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