Complete MPLS TE Tunnel Preemption Guide Part 4
Welcome to Part 4 of the MPLS Traffic Engineering series.
In the previous tutorials we learned:
- MPLS core configuration
- Explicit MPLS TE tunnels
- Dynamic MPLS TE tunnels
- RSVP bandwidth reservation
- CSPF path computation
In this tutorial we focus on one of the most important MPLS TE concepts:
MPLS Traffic Engineering Tunnel Preemption
🎯 What You Will Learn
- What MPLS TE preemption is
- How tunnel priorities work
- Why tunnels fail to come up
- How RSVP bandwidth exhaustion occurs
- How higher priority tunnels preempt lower priority tunnels
- How MPLS TE dynamically reallocates bandwidth
- How routing tables change after preemption
- How CSPF handles bandwidth conflicts
Table of Contents
1. MPLS TE Preemption Introduction
MPLS Traffic Engineering uses:
- RSVP bandwidth reservation
- Tunnel priorities
- Constraint-based routing
to intelligently allocate network resources.
Why Preemption Exists
Bandwidth is limited.
When a new tunnel requests bandwidth:
- The router checks available RSVP bandwidth
- If enough bandwidth exists, the tunnel comes up
- If insufficient bandwidth exists, the tunnel fails
- Unless the tunnel has higher priority
What is Tunnel Preemption?
Preemption allows:
Higher Priority Tunnels
to remove:
Lower Priority Tunnels
from the network.
Preemption Formula
$$ Higher\\ Priority > Lower\\ Priority $$Priority Rule
$$ Lower\\ Number = Higher\\ Priority $$Bandwidth Reservation Scenario
Earlier tunnels already consumed RSVP bandwidth.
Now a new tunnel requests:
$$ 300\\ kbps $$toward:
$$ 3.3.3.3 $$The network initially cannot satisfy this request.
2. Task 1 - Configure Dynamic Tunnel R1 → R3
We now configure a dynamic MPLS TE tunnel from:
$$ R1 \rightarrow R3 $$with:
- 300 kbps RSVP reservation
- Dynamic path computation
- Priority 3
Initial Tunnel Configuration
Code Example
tunnel mpls traffic-eng path-option 1 dynamic
R1 interface Tunnel13 ip unnumbered Loopback0 tunnel destination 3.3.3.3 tunnel mode mpls traffic-eng tunnel mpls traffic-eng bandwidth 300 tunnel mpls traffic-eng path-option 1 dynamic tunnel mpls traffic-eng priority 3 3 tunnel mpls traffic-eng autoroute announce
Detailed Explanation
bandwidth 300
Requests RSVP reservation of 300 kbps.
priority 3 3
Defines:
- Setup priority = 3
- Hold priority = 3
This is not high enough to preempt existing tunnels.
path-option 1 dynamic
Uses CSPF to dynamically calculate paths.
Bandwidth Allocation Formula
$$ Available\\ BW = Total\\ BW - Reserved\\ BW $$3. Task 2 - Verify Tunnel Failure
Now verify if the tunnel came up.
Verification Command
show ip route
Expected Result:
The tunnel does NOT come up.
Why Did the Tunnel Fail?
The network already reserved bandwidth for:
- Tunnel14
- Tunnel12
- Tunnel21
The remaining RSVP bandwidth is insufficient.
Failure Formula
$$ Requested\\ BW > Available\\ BW $$Sample Verification Output
show ip route Output
R1#show ip route No route installed via Tunnel13
How CSPF Evaluates the Tunnel
CSPF checks:
- Shortest available path
- Available RSVP bandwidth
- Tunnel priorities
- TE constraints
Since sufficient bandwidth is unavailable:
The tunnel remains down.
CSPF Formula
$$ Path = Lowest\\ Cost + Available\\ Bandwidth $$4. Task 3 - Modify Tunnel Priority
Now we increase the tunnel priority.
We change:
$$ Priority = 3 $$to:
$$ Priority = 2 $$This makes Tunnel13:
Higher Priority
than existing tunnels.
