Name: The Foundation of Iron — Week 19 — Routing
Author: Amine RAITI

RATIO

THE FOUNDATION OF IRON · COURSE MATERIAL · WEEK 19

◆◆◆

ROUTING
INTER-VLAN AND ROUTING TABLES
Week 19 of 26 · Block 6 — Foundational Networking
10h theory · 25h practice

◆

◆ WEEKLY LEARNING OBJECTIVES

1. Understand how routing works and a router's role at OSI layer 3
2. Read and interpret a routing table
3. Configure static routes and a default route
4. Configure inter-VLAN routing (router-on-a-stick)
5. Diagnose a connectivity loss due to a missing route

◆◆◆

⚠ WARNING — SHELF LIFE OF VERSIONS REFERENCED HERE

Router configuration syntax varies by vendor and version. This material presents generic concepts (static routing, inter-VLAN) applicable to any device or simulator — the instructor adapts commands to the tool actually available.

◆

Amine RAITI · Infrastructure Architect & SRE
Public document · CC BY-NC-SA 4.0 · AI Powered by Amine
Opération Dindon

RATIO

COURSE OUTLINE · 10H

THEORY GUIDING THREAD — ROUTING

◆

19.1 · How routing works3h

— The router operates at OSI layer 3 (link with Week 17), makes forwarding decisions packet by packet
— Routing table: list of known networks and the next hop to reach them
— Difference between directly connected network / static route / dynamic route

19.2 · Static routing3h

— Manual configuration of a static route: destination network, mask, next-hop or exit interface
— Default route (0.0.0.0/0): used when no specific route matches
— Routing table construction and reading exercises

19.3 · Inter-VLAN routing3h

— Why VLANs cannot talk directly (link with Week 18 — isolated at layer 2) and need a router or layer-3 interface to communicate
— Router-on-a-stick: sub-interfaces on a router connected in trunk to the switch
— Layer-3 switch: more efficient alternative in real environments

19.4 · Introduction to dynamic routing protocols1h

— Conceptual notion: dynamic protocols automate building the routing table
— Common examples (OSPF, BGP) — conceptual introduction only, no configuration at this stage

RATIO

EXERCISE 1 · STATIC ROUTING IN A SIMULATOR · 12H

Equipment: network simulator, provided topology (3 networks, 2 routers, machines in each network).

(2h) Analysing the topology: identify the networks, router interfaces, initial routing tables (only directly connected networks).

(3h) Configuring static routes on each router to enable communication between all networks, testing full connectivity (ping between all machines).

(3h) Configuring inter-VLAN routing (router-on-a-stick) on the Week 18 VLAN topology: allow communication between VLAN 10 and VLAN 20 via a router sub-interface.

(2h) Testing inter-VLAN communication, verifying that security segmentation remains possible (basic ACL to allow some flows and block others).

(2h) Simulated failure exercise: remove a static route, observe the connectivity loss, identify the problem in the routing table, fix it.

SOLUTION — EXERCISE 1

Expected inter-VLAN configuration: the router has one sub-interface per VLAN (with the corresponding 802.1Q tag and an IP address in each VLAN subnet), the trunk port of the switch connected to the router allows all VLANs. Machines in each VLAN use the corresponding router sub-interface IP address as their gateway.

RATIO

EXERCISE 2 · ROUTING TABLES AND TRAFFIC ANALYSIS · 13H

Equipment: network simulator, Wireshark on the VMs from previous weeks.

(3h) Reading and interpreting 5 complex provided routing tables from different topologies — identify the next-hop used for each destination.

(4h) Capturing routing traffic on existing VMs: observe how packets traverse interfaces, identify MAC address changes at each hop (the MAC address changes at every router, the IP address stays constant).

(3h) Building a multi-router topology in the simulator (3 routers in series), configuring full static routing, testing end-to-end connectivity.

(3h) Documenting the final topology with each router's routing table and the path taken by a packet from each source to each destination.

SOLUTION — EXERCISE 2

Fundamental point to anchor: on a multi-router network, the source and destination IP addresses of a packet never change throughout its journey — only the Ethernet frame's MAC addresses change at each hop (they identify the local link, not the global path). This is the central conceptual difference between layer 2 (local addressing) and layer 3 (global addressing).

◆ SUMMARY SHEET — WEEK 19 SELF-ASSESSMENT

1. I can explain a router's role and its operation at OSI layer 3.

2. I can read and interpret a routing table.

3. I can configure a static route on a router.

4. I can configure a default route.

5. I can configure inter-VLAN routing (router-on-a-stick).

6. I can explain why the MAC address changes at each hop but not the IP address.

7. I can diagnose a connectivity loss due to a missing route.

8. I know the principle of dynamic routing protocols.