Network Topologies N10-009

Network+ N10-009 — Module 2
Network Topologies:
The Shape of Your Network
How devices connect determines performance, redundancy, and cost. Every network has a shape — knowing it is the first step to understanding its failure modes.
10 Slides Physical & Logical LAN, WAN, Data Center Exam Domain 1.2
Topology 1 of 6
Star / Hub-and-Spoke
SWITCH / HUB PC-1 Workstation PC-2 Printer PC-3 Laptop PC-4 Server PC-5 Camera PC-6 VoIP
Most common topology in modern LANs. Every office with a switch is a star.
Outer nodes have no direct connection to each other. All traffic flows through the center.
Easy to add or remove devices without affecting other nodes.
A single failed end node affects only that node. Isolation is trivial.
Single point of failure: if the central switch goes down, the entire network goes down.
Hub vs. Switch
A hub broadcasts to all ports (one collision domain). A switch makes per-MAC forwarding decisions (separate collision domain per port). Both create a star physical topology.
Real-World
Every Ethernet office LAN. Your home router is the center. All devices radiate out to it.
Topology 2 of 6
Mesh: Full and Partial
Full Mesh — 5 nodes, 10 links
R1 R2 R3 R4 R5 n(n-1)/2 = 5(4)/2 = 10 links
Partial Mesh — selective redundancy
R1 R2 R3 R4 R5 failed link reroute path
Full mesh: every node connects to every other. Maximum redundancy, no single point of failure.
Cost scales as n(n-1)/2 — 5 nodes = 10 links. 10 nodes = 45 links. Expensive at scale.
Partial mesh: pragmatic. Protect the critical paths. Accept some redundancy loss.
Where You See Mesh
WAN connections between branch offices. ISP backbone networks. The internet itself is a partial mesh of interconnected autonomous systems. Full mesh is rare outside of 3-4 node setups.
Topology 3 of 6 — Legacy
Bus Topology: One Cable, All Problems
T terminator T terminator BREAK = all down PC-1 Workstation PC-2 Server PC-3 offline PC-4 offline PC-5 offline Shared coaxial backbone — one break takes down the entire segment
All devices share a single coaxial cable. Data travels in both directions from the source.
All devices compete for the medium simultaneously — collision-prone. CSMA/CD required.
Single cable break takes everything offline. No failover path exists.
Terminators at both ends absorb the signal. Missing terminator = reflections = corrupted data.
Legacy Warning
Bus is effectively dead. Used in 10BASE2 (thinnet, RG-58) and 10BASE5 (thicknet, RG-8). You will see this on the exam. You will not see it in any modern network.
Exam Tip
The exam may describe a network where "one cable failure brings everything down." That is a bus topology.
Topology 4 of 6 — Legacy
Ring Topology: Token Passing
Single Ring — Token Ring / 802.5
token A PC-1 B PC-2 C PC-3 D FAIL E down 1 node fails = ring broken = all go down
Dual Ring — FDDI (self-healing)
A B C D E primary (CW) secondary (CCW)
Devices form a closed loop. Data travels in one direction using token passing. A device must hold the token to transmit.
Single ring: one node failure breaks the entire ring. No path around the break.
FDDI dual ring: primary breaks, network wraps back on the secondary ring automatically — self-healing.
Legacy Status
Token Ring (802.5) and FDDI are exam topics, not modern deployments. Cisco SONET/SDH uses ring logic for WAN carrier circuits. Know the concept; you will not deploy it.
Topology 5 of 6
Point-to-Point: Direct Link
Dedicated 1 Gbps ROUTER A Site A 10.1.0.1 ROUTER B Site B 10.1.0.2 Physical medium: fiber, copper, or wireless Examples: T1, T3, leased line, fiber cross-connect, microwave Typical subnet: /30 or /31 (only 2 usable host addresses needed)
The simplest topology: exactly two nodes, one link. No shared medium, no collisions.
Bandwidth is dedicated — both nodes get the full capacity of the link at all times.
Common for WAN uplinks: ISP circuit to your router, inter-office fiber, building-to-building wireless.
Use a /30 (2 host addresses) or /31 (RFC 3021) to minimize wasted address space.
Single point of failure: if the link or either device fails, connectivity is gone. No reroute option.
Where You See It
ISP uplink at your network edge. BGP peering sessions between autonomous systems. Serial WAN links (T1, T3). Any two routers connected back-to-back across a leased line or fiber run.
