IS-IS Routing Protocol: Design Principles for Scalable Enterprise Networks

Author Ermias Teffera

At ITVue Networks, designing resilient and scalable enterprise networks requires efficient routing protocols. IS-IS (Intermediate System to Intermediate System) is a link-state routing protocol widely used in large service provider and enterprise networks due to its scalability, fast convergence, and hierarchical design capabilities. In this blog, we’ll explore IS-IS design principles, area hierarchy, best practices, and provide Cisco examples with diagrams.

What is IS-IS?

IS-IS is a link-state interior gateway protocol (IGP) used to determine the best path within an autonomous system. Key features:

• Fast convergence with link-state updates

• Hierarchical design with Level 1 (intra-area) and Level 2 (inter-area) routing

• Uses CLNS addresses (NSAP) but can also carry IPv4/IPv6 via integrated IS-IS

• Loop-free routing and efficient path selection

IS-IS Hierarchical Design

IS-IS scales networks using a two-level hierarchy:

1. Level 1 (L1) Routers

   
   

   • Route within a single area

   • Similar to OSPF non-backbone areas

   
   

2. Level 2 (L2) Routers

   
   

   • Connects areas (backbone)

   • Similar to OSPF Area 0 backbone

     
     

3. Level 1-2 Routers

   
   

   • Participate in both intra-area and inter-area routing

   • Typically distribution routers connecting access areas to backbone

IS-IS Design Principles

1. Area Planning

• Assign small areas to minimize LSP flooding and CPU load

• Maintain level separation: L1 routers only know about intra-area routes

• L2 routers carry inter-area routes to connect areas efficiently

  
  

2. Metric Calculation

• IS-IS uses cost metrics similar to OSPF

• Interface cost = configurable value based on link bandwidth

• Supports equal-cost multipath (ECMP) for traffic balancing

  
  

3. Route Summarization

• Summarize routes at L1/L2 boundary routers

• Reduces size of routing tables in backbone L2 routers

  
  

4. Redundancy and High Availability

• Use multiple L2 routers in the backbone for redundancy

• Combine with EtherChannel or LACP to aggregate uplinks from distribution routers

• Use adjacency timers for fast convergence

IS-IS Network Diagram

• L1 routers at access layer

• L1-2 routers at distribution layer connecting to L2 backbone

• Supports hierarchical scaling and efficient LSP propagation

Cisco IS-IS Configuration Example

• net is the NSAP address (can be automatically generated)

• is-type level-1-2 allows router to participate in both intra-area and inter-area routing

Best Practices for IS-IS

• Use Level 1 and Level 2 separation to reduce flooding

• Summarize routes at boundaries to minimize table size

• Enable ECMP for load balancing across multiple L2 paths

• Monitor adjacencies with show isis neighbor and route tables with show isis route

• Combine with EtherChannel for aggregated uplinks between distribution and core

Real-World Example

In a large campus or service provider network:

• Each building has L1 access routers connecting client devices

• Distribution routers are L1-2 and connect to the backbone L2 routers

• L2 backbone routers provide high-speed inter-area routing

• EtherChannel uplinks and ECMP across L2 routers ensure bandwidth optimization, redundancy, and minimal downtime

Conclusion

IS-IS provides a highly scalable and loop-free routing environment for large enterprise and service provider networks. By implementing hierarchical L1/L2 design, route summarization, and link aggregation, ITVue Networks ensures clients’ networks are robust, efficient, and ready for growth.