IS-IS and the OSI Reference Model

Understanding Layer 2 Operation

Unlike most IP routing protocols that operate at Layer 3, IS-IS operates directly at the Data Link Layer (Layer 2). This unique characteristic provides several advantages and requires understanding of OSI model integration.

OSI Layer Integration

7

Application

Network applications and services

6

Presentation

Data formatting and encryption

5

Session

Session management

4

Transport

End-to-end delivery (TCP/UDP)

3

Network

IP routing and addressing

OSPF, RIP operate here
2

Data Link

Frame formatting and error detection

IS-IS operates here
1

Physical

Physical transmission media

Advantages of Layer 2 Operation

Protocol Independence
  • Independent of Layer 3 protocols
  • Can carry multiple protocols (IP, IPv6, CLNP)
  • No IP addressing required for protocol operation
Network Efficiency
  • No IP header overhead for IS-IS packets
  • Direct encapsulation in Layer 2 frames
  • More efficient bandwidth utilization

IS-IS Addressing

Network Service Access Point (NSAP)

IS-IS uses OSI-style addressing based on Network Service Access Points (NSAPs). The NSAP address structure is hierarchical and provides both area and system identification.

NSAP Address Structure

AFI
1 byte
Authority & Format Identifier
IDI
Variable
Initial Domain Identifier
HO-DSP
Variable
High Order Domain Specific Part
Area ID
1-13 bytes
Area Identifier
System ID
6 bytes
System Identifier
NSEL
1 byte
Network Selector (always 00 for NET)

Network Entity Title (NET)

In IS-IS, routers are identified by a Network Entity Title (NET), which is a special type of NSAP address where the NSEL field is set to 00.

NET Address Examples

# Simple NET format
49.0001.1921.6800.1001.00
│   │    │         │    │
│   │    │         │    └── NSEL (always 00)
│   │    │         └─────── System ID (6 bytes)
│   │    └───────────────── Area ID
│   └────────────────────── AFI (49 = locally administered)
└────────────────────────── Private use AFI

# Cisco-style NET
49.0001.0000.0000.0001.00
Area: 49.0001
System ID: 0000.0000.0001

System ID Structure

Component Length Description Example
System ID 6 bytes Unique identifier within the domain 1921.6800.0001
Pseudonode ID 1 byte Identifies pseudonode on broadcast networks 01, 02, etc. (00 for router)
LSP Number 1 byte Fragment number for large LSPs 00, 01, 02, etc.

IS-IS Hierarchy

Two-Level Hierarchy

IS-IS uses a two-level hierarchical design to support scalable routing in large networks. This design separates intra-area routing (Level-1) from inter-area routing (Level-2).

Level-1 and Level-2 Routing

Area 49.0001

L1
L1
L1-L2

Level-2 Backbone

L2
L2

Area 49.0002

L1-L2
L1
L1

Router Types

Router Type Function Database Adjacencies
Level-1 Intra-area routing only Level-1 LSDB Level-1 only
Level-2 Inter-area routing only Level-2 LSDB Level-2 only
Level-1-2 Both intra and inter-area Both L1 and L2 LSDB Both L1 and L2

Hierarchical Routing Benefits

Scalability

Reduces routing table size and LSP flooding scope

Performance

Faster convergence within areas, reduced CPU overhead

Stability

Area isolation prevents topology changes from affecting entire domain

Adjacency Formation

Adjacency Process

IS