Enterprise Architecture
Three-Tier Hierarchical Model
CORE DIST ACCESS CORE-1 high-speed CORE-2 redundant DIST-1 DIST-2 DIST-3 DIST-4 ACC-1 ACC-2 ACC-3 ACC-4 ACC-5 ACC-6 ACC-7 ACC-8 - - - - - - Users connect here - - - - - -
Core: high-speed backbone switching. Pure throughput. No policy, no ACLs. Connects distribution blocks together.
Distribution: routing between VLANs, ACL enforcement, QoS policy. Aggregates access layer traffic upward.
Access: port density, PoE, VLAN assignment. Where end-user devices (PCs, phones, APs) physically connect.
Each distribution switch dual-homes to both core switches. Redundancy at every tier boundary.
Scales to thousands of users by adding distribution/access blocks without re-architecting the core.
Collapsed Core
Smaller networks collapse Core + Distribution into one layer. Two switches serve both roles. Reduces cost; reduces redundancy separation.
Data Center Architecture
Spine-and-Leaf: East-West Fabric
SPINE LEAF SPINE-1 100G uplinks SPINE-2 100G uplinks SPINE-3 100G uplinks LEAF-1 LEAF-2 LEAF-3 LEAF-4 LEAF-5 servers VMs storage servers border Every leaf is exactly 2 hops from every other leaf No leaf-to-leaf links. No spine-to-spine links. Pure fabric.
Every leaf connects to every spine. No leaf connects to another leaf. No spine connects to another spine.
Any leaf-to-leaf path is always exactly 2 hops: Leaf → Spine → Leaf. Predictable, uniform latency.
Scale out: add a leaf for more host ports. Add a spine for more bandwidth. No re-cabling the rest.
Built for east-west traffic: server-to-server, VM migration, distributed storage, containerized microservices.
vs. Three-Tier
Three-tier was built for north-south (user to server). Spine-leaf was built for east-west (server to server). Cloud and virtualization broke the north-south model.
Real-World
Cisco ACI, Arista CloudVision, AWS VPC fabric. Standard in any modern data center since ~2012.
Traffic Patterns
North-South vs. East-West Traffic
North-South — Client to Server
INTERNET / EDGE CORE DISTRIBUTION ACCESS SERVER North - South Flow Traditional: user fetches data from server Three-tier optimized for this pattern
East-West — Server to Server
SPINE LAYER LEAF-1 LEAF-2 LEAF-3 LEAF-4 SRV-A SRV-B SRV-C SRV-D SRV-A to SRV-D: 2 hops Modern: VM migration, micro-services, distributed storage, container networking Spine-leaf optimized for this pattern
The Shift
Virtualization and cloud broke the north-south model. A single physical server now hosts dozens of VMs that need to talk to each other. That traffic is east-west. Three-tier hairpins it through distribution and core. Spine-leaf handles it in 2 hops.
North-South: across network boundaries. Client outside → resource inside. Firewall policies apply here.
East-West: within the data center. Server to server. Must be fast and low-latency.
Micro-segmentation tools (NSX, ACI) enforce policy on east-west traffic without choking throughput.
Summary
Topology Quick Reference
Star
Most common LAN
Central SPOF
Full Mesh
n(n-1)/2 links
Max redundancy
Bus
Shared coax, legacy
10BASE2 / 10BASE5
Ring
Token passing, legacy
FDDI = dual ring
Three-Tier
CORE DIST-1 DIST-2
Enterprise LAN
North-South optimized
Spine-Leaf
SPINE-1 SPINE-2 LEAF-1 LEAF-2 LEAF-3
Data center fabric
East-West optimized
1 Star is the answer to "most common modern LAN topology." The central switch is the single point of failure.
2 Full mesh formula: n(n-1)/2. Partial mesh is the practical WAN compromise. Maximum redundancy, maximum cost.
3 Bus = shared coax, terminators at both ends, one break kills all. Ring = token passing, one break = network down (unless FDDI dual ring).
4 Three-tier: Core (speed) → Distribution (policy) → Access (users). Scales via access/distribution blocks. North-south optimized.
5 Spine-leaf: every leaf exactly 2 hops from every other. No leaf-to-leaf, no spine-to-spine links. East-west optimized. Modern data center standard.