Add all 8 course modules with converted markdown links

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

modules/02-mpls.md

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+# Module 2: MPLS — Labeling the Backbone
+
+> **Course**: [ISP Backbone Lab Course](../README.md)
+> **Previous**: [Module 1: IS-IS](01-isis.md)
+> **Next**: [Module 3: iBGP](03-ibgp.md)
+
+---
+
+## Network Diagram
+
+![MPLS Label Switched Path](../diagrams/Module2_MPLS_LabelPath.png)
+*MPLS label switched path showing Push → Swap → Swap → PHP Pop → IP Lookup*
+
+---
+
+## What Is MPLS and Why Do ISPs Use It?
+
+MPLS (Multi-Protocol Label Switching) is the **heart of every modern ISP backbone**. Here's the key insight:
+
+> **Core routers (P routers) don't need to know about customer routes.**
+
+Without MPLS, every P router would need a full routing table — millions of routes. That's slow, expensive, and a security risk. Instead:
+
+1. PE routers push an **MPLS label** onto packets entering the core
+2. P routers only look at the label — a simple number — and swap/forward it
+3. The PE on the other side pops the label and delivers to the customer
+
+Think of it like shipping containers. The cargo ship (P router) doesn't care what's inside the container. It just reads the shipping label and moves it to the right port.
+
+## LDP (Label Distribution Protocol)
+
+LDP is how routers agree on which labels to use. It's simple:
+
+1. Router A tells Router B: "If you want to reach my loopback 10.0.0.1, use label 24"
+2. Router B remembers this and tells its own neighbors: "To reach 10.0.0.1, send it to me with label 30" (its own locally assigned label)
+3. This builds a **Label Switched Path (LSP)** across the network
+
+## Lab 2 Config: Enable MPLS/LDP
+
+**On every P and PE router (add to existing config):**
+
+```
+mpls ip
+mpls ldp router-id Loopback0 force
+!
+interface GigabitEthernet0/1
+ mpls ip
+!
+interface GigabitEthernet0/2
+ mpls ip
+!
+interface GigabitEthernet0/3
+ mpls ip
+```
+
+**That's it.** MPLS is beautifully simple to enable. The magic is in what it *enables* (VPNs, TE, etc.)
+
+## Key Concept: PHP (Penultimate Hop Popping)
+
+When a packet is one hop away from its destination, the second-to-last router **pops the label** instead of swapping it. Why? So the destination router only has to do one lookup (IP) instead of two (label + IP). This is called PHP and you'll see it as `implicit-null` or label `3` in the LFIB.
+
+## Verification Commands
+
+```
+show mpls interfaces            ! Which interfaces have MPLS enabled?
+show mpls ldp neighbor          ! LDP sessions established?
+show mpls ldp bindings          ! What labels are assigned to what prefixes?
+show mpls forwarding-table      ! The LFIB — the actual label switching table
+traceroute 10.0.0.14 source 10.0.0.11  ! Should show MPLS labels in the path!
+```
+
+## Understanding Check
+
+1. Why don't P routers need customer routes?
+2. What is the LFIB and how is it different from the RIB/FIB?
+3. Explain PHP — why does the second-to-last hop pop the label?
+4. What would break if you forgot `mpls ldp router-id Loopback0 force`?
+
+---
+
+> **Next Module**: [Module 3: iBGP — The Brain of the ISP →](03-ibgp.md)