a route troubleshooting checklist. 10 min · Advanced
- RIP explained: RIPv2 configuration, timers, loop prevention and troubleshootingHow the Routing Information Protocol works, the difference between RIPv1, RIPv2 and RIPng, its timers and loop-prevention rules, and how to configure passive interfaces, default routes, summaries, offset-lists and load sharing on Cisco IOS. 9 min · Beginner
- IS-IS fundamentals: NET addresses, levels, DIS and pseudonode, metrics and configurationHow IS-IS works as a link-state protocol: NET addressing, level 1 and level 2 routing, packet types, the designated intermediate system and pseudonode on LANs, narrow and wide metrics, and a basic Cisco IOS configuration. 10 min · Advanced
- IS-IS authentication, redistribution, summarisation, filtering and route leakingHow to secure IS-IS adjacencies and LSPs with authentication, redistribute routes into IS-IS, summarise at level boundaries, filter routes and leak level 2 routes into level 1 for better exit selection, with Cisco IOS examples. 9 min · Advanced
- HSRP and VRRP configuration, tracking and troubleshooting on Cisco IOSStep-by-step HSRP and VRRP configuration with priorities, pre-emption, interface tracking and authentication, the differences between HSRP versions 1 and 2 and VRRPv3, and a checklist for fixing common first hop redundancy faults. 9 min · Intermediate
- Configuring NAT on Cisco IOS: static NAT, dynamic NAT, PAT and port forwardingHow to configure and verify static NAT, dynamic NAT with a pool, PAT (overload) and port forwarding on a Cisco router, what inside local and inside global mean, and how to troubleshoot translations that do not work. 9 min · Intermediate
- BGP fundamentals: autonomous systems, path vector routing, eBGP, iBGP and multihomingWhat the Border Gateway Protocol does, autonomous system numbers including private and 4-byte ASNs, how path vector routing prevents loops, the difference between eBGP and iBGP, and when single-homed, dual-homed and multihomed sites need BGP. 9 min · Intermediate
- Configuring BGP on Cisco IOS: neighbours, network statements, next hop and reading the BGP tableA practical guide to configuring eBGP and iBGP on Cisco IOS, advertising networks with network statements and aggregates, eBGP multihop to loopbacks, the iBGP next-hop problem and next-hop-self, auto-summary, and how to read show ip bgp output. 10 min · Intermediate
- BGP neighbour states, messages and troubleshooting sessions and missing routesThe BGP finite state machine from Idle to Established, the five BGP message types, active and passive session roles, a checklist for sessions that will not come up, why advertised routes go missing, and soft reconfiguration versus route refresh. 9 min · Intermediate
- BGP path attributes and best path selection: weight, local preference, AS path, MED and moreThe BGP path attributes and the order Cisco routers use to choose the best path: weight, local preference, locally originated, AIGP, AS path length and prepending, origin, MED, eBGP over iBGP, IGP metric and tie-breakers, plus BGP multipath. 10 min · Advanced
- BGP communities and route filtering: prefix lists, AS path regex, route maps and avoiding transitHow BGP communities tag routes, the well-known NO_EXPORT, NO_ADVERTISE and LOCAL_AS communities, filtering with prefix lists, AS path regular expressions and route maps, IPv6 route filtering, legacy extended ACL filtering, and how to stop your network becoming a transit AS. 10 min · Advanced
- Scaling iBGP: full mesh, route reflectors, confederations, peer groups and synchronisationWhy iBGP needs a full mesh, how route reflectors and their client rules work, how confederations split an AS into member ASes, how peer groups and templates simplify configuration, and what the old BGP synchronisation rule did. 9 min · Advanced
- Advanced BGP features: MP-BGP, private AS removal, allowas-in, AS override, aggregation, dampening and backdoorsA tour of the BGP features you meet in CCNP and provider networks: multiprotocol BGP address families, removing private ASNs, allowas-in and as-override for sites sharing an ASN, 4-byte ASN compatibility, aggregation with AS_SET, route dampening and backdoor routes. 10 min · Advanced
- BGP security and convergence: route leaks, RPKI, next-hop tracking, Add-Path and PICHow route leaks and hijacks happen, how RPKI route origin validation works and how to deploy it, session protections such as TTL security and maximum-prefix, and the features that make BGP converge faster: next-hop tracking, BFD, Add-Path and Prefix Independent Convergence. 10 min · Advanced
- MPLS fundamentals: labels, LSRs, label operations and LDPWhy service providers use Multiprotocol Label Switching, the MPLS label format, the roles of CE, PE and P routers, push, swap and pop operations, penultimate hop popping, how LDP discovers neighbours and distributes labels, and how to filter LDP labels. 9 min · Advanced
- VRFs and MPLS Layer 3 VPNs: route distinguishers, route targets and MP-BGPHow VRFs create separate routing tables on one router, VRF-lite without MPLS, and how an MPLS L3VPN uses route distinguishers, route targets, VPNv4 routes in MP-BGP and a two-label stack to give many customers private any-to-any networks. 10 min · Advanced
- MPLS L3VPN PE-CE routing and route leaking: BGP, OSPF sham links, EIGRP, RIP and extranetsHow customer routers exchange routes with an MPLS VPN using static routes, eBGP, OSPF, EIGRP or RIP, why OSPF sites need sham links when a backdoor exists, how default routes reach VRFs, and how extranets, import and export maps and VRF-lite route leaking share routes between VRFs. Also MPLS over FlexVPN. 10 min · Advanced
- MPLS Layer 2 VPNs and IPv6 over MPLS: AToM pseudowires, VPLS, EVPN, 6PE and 6VPEHow Any Transport over MPLS builds point-to-point pseudowires, how VPLS and EVPN extend that to multipoint Ethernet, and how 6PE and 6VPE carry IPv6 and IPv6 VPNs across an IPv4-only MPLS core without upgrading the P routers. 8 min · Advanced