Updated Configuration
R1 interface Tunnel13 tunnel mpls traffic-eng priority 2 2
What Happens Now?
The router performs:
- Bandwidth reevaluation
- Tunnel preemption
- RSVP reallocation
Lower priority tunnels may now be removed.
Preemption Formula
$$ Priority\\ 2 > Priority\\ 3 $$Result
Tunnel13 successfully comes up.
This occurs because:
- Higher priority tunnels can reclaim bandwidth
- Lower priority tunnels may be rerouted or removed
- RSVP reservations are recalculated dynamically
5. Verify Tunnel Preemption
Verify Routing Table
show ip route
Expected Routing Output
R1#show ip route 3.3.3.3/32 via Tunnel13
Verify Tunnel Status
show mpls traffic-eng tunnels
Tunnel Verification Output
R1#show mpls traffic-eng tunnels Tunnel13: Admin: up Oper: up Path option 1: dynamic Priority: 2 2
Verify RSVP Reservations
show ip rsvp reservation
Understanding Tunnel Priorities
| Priority | Meaning |
|---|---|
| 0 | Highest Priority |
| 1 | Very High Priority |
| 2 | High Priority |
| 3 | Medium Priority |
| 7 | Lowest Priority |
Priority Scale Formula
$$ 0 < 1 < 2 < 3 < 7 $$6. MPLS TE Mathematics
RSVP Reservation Formula
$$ Reserved\\ BW = \sum Tunnel\\ Reservations $$Bandwidth Availability Formula
$$ Free\\ BW = Total\\ BW - Reserved\\ BW $$TE Path Selection Formula
$$ Best\\ Path = Lowest\\ Cost + Available\\ BW $$Preemption Formula
$$ Higher\\ Priority\\ Tunnel \rightarrow Lower\\ Priority\\ Tunnel\\ Removal $$Priority Logic Formula
$$ Lower\\ Value = Higher\\ Priority $$CSPF Decision Formula
$$ Path = SPF + RSVP\\ Constraints $$7. Troubleshooting MPLS TE Preemption
| Problem | Cause | Solution |
|---|---|---|
| Tunnel Down | No available bandwidth | Increase priority or reduce bandwidth |
| No RSVP Reservation | RSVP disabled | Enable RSVP bandwidth |
| No TE Tunnel | MPLS TE disabled | Enable MPLS TE globally |
| No Route via Tunnel | Missing autoroute announce | Enable autoroute |
| No Dynamic Path | OSPF TE missing | Enable OSPF TE extensions |
Useful Verification Commands
show mpls traffic-eng tunnels show mpls traffic-eng tunnels detail show ip rsvp reservation show ip route show ip ospf mpls traffic-eng show mpls forwarding-table
💡 Key Takeaways
- MPLS TE supports tunnel preemption
- Lower priority numbers mean higher importance
- RSVP controls bandwidth reservations
- Bandwidth exhaustion prevents tunnel creation
- Higher priority tunnels can reclaim resources
- CSPF dynamically computes paths
- Autoroute announce injects tunnels into routing
- MPLS TE enables intelligent WAN optimization
8. Related Articles
- Part 1 - Complete MPLS Core Configuration Guide
- Part 2 - Complete MPLS Traffic Engineering Guide
- Part 3 - Complete MPLS Dynamic Traffic Engineering Guide
- Complete MPLS IS-IS Traffic Engineering Configuration Guide Part 5 | Cisco MPLS TE Lab
Final Conclusion
In this MPLS Traffic Engineering Part 4 tutorial we explored:
- MPLS TE tunnel priorities
- Bandwidth exhaustion
- RSVP reservation conflicts
- Tunnel preemption
- Dynamic bandwidth reallocation
- CSPF optimization
These concepts are critical in real-world:
- ISP backbones
- Enterprise WANs
- Carrier MPLS networks
- Cloud infrastructures
MPLS TE preemption allows networks to:
- Guarantee bandwidth
- Protect critical traffic
- Optimize congestion management
- Dynamically allocate resources
- Maintain SLA compliance
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