Complete MPLS IS-IS Traffic Engineering Guide Part 5
Welcome to Part 5 of the MPLS Traffic Engineering series.
In the earlier tutorials we implemented:
- MPLS Core Configuration
- RSVP Bandwidth Reservation
- Explicit MPLS TE Tunnels
- Dynamic MPLS TE Tunnels
- TE Tunnel Preemption
In this tutorial we replace:
OSPF
with:
IS-IS
for MPLS Traffic Engineering.
🎯 What You Will Learn
- Why IS-IS is preferred in Service Provider networks
- How to configure IS-IS Level-1 adjacencies
- How IS-IS differs from OSPF
- How NET addresses work
- What Wide Metrics are
- Why MPLS TE requires Wide Metrics
- How MPLS TE integrates with IS-IS
- How to verify MPLS TE tunnels using IS-IS
Table of Contents
1. IS-IS Introduction
IS-IS stands for:
Intermediate System to Intermediate System
It is one of the most widely deployed routing protocols in:
- ISP Backbones
- Carrier MPLS Networks
- Large Service Provider infrastructures
Why Service Providers Prefer IS-IS
| Feature | IS-IS | OSPF |
|---|---|---|
| Scalability | Excellent | Good |
| TLV Extensibility | Very Flexible | Moderate |
| MPLS TE Integration | Excellent | Excellent |
| SP Adoption | Very High | Moderate |
| Protocol Layer | Layer 2 | Layer 3 |
IS-IS Hierarchy
IS-IS supports:
- Level-1 Routing
- Level-2 Routing
- Level-1-2 Routing
In this lab:
Only Level-1 Adjacencies
are configured.
IS-IS Area Formula
$$ NET = Area\\ ID + System\\ ID + NSEL $$2. Task 1 - Remove OSPF Configuration
We first remove OSPF from all routers.
MPLS TE will now use:
IS-IS TE Extensions
R1 Remove OSPF
Code Example
no router ospf 1
R1 no router ospf 1
R2 Remove OSPF
R2 no router ospf 1
R3 Remove OSPF
R3 no router ospf 1
R4 Remove OSPF
R4 no router ospf 1
Migration Formula
$$ OSPF \rightarrow IS\text{-}IS $$3. Task 2 - Configure IS-IS on Core Routers
Now we configure:
- Single Area IS-IS
- Level-1 only routing
- Wide Metrics
- Loopback advertisements
Understanding the NET Address
Example:
49.0000.1111.1111.1111.00
| Field | Value |
|---|---|
| Area ID | 49.0000 |
| System ID | 1111.1111.1111 |
| NSEL | 00 |
NET Structure Formula
$$ NET = Area + SystemID + NSEL $$Why Wide Metrics?
Traditional IS-IS metrics support:
$$ 0 - 63 $$Wide Metrics support:
$$ 0 - 16,777,215 $$MPLS TE requires Wide Metrics for:
- Bandwidth calculations
- Traffic Engineering attributes
- Advanced TE computations
Wide Metric Formula
$$ Wide\\ Metrics > Narrow\\ Metrics $$R1 IS-IS Configuration
R1 router isis net 49.0000.1111.1111.1111.00 is-type level-1 metric-style wide
R2 IS-IS Configuration
R2 router isis net 49.0000.2222.2222.2222.00 is-type level-1 metric-style wide ! interface Loopback0 ip router isis ! interface E0/0 ip router isis ! interface E0/1 ip router isis ! interface E0/2 ip router isis
R3 IS-IS Configuration
R3 router isis net 49.0000.3333.3333.3333.00 is-type level-1 metric-style wide ! interface Loopback0 ip router isis ! interface E0/0 ip router isis ! interface E0/1 ip router isis ! interface E0/2 ip router isis
R4 IS-IS Configuration
R4 router isis net 49.0000.4444.4444.4444.00 is-type level-1 metric-style wide ! interface Loopback0 ip router isis ! interface E0/0 ip router isis ! interface E0/1 ip router isis ! interface E0/2 ip router isis
How IS-IS Adjacencies Form
Routers exchange:
- Hello packets
- LSPs
- CSNPs
- PSNPs
Since all routers are:
Level-1
they establish only:
L1 Adjacencies
Adjacency Formula
$$ L1 + Same\\ Area = Adjacency $$4. Task 3 - Enable MPLS TE with IS-IS
Now we enable:
MPLS Traffic Engineering Extensions
inside IS-IS.
