Inter-VLAN Routing — SVI & Router-on-a-Stick
Switch Virtual Interface (SVI) – inter-VLAN routing A Switch Virtual Interface (SVI) is a logical Layer 3 interface on a Cisco Layer 3 switch, giving a VLAN an IP address so the switch can act as the default gateway for devices in that VLAN. Primary use: inter-VLAN routing – communication between VLANs without an external […]
HSRP Deep Dive
HSRP (Hot Standby Router Protocol) creates redundancy between routers by assigning a virtual IP and MAC address. One router is active, others standby; if the active fails, the standby takes over automatically, minimizing downtime. Facts Topic HSRP Standard Cisco proprietary Multicast address 224.0.0.2 (v1) / 224.0.0.102 (v2) Transport UDP 1985 Virtual address One virtual IPv4 […]
FHRP Compared — HSRP vs VRRP vs GLBP
FHRP (First Hop Redundancy Protocol) is a family of protocols – HSRP, VRRP and GLBP – that maintain network continuity by assigning a virtual IP and MAC address to several routers. One router is active, the others standby; on failure a standby takes over automatically, giving higher availability. Rule of thumb: Cisco-only + simple → […]
EIGRP & DUAL — The Feasibility Condition
EIGRP uses DUAL (Diffusing Update Algorithm) to select the best path and guarantee fast, loop-free routing per destination inside an autonomous system. Facts Advanced Distance Vector (hybrid) → the DUAL algorithm Protocol 88 in the IP header · Multicast 224.0.0.10 (224.0.0.9 old EIGRP routers) AD: internal 90 · external 170 Core concepts Term Short explanation […]
OSPF Core — LSAs, Timers, Router ID
OSPF is a link-state IGP that uses cost as its metric and requires Area 0 as the backbone. Facts Type: link-state → Dijkstra’s algorithm (SPF); uses LSAs (Link-State Advertisements) Protocol 89 in the IP header – OSPF uses no TCP/UDP AD: 110 (internal + external) Multicast: 224.0.0.5 (AllSPFRouters), 224.0.0.6 (DR/BDR) Router ID election Manually configured […]
Static Routes — Default, Floating, Recursive
A static route is manually configured by the administrator. The router does not learn it dynamically and does not update it automatically. ip route <destination-net> <subnet-mask> <next-hop | interface> Default route (0.0.0.0/0) Used when the router has no more specific route – matches all destinations Also called gateway of last resort Used: toward the internet, […]
Best Path Selection: LPM, AD, Metric, ECMP
Longest Prefix Match → Administrative Distance → Metric → ECMP → FIB (CEF) When a router receives an IP packet it determines one best route using a fixed prioritized decision chain. The process runs in the control plane (RIB) and is then used in the data plane (FIB/CEF). 1. Longest Prefix Match (LPM) The router […]
Root Guard
What is Root Guard? Root Guard prevents a port from becoming a Root Port. Which problem does it solve? Protects against: A wrongly placed Root Bridge An access/distribution switch winning the root election What happens technically? The port receives a BPDU that is “better” than the current root The port is put into root-inconsistent No […]
BPDU Guard
What is BPDU Guard? BPDU Guard shuts a port down when a BPDU is received. Which problem does it solve? Prevents that: A switch gets connected to an access port An access port becomes part of the STP topology What happens technically? The port is typically PortFast A BPDU is received The port is put […]
PortFast
What is PortFast? PortFast makes a port go straight to forwarding. Which problem does it solve? Long wait at link-up (30 seconds through Listening + Learning) Unnecessary TCNs when PCs go up/down What happens technically? Listening & Learning are skipped The port goes directly to forwarding STP is still active – PortFast is not less […]