82 lines
2.9 KiB
Markdown
82 lines
2.9 KiB
Markdown
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# Module 2: MPLS — Labeling the Backbone
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> **Course**: [ISP Backbone Lab Course](../README.md)
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> **Previous**: [Module 1: IS-IS](01-isis.md)
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> **Next**: [Module 3: iBGP](03-ibgp.md)
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---
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## Network Diagram
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*MPLS label switched path showing Push → Swap → Swap → PHP Pop → IP Lookup*
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---
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## What Is MPLS and Why Do ISPs Use It?
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MPLS (Multi-Protocol Label Switching) is the **heart of every modern ISP backbone**. Here's the key insight:
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> **Core routers (P routers) don't need to know about customer routes.**
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Without MPLS, every P router would need a full routing table — millions of routes. That's slow, expensive, and a security risk. Instead:
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1. PE routers push an **MPLS label** onto packets entering the core
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2. P routers only look at the label — a simple number — and swap/forward it
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3. The PE on the other side pops the label and delivers to the customer
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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.
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## LDP (Label Distribution Protocol)
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LDP is how routers agree on which labels to use. It's simple:
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1. Router A tells Router B: "If you want to reach my loopback 10.0.0.1, use label 24"
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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)
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3. This builds a **Label Switched Path (LSP)** across the network
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## Lab 2 Config: Enable MPLS/LDP
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**On every P and PE router (add to existing config):**
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```
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mpls ip
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mpls ldp router-id Loopback0 force
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!
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interface GigabitEthernet0/1
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mpls ip
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!
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interface GigabitEthernet0/2
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mpls ip
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!
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interface GigabitEthernet0/3
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mpls ip
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```
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**That's it.** MPLS is beautifully simple to enable. The magic is in what it *enables* (VPNs, TE, etc.)
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## Key Concept: PHP (Penultimate Hop Popping)
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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.
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## Verification Commands
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```
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show mpls interfaces ! Which interfaces have MPLS enabled?
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show mpls ldp neighbor ! LDP sessions established?
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show mpls ldp bindings ! What labels are assigned to what prefixes?
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show mpls forwarding-table ! The LFIB — the actual label switching table
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traceroute 10.0.0.14 source 10.0.0.11 ! Should show MPLS labels in the path!
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```
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## Understanding Check
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1. Why don't P routers need customer routes?
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2. What is the LFIB and how is it different from the RIB/FIB?
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3. Explain PHP — why does the second-to-last hop pop the label?
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4. What would break if you forgot `mpls ldp router-id Loopback0 force`?
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---
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> **Next Module**: [Module 3: iBGP — The Brain of the ISP →](03-ibgp.md)
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