Complete Cisco IS-IS Multi Area Routing Configuration Guide
IS-IS is one of the most powerful and scalable Interior Gateway Protocols used in enterprise and service provider networks. Many engineers consider IS-IS more scalable than OSPF in very large infrastructures.
This complete guide explains every concept from basic Level-1 routing all the way to inter-area Level-2 routing. The tutorial also includes SPF mathematics, NET addressing, System ID explanation, CLI outputs, troubleshooting, and real Cisco IOS configuration examples.
🎯 What You Will Learn
- What IS-IS is
- Level-1 and Level-2 routing
- NET addressing
- System IDs
- SPF algorithm
- Area design
- Inter-area routing
- Cisco IOS configuration
- Verification commands
- Troubleshooting methods
Table of Contents
Introduction to IS-IS
IS-IS stands for Intermediate System to Intermediate System. It is a link-state routing protocol originally designed for the OSI model. Later it became one of the most widely used routing protocols for IP networks.
IS-IS uses Link State Packets to share topology information. Every router builds an identical topology database and independently calculates the shortest path.
Shortest Path Formula
$$ D(v)=min(D(v),D(u)+c(u,v)) $$Where:
- $D(v)$ = Current shortest distance
- $D(u)$ = Neighbor distance
- $c(u,v)$ = Link cost
Level-1 vs Level-2 Routing
IS-IS supports hierarchical routing using levels.
| Level | Purpose |
|---|---|
| Level-1 | Intra-area routing |
| Level-2 | Inter-area routing |
| Level-1-2 | Both intra-area and inter-area routing |
In this lab:
- R1-R5 are Level-1 routers
- R6 and R7 are Level-1-2 routers
- R9 and R10 are Level-2 routers
💡 Important Concept
Level-1 routers know only their local area topology. Level-2 routers connect different IS-IS areas together.
Understanding NET Addressing
Every IS-IS router requires a NET address.
Example:
$$ 49.0010.1111.1111.1111.00 $$This contains:
- 49 = AFI
- 0010 = Area ID
- 1111.1111.1111 = System ID
- 00 = NSEL
Important Note
Every router must have a unique System ID. Duplicate System IDs cause adjacency failures.
SPF Mathematics
IS-IS uses Dijkstra SPF algorithm.
SPF Cost Calculation
$$ Total\ Cost = \sum_{i=1}^{n} Cost_i $$Example:
- R1 to R2 = 10
- R2 to R3 = 20
- R3 to R4 = 30
Then:
$$ 10+20+30=60 $$Bandwidth Formula
$$ Cost=\frac{10^8}{Bandwidth} $$Task 1 - Area 49.0010
Routers R1 through R6 belong to Area 49.0010.
R6 operates as a Level-1-2 router.
R1 Configuration
CLI Example:
Router(config)# router isis Router(config-router)# net 49.0010.1111.1111.1111.00
router isis net 49.0010.1111.1111.1111.00 is-type level-1 interface lo0 ip router isis interface lo1 ip router isis interface lo2 ip router isis interface lo3 ip router isis
R1 Explanation
The command:
is-type level-1
forces R1 to establish only Level-1 adjacencies.
Loopbacks are advertised using:
ip router isis
R2 Configuration
router isis net 49.0010.2222.2222.2222.00 is-type level-1 interface lo0 ip router isis interface lo4 ip router isis interface e0/0 ip router isis
R3 Configuration
router isis net 49.0010.3333.3333.3333.00 is-type level-1 interface lo0 ip router isis interface e0/0 ip router isis interface e0/1 ip router isis
R4 Configuration
router isis net 49.0010.4444.4444.4444.00 is-type level-1 interface lo0 ip router isis interface e0/0 ip router isis interface e0/1 ip router isis interface e0/2 ip router isis
R5 Configuration
router isis net 49.0010.5555.5555.5555.00 is-type level-1 interface lo0 ip router isis interface e0/0 ip router isis interface e0/1 ip router isis
R6 Configuration
router isis net 49.0010.6666.6666.6666.00 interface lo0 ip router isis interface e0/0 ip router isis interface e0/1 ip router isis
💡 Why R6 Is Special
R6 does not have:
is-type level-1
This means Cisco IOS automatically treats R6 as a Level-1-2 router.
Task 2 - Area 49.0020
This area contains R7 and R8.
R7 Configuration
router isis net 49.0020.7777.7777.7777.00 interface lo0 ip router isis
R8 Configuration
router isis net 49.0020.8888.8888.8888.00 is-type level-1 interface lo0 ip router isis interface e0/1 ip router isis interface e0/0 ip router isis
Inter-Area Routing Concept
$$ Area\ 49.0010 \leftrightarrow Area\ 49.0020 $$R6 and R7 help exchange inter-area routing information.
Task 3 - Custom System IDs
R11 Configuration
router isis net 49.0040.0011.0011.0011.00 interface lo0 ip router isis interface e0/1 ip router isis
R12 Configuration
router isis net 49.0040.0012.0012.0012.00 is-type level-1 interface lo0 ip router isis interface e0/0 ip router isis
Task 4 - Level-2 Routing
R9 and R10 establish only Level-2 adjacencies.
R9 Configuration
router isis net 49.0030.9999.9999.9999.00 is-type level-2 interface lo0 ip router isis interface e0/1 ip router isis
R10 Configuration
router isis net 49.0030.1010.1010.1010.00 is-type level-2 interface lo0 ip router isis interface e0/0 ip router isis
Level-2 Backbone Formula
$$ Backbone = \sum InterArea\ Paths $$Verification Commands
Verify Neighbors
show isis neighbors
R1# show isis neighbors System Id Type Interface IP Address State Holdtime Circuit Id R2 L1 Et0/0 10.1.1.2 UP 24 R2.01
Verify Database
show isis database
R6# show isis database IS-IS Level-1 Link State Database LSPID Seq Num Checksum Holdtime R1.00-00 0x15 0xA123 1074 R2.00-00 0x11 0xB234 1021
Verify Routes
show ip route isis
R4# show ip route isis
i L1 10.1.1.1/32 [115/10]
via 10.0.0.1, Ethernet0/0
Troubleshooting IS-IS
No Adjacency Formation
- Area mismatch
- Different IS types
- Missing ip router isis
- Interface shutdown
Missing Routes
- Loopback not advertised
- LSP flooding issue
- Authentication mismatch
Database Synchronization
$$ LSDB_{R1}=LSDB_{R2}=LSDB_{R3} $$All routers must maintain identical topology databases.
Related Articles
Conclusion
IS-IS remains one of the most scalable routing protocols available today. It is heavily used in enterprise and service provider infrastructures.
This tutorial covered:
- Level-1 routing
- Level-2 routing
- Inter-area communication
- NET addressing
- SPF mathematics
- CLI verification
- Troubleshooting
Practice these labs repeatedly to build strong routing expertise.
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