Complete MPLS Dynamic Traffic Engineering Guide Part 3
Welcome to Part 3 of the MPLS Traffic Engineering series.
In Part 2 we configured:
- Explicit MPLS TE tunnels
- RSVP bandwidth reservations
- Manual path selection
- Traffic engineered LSPs
In this tutorial we move toward:
Dynamic MPLS Traffic Engineering
Instead of manually defining tunnel paths, routers will dynamically calculate optimal paths using:
- Available bandwidth
- OSPF TE database
- RSVP reservations
- Constraint-based shortest path first (CSPF)
🎯 What You Will Learn
- What Dynamic MPLS TE is
- Difference between explicit and dynamic tunnels
- How CSPF works
- How bandwidth-aware routing functions
- How RSVP affects tunnel selection
- How TE tunnels automatically choose paths
- How autoroute announce injects tunnels into routing
- How to verify dynamic MPLS tunnels
Table of Contents
1. Dynamic MPLS TE Introduction
Dynamic MPLS Traffic Engineering automatically computes tunnel paths using:
- Link metrics
- Bandwidth availability
- Constraint calculations
- Traffic Engineering Database (TED)
This process is called:
Constraint-Based Shortest Path First (CSPF)
CSPF extends traditional SPF calculations by considering bandwidth constraints.
Traditional SPF Formula
$$ Best\\ Path = Lowest\\ OSPF\\ Cost $$CSPF Formula
$$ Best\\ Path = Lowest\\ Cost + Bandwidth\\ Constraints $$Explicit Tunnel vs Dynamic Tunnel
| Feature | Explicit Tunnel | Dynamic Tunnel |
|---|---|---|
| Path Selection | Manual | Automatic |
| Flexibility | Low | High |
| Scalability | Limited | Excellent |
| Bandwidth Awareness | Manual | Automatic |
| Adaptation | Static | Dynamic |
2. Task 1 - Delete Explicit MPLS TE Tunnels
We first remove the tunnels configured in Part 2.
Those tunnels used:
- Explicit paths
- Manually defined hops
- Static engineering
Now we move to:
Dynamic Path Computation
R1 Remove Explicit Tunnels
Code Example
no interface Tunnel14
R1 no interface Tunnel14 no interface Tunnel13
R2 Remove Explicit Tunnel
R2 no interface Tunnel21
R4 Remove Explicit Tunnel
R4 no interface Tunnel41
Dynamic TE Formula
$$ Path\\ Selection = CSPF\\ Algorithm $$3. Task 2 - Configure Dynamic Tunnel R1 → R4
Now we configure a dynamic MPLS TE tunnel from:
$$ R1 \rightarrow R4 $$with:
- 128 kbps reservation
- Dynamic path calculation
- Automatic route injection
Key Difference
Instead of:
explicit name
we now use:
path-option 1 dynamic
R1 Dynamic Tunnel Configuration
R1 interface Tunnel14 ip unnumbered Loopback0 tunnel destination 4.4.4.4 tunnel mode mpls traffic-eng tunnel mpls traffic-eng bandwidth 128 tunnel mpls traffic-eng path-option 1 dynamic tunnel mpls traffic-eng priority 3 3 tunnel mpls traffic-eng autoroute announce
Detailed Tunnel Explanation
path-option 1 dynamic
Enables CSPF-based automatic path computation.
bandwidth 128
Requests 128 kbps RSVP reservation.
autoroute announce
Injects the tunnel into the routing table.
priority 3 3
Defines setup and hold priority.
Bandwidth Reservation Formula
$$ Requested\\ BW \le Available\\ Link\\ BW $$4. Task 3 - Configure Dynamic Tunnel R2 → R1
This tunnel reserves:
$$ 400\\ kbps $$toward:
$$ 1.1.1.1 $$R2 Dynamic Tunnel Configuration
R2 interface Tunnel21 ip unnumbered Loopback0 tunnel destination 1.1.1.1 tunnel mode mpls traffic-eng tunnel mpls traffic-eng bandwidth 400 tunnel mpls traffic-eng path-option 1 dynamic tunnel mpls traffic-eng priority 3 3 tunnel mpls traffic-eng autoroute announce
Why This Matters
The router now dynamically calculates:
- Available bandwidth
- Least congested path
- TE-compliant path
Constraint Formula
$$ CSPF = SPF + RSVP\\ Constraints $$5. Task 4 - Configure Dynamic Tunnel R1 → R2
This tunnel has:
- 400 kbps reservation
- Priority 2
- Automatic TE path selection
Why Higher Priority Matters
Higher priority tunnels can:
- Preempt lower priority tunnels
- Reserve scarce bandwidth
- Receive preferential treatment
Priority Formula
$$ Lower\\ Number = Higher\\ Priority $$R1 Tunnel 12 Configuration
R1 interface Tunnel12 ip unnumbered Loopback0 tunnel destination 2.2.2.2 tunnel mode mpls traffic-eng tunnel mpls traffic-eng bandwidth 400 tunnel mpls traffic-eng path-option 1 dynamic tunnel mpls traffic-eng priority 2 2 tunnel mpls traffic-eng autoroute announce
How CSPF Chooses Paths
The router evaluates:
- OSPF cost
- Available RSVP bandwidth
- Administrative constraints
- Tunnel priorities
CSPF Decision Formula
$$ Best\\ TE\\ Path = Lowest\\ Cost + Available\\ BW $$6. Verification and Path Analysis
Now verify tunnel creation and selected paths.
