87 lines
2.8 KiB
Markdown
87 lines
2.8 KiB
Markdown
# Module 6: Segment Routing
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> **Course**: [ISP Backbone Lab Course](../README.md)
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> **Previous**: [Module 5: eBGP](05-ebgp.md)
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> **Next**: [Module 7: Traffic Engineering](07-traffic-engineering.md)
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---
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## Network Diagram
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*Segment Routing SID assignments (SRGB 16000+) with Prefix SIDs and Adjacency SIDs on all nodes*
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---
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## The Evolution: LDP → SR
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Remember how LDP assigns labels? It works, but it has problems:
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- LDP creates a **full mesh of label bindings** — every router gets a label for every prefix
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- LDP is a **separate protocol** that must stay in sync with IS-IS (if they disagree, traffic blackholes)
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- No built-in traffic engineering
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**Segment Routing (SR)** fixes all of this by embedding labels directly into IS-IS (or OSPF). No more LDP. No more synchronization issues.
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## How SR Works
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Instead of LDP negotiating labels between neighbors, each router advertises a **Segment ID (SID)** via IS-IS:
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- **Prefix SID** = A label for reaching a specific router's loopback (replaces what LDP does)
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- **Adjacency SID** = A label for a specific link (used for traffic engineering)
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The SID is an **index** added to a globally configured **SRGB (Segment Routing Global Block)**. Default SRGB: 16000–23999.
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Example: If P1's Prefix SID index is 1, its label is **16001** everywhere in the network. No negotiation needed.
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## Lab 6 Config: Enable Segment Routing
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**On every router (replace LDP):**
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```
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! Remove LDP (optional — SR and LDP can coexist, but pure SR is cleaner)
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no mpls ldp router-id Loopback0 force
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!
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! Enable SR in IS-IS
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router isis YOURSP
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segment-routing mpls
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segment-routing prefix-sid-map advertise-local
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!
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interface Loopback0
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ip router isis YOURSP
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isis prefix-sid index <UNIQUE_NUMBER>
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```
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**SID Index Assignments:**
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| Router | Loopback | Prefix SID Index | Resulting Label |
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|--------|----------|-----------------|----------------|
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| P1 | 10.0.0.1 | 1 | 16001 |
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| P2 | 10.0.0.2 | 2 | 16002 |
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| P3 | 10.0.0.3 | 3 | 16003 |
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| P4 | 10.0.0.4 | 4 | 16004 |
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| P-CORE | 10.0.0.5 | 5 | 16005 |
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| PE-EDGE1 | 10.0.0.11 | 11 | 16011 |
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| PE-EDGE2 | 10.0.0.12 | 12 | 16012 |
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| PE-EDGE3 | 10.0.0.13 | 13 | 16013 |
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| PE-EDGE4 | 10.0.0.14 | 14 | 16014 |
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## Verification
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```
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show isis segment-routing prefix-sid-map
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show mpls forwarding-table ! Labels should now be 16xxx
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show isis database detail ! Look for SR TLVs in the LSPs
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show segment-routing mpls state
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```
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## Understanding Check
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1. Why is SR simpler than LDP? What failure mode does it eliminate?
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2. What's the SRGB and why does it need to be the same on every router?
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3. What's the difference between a Prefix SID and an Adjacency SID?
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4. Can SR and LDP coexist? When would you want that?
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---
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> **Next Module**: [Module 7: Traffic Engineering →](07-traffic-engineering.md)
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