This allows IS-IS to advertise:
- Bandwidth
- TE metrics
- Link attributes
- RSVP information
R1 MPLS TE IS-IS Configuration
Code Example
mpls traffic-eng level-1
R1 router isis mpls traffic-eng router-id Loopback0 mpls traffic-eng level-1
R2 MPLS TE IS-IS Configuration
R2 router isis mpls traffic-eng router-id Loopback0 mpls traffic-eng level-1
R3 MPLS TE IS-IS Configuration
R3 router isis mpls traffic-eng router-id Loopback0 mpls traffic-eng level-1
R4 MPLS TE IS-IS Configuration
R4 router isis mpls traffic-eng router-id Loopback0 mpls traffic-eng level-1
MPLS TE Database Formula
$$ TED = IS\text{-}IS + TE\\ Extensions $$Why Loopback0 is Used as TE Router-ID
Loopbacks provide:
- Stable identifiers
- Reliable TE endpoints
- Persistent tunnel destinations
5. Verify MPLS TE Tunnels
Now verify:
- IS-IS routing
- MPLS TE tunnels
- Routing table entries
Verify Routing Table
show ip route
Sample Output
R1#show ip route 3.3.3.3/32 via Tunnel13 4.4.4.4/32 via Tunnel14
Verify MPLS TE Tunnel Status
show mpls traffic-eng tunnel brief
Sample Tunnel Output
R1#show mpls traffic-eng tunnel brief Tunnel13 Up Tunnel14 Up
R2 Verification
show ip route show mpls traffic-eng tunnel brief
R2 Tunnel Verification
R2#show mpls traffic-eng tunnel brief Tunnel21 Up
Verify IS-IS Adjacencies
show isis neighbors
Neighbor Output
R1#show isis neighbors System Id Interface State Type 2222.2222.2222 Et0/0 UP L1 3333.3333.3333 Et0/1 UP L1 4444.4444.4444 Et0/2 UP L1
IS-IS Reachability Formula
$$ Reachability = SPF + LSP\\ Database $$6. IS-IS and MPLS TE Mathematics
Wide Metric Formula
$$ 0 \le Metric \le 16,777,215 $$IS-IS SPF Formula
$$ Best\\ Path = Lowest\\ IS\text{-}IS\\ Metric $$TE Path Formula
$$ TE\\ Path = CSPF + RSVP $$Bandwidth Formula
$$ Available\\ BW = Total\\ BW - Reserved\\ BW $$IS-IS Database Formula
$$ LSDB = \sum LSPs $$MPLS TE Formula
$$ MPLS\\ TE = IS\text{-}IS + RSVP + CSPF $$7. Troubleshooting IS-IS MPLS TE
| Problem | Cause | Solution |
|---|---|---|
| No IS-IS Adjacency | Wrong Area or Level | Verify NET and IS-Type |
| No TE Tunnel | TE extensions missing | Enable MPLS TE Level-1 |
| No Reachability | Interface not enabled | Use ip router isis |
| TE Tunnel Down | No RSVP bandwidth | Verify RSVP reservations |
| No Wide Metrics | metric-style wide missing | Enable Wide Metrics |
Useful Verification Commands
show isis neighbors show isis database show ip route show mpls traffic-eng tunnel brief show mpls traffic-eng tunnels detail show ip rsvp reservation
💡 Key Takeaways
- IS-IS is widely used in Service Provider MPLS networks
- Wide Metrics are required for MPLS TE
- IS-IS Level-1 creates intra-area adjacencies
- MPLS TE integrates directly with IS-IS
- Loopbacks provide stable TE Router IDs
- IS-IS advertises TE attributes using TLVs
- MPLS TE tunnels continue working after migration from OSPF
- IS-IS scales extremely well in large MPLS networks
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
- Part 4 - Complete MPLS TE Tunnel Preemption Guide
Final Conclusion
In this MPLS IS-IS Traffic Engineering Part 5 tutorial we implemented:
- Migration from OSPF to IS-IS
- IS-IS Level-1 routing
- Wide Metrics
- MPLS TE integration with IS-IS
- TE tunnel verification
- IS-IS TE extensions
IS-IS remains one of the most important protocols in:
- ISP Backbones
- Carrier Ethernet
- Large MPLS infrastructures
- Cloud provider networks
Its flexibility, scalability, and TE support make it ideal for:
- MPLS Traffic Engineering
- RSVP deployments
- Segment Routing
- Large-scale routing domains
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