IS-IS — Intermediate System to Intermediate System

Overview

IS-IS is a link-state routing protocol originally designed for OSI networks (CLNP) and later adapted for IP (Integrated IS-IS, RFC 1195). It’s widely used by ISPs and large-scale networks, often preferred over OSPF for its simplicity at scale.

• OSI standard: ISO 10589
• IP extension: RFC 1195 (Integrated IS-IS)
• Metric: Configurable cost (default 10 per interface)
• AD: 115
• Algorithm: Dijkstra’s SPF
• Transport: Layer 2 (directly on data link, no IP dependency)

Why IS-IS Matters

• Preferred by ISPs and cloud providers (Google, Facebook, large ISPs use IS-IS)
• Simpler area design than OSPF (backbone is Level 2, not Area 0)
• Runs directly on Layer 2 — no IP dependency (works even if IP is broken)
• Better scalability for very large flat networks
• TLV-based encoding makes it extensible (easy to add new features)

IS-IS vs OSPF

IS-IS Levels

graph TD
    subgraph "Level 1 (Intra-area)"
        L1R1[Router L1]
        L1R2[Router L1]
    end
    subgraph "Level 2 (Backbone)"
        L2R1[Router L2]
        L2R2[Router L2]
    end
    subgraph "Level 1/2 (ABR equivalent)"
        L12R1[Router L1/2]
        L12R2[Router L1/2]
    end
    L1R1 --> L12R1
    L1R2 --> L12R1
    L12R1 --> L2R1
    L12R2 --> L2R2
    L2R1 --> L2R2
    L12R1 --> L12R2

Key Difference from OSPF

• In OSPF, all areas must physically connect to Area 0
• In IS-IS, Level 2 routers form a contiguous backbone (areas are on links, not interfaces)
• This makes IS-IS area design more flexible

Network Entity Title (NET)

IS-IS uses NET addresses instead of IP router IDs. A NET is typically 8-20 bytes:

49.0001.0000.0000.0001.00
│   │      │              │
│   │      │              N-selector (always 00)
│   │      System ID (6 bytes, unique)
│   Area ID (variable length)
AFI (49 = private)

Example: 49.0001.0000.0000.0001.00

IS-IS Packet Types

IS-IS Neighbor States

stateDiagram-v2
    [*] --> Down
    Down --> Initializing: Hello received
    Initializing --> Up: Adjacency established
    Up --> [*]: Session active

Unlike OSPF (which has 7 states), IS-IS has a simpler 3-state model.

IS-IS Configuration (Cisco)

router isis COMPANY
  net 49.0001.0000.0000.0001.00
  is-type level-2-only
  metric-style wide

interface GigabitEthernet0/0
  ip router isis COMPANY
  isis circuit-type level-2-only
  isis metric 10 level-2

IS-IS Metric Types

Wide metrics (RFC 5305) are preferred for modern networks, supporting traffic engineering.

IS-IS Authentication

Interview Questions

1. Q: Why do ISPs prefer IS-IS over OSPF? A: IS-IS runs on Layer 2 (no IP dependency), has simpler area design, is more extensible (TLV-based), handles large flat networks better, and supports IPv6 natively without a separate process.

2. Q: What is the difference between Level 1, Level 2, and Level 1/2 routers? A: Level 1 routers only know routes within their area. Level 2 routers only know inter-area routes (backbone). Level 1/2 routers know both and act as default gateways for Level 1 routers.

3. Q: How does IS-IS differ from OSPF in area design? A: OSPF assigns areas to interfaces — a router is an ABR if it has interfaces in multiple areas. IS-IS assigns levels to links — a router is Level 1/2 if it has links at both levels. This is more flexible.

4. Q: What is a NET in IS-IS? A: A Network Entity Title is IS-IS’s identifier, analogous to OSPF’s Router ID. It contains an Area ID, a 6-byte System ID (unique), and a N-selector (always 00).

5. Q: Why doesn’t IS-IS need IP to function? A: IS-IS PDUs are encapsulated directly in Layer 2 frames (like Ethernet). They don’t require IP addressing to establish adjacencies. This means IS-IS can run even if IP is misconfigured.

6. Q: What are CSNP and PSNP? A: CSNP (Complete Sequence Number PDU) contains a summary of all LSPs in the LSDB, sent periodically by the DIS. PSNP (Partial Sequence Number PDU) requests specific LSPs or acknowledges receipt.

Common Mistakes

• Confusing IS-IS levels with OSPF areas (they work differently)
• Forgetting that IS-IS runs on Layer 2, not Layer 3
• Not understanding the NET format
• Assuming IS-IS and OSPF have the same area interconnection requirements
• Forgetting that IS-IS metric-style narrow has a max of 63

Summary

IS-IS is a link-state protocol that runs directly on Layer 2, uses levels (1, 2, 1/2) instead of numbered areas, and is preferred by ISPs for its scalability and extensibility. Its TLV-based design makes it easy to extend for new features like traffic engineering and IPv6.

Cross-References

• Routing Overview
• OSPF — Most comparable protocol
• BGP — Used alongside IS-IS in ISP networks
• Static vs Dynamic
• SDN — Modern network control

Cross References

• OSPF
• BGP