Verify MPLS TE Tunnels
Code Example
show mpls traffic-eng tunnel
R1 Verification
show mpls traffic-eng tunnel
R1#show mpls traffic-eng tunnel Tunnel14: Admin: up Oper: up Path option 1: dynamic Tunnel12: Admin: up Oper: up Path option 1: dynamic
R2 Verification
show mpls traffic-eng tunnel
R2#show mpls traffic-eng tunnel Tunnel21: Admin: up Oper: up Path option 1: dynamic
Verify Tunnel Paths
show mpls traffic-eng tunnels detail
Detailed Tunnel Output
R1#show mpls traffic-eng tunnels detail Tunnel14 Explicit Route: Dynamic Path Option Path: R1 -> R2 -> R4
Verify Routing Table
show ip route
Routing Output
R1#show ip route 4.4.4.4/32 via Tunnel14 2.2.2.2/32 via Tunnel12
Routing Injection Formula
$$ Autoroute\\ Announce = Tunnel\\ in\\ RIB $$7. MPLS TE Mathematics
Traffic Engineering Formula
$$ Traffic\\ Optimization = Constraint\\ Based\\ Routing $$Available Bandwidth Formula
$$ Available\\ BW = Total\\ BW - Reserved\\ BW $$RSVP Reservation Formula
$$ Total\\ Reservations = \sum Tunnel\\ Requests $$TE Tunnel Formula
$$ TE\\ Tunnel = RSVP + CSPF + MPLS $$Dynamic Path Formula
$$ Path = Lowest\\ Cost\\ Satisfying\\ Constraints $$Preemption Formula
$$ High\\ Priority > Low\\ Priority $$8. Troubleshooting Dynamic TE
| Problem | Cause | Solution |
|---|---|---|
| Tunnel Down | No RSVP bandwidth | Enable RSVP on interfaces |
| No Dynamic Path | OSPF TE missing | Enable OSPF TE extensions |
| Bandwidth Failure | Insufficient RSVP BW | Reduce requested bandwidth |
| No Autoroute | Missing autoroute announce | Enable autoroute |
| No TE Tunnel | MPLS TE disabled | Enable MPLS TE globally |
Useful Verification Commands
show mpls traffic-eng tunnels show mpls traffic-eng tunnels detail show ip rsvp reservation show ip ospf mpls traffic-eng show ip route show mpls forwarding-table
💡 Key Takeaways
- Dynamic MPLS TE uses CSPF
- Bandwidth affects path selection
- RSVP reserves bandwidth dynamically
- TE tunnels can automatically reroute
- Autoroute announce injects tunnels into routing
- Dynamic tunnels scale better than explicit tunnels
- OSPF TE extensions advertise bandwidth information
- MPLS TE optimizes WAN utilization
9. Related Articles
- Part 1 - Complete MPLS Core Configuration Guide
- Part 2 - Complete MPLS Traffic Engineering Guide
- Complete MPLS Traffic Engineering Tunnel Preemption Guide | Dynamic MPLS TE Lab Part 4
Final Conclusion
In this MPLS Traffic Engineering Part 3 tutorial we implemented:
- Dynamic MPLS TE tunnels
- CSPF path calculation
- Bandwidth-aware routing
- Automatic TE path selection
- RSVP optimization
- Dynamic traffic engineering
Dynamic MPLS TE is widely deployed in:
- Enterprise WANs
- ISP backbones
- Cloud provider networks
- Carrier MPLS infrastructures
It provides:
- Automatic optimization
- Better scalability
- Intelligent bandwidth utilization
- High availability
- Efficient traffic distribution
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