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 → […]
VRF — Several Routers in One
VRF (Virtual Routing and Forwarding) lets one router keep several separate routing tables, so traffic is logically isolated on the same physical device. Facts Function: several independent routing tables on one router · OSI: Layer 3 Standard: no official RFC for VRF-Lite (MPLS L3VPN = RFC 4364) IP overlap: ✅ allowed · RD (Route Distinguisher) […]
Advanced BGP — Route Reflectors, Protection, Policy
Where basic BGP is about neighbors and exchanging routes, advanced BGP is about scalability and granular control. The three pillars: scaling (route reflectors, aggregation), protection (maximum prefix) and policy control (prefix-lists, route-maps, regex, communities, PBR). 1. Scaling iBGP – Route Reflectors The challenge: split horizon & full mesh. In iBGP, a route learned from an […]
BGP Basics — Attributes & Aggregation
BGP is a path-vector routing protocol used between autonomous systems to select routes based on policies rather than shortest path. Facts Point Detail Type Path vector → best-path selection (attributes) “Advertisements” NLRI (Network Layer Reachability Information) + path attributes in UPDATE messages (AS-PATH, NEXT-HOP, LOCAL_PREF, MED) Protocol TCP port 179 (reliable session) AD eBGP 20 […]
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 Area Types & Where Filtering Works
OSPF area types control which LSAs may exist in an area – to reduce routing-table size, LSA flooding and to improve stability and performance. Area type overview Area type Allowed LSAs External routes Default route ASBR in area Cisco command Normal area 1, 2, 3, 4, 5 ✔️ ❌ ✔️ *(default)* Stub area 1, 2, […]
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 […]
STP Design & Topology Choices
STP design is about control: who is root, and how traffic flows. Design choice Why Primary Root Bridge (deterministic) Predictable traffic paths Secondary Root Fast failover Per-VLAN roots Load balancing MST mapping Scalability Exam memory hook ⚠️ The root is placed – not chosen by chance Pitfalls 🚨 No secondary root configured Default priority everywhere […]
STP Troubleshooting
STP failures are most often caused by missing BPDUs or bad links. Problem Solution Missing BPDUs Loop Guard One-way (unidirectional) link UDLD Duplex mismatch Fix duplex CAM flapping Check for loops Commands show spanning-tree show spanning-tree inconsistentports Exam memory hook ⚠️ No BPDU = suspected loop Pitfalls 🚨 Only looking at forwarding ports Forgetting to […]
UDLD — Unidirectional Link Detection
UDLD (Unidirectional Link Detection) detects one-way links – when A → B works but B → A does not. Typically on fiber. Which problem does it solve? ⚠️ A unidirectional link is dangerous because: The interface shows up/up STP can fail → loops or blackholes STP sees the link as up and blocks the wrong […]