What is IS-IS?

Welcome to ISIS.guru!

This comprehensive tutorial will take you from zero IS-IS knowledge to expert-level understanding. We'll cover everything from basic concepts to packet-level analysis, complete with configuration examples and interactive tools.

Intermediate System to Intermediate System (IS-IS) Overview

IS-IS is a link-state routing protocol that uses the Shortest Path First (SPF) algorithm to determine the best path to a destination. Originally developed for the OSI protocol suite, IS-IS has been adapted for IP routing and is widely used in large-scale networks and service provider environments.

Key Characteristics

  • Link-State Protocol: Each router maintains a complete topology database
  • Hierarchical Design: Uses Level-1 and Level-2 areas for scalability
  • Fast Convergence: Quickly adapts to network changes
  • Load Balancing: Supports equal-cost multi-path routing
  • Scalable: Efficient in large networks through area design
  • Extensible: Supports various address families (IPv4, IPv6, MPLS)

IS-IS vs Other Routing Protocols

Aspect IS-IS OSPF EIGRP
Algorithm Dijkstra SPF Dijkstra SPF DUAL
Hierarchy Level-1/Level-2 Area-based Autonomous System
Convergence Fast Fast Very Fast
Scalability Excellent Good Good
Vendor Support Multi-vendor Multi-vendor Cisco proprietary
Transport Data Link Layer IP IP

Why Learn IS-IS?

IS-IS is essential for:

  • Large-scale enterprise networks requiring robust routing
  • Service provider networks and ISP infrastructures
  • MPLS networks and traffic engineering deployments
  • Multi-protocol environments (IPv4, IPv6, MPLS)
  • Networks requiring fast convergence and high availability

The IS-IS Process

Adjacency Formation
→
Database Synchronization
→
SPF Calculation
→
Route Installation

The IS-IS process involves four main phases:

  1. Adjacency Formation: Hello packets establish neighbor relationships
  2. Database Synchronization: LSPs are exchanged to build topology database
  3. SPF Calculation: Shortest Path First algorithm computes best paths
  4. Route Installation: Selected routes are installed in the routing table

Quick Start Example

Here's a basic IS-IS configuration to get you started:

# Cisco Configuration
router isis
 net 49.0001.1921.6800.1001.00
 is-type level-2-only
 metric-style wide

interface GigabitEthernet0/0
 ip address 10.1.1.1 255.255.255.252
 ip router isis
 isis circuit-type level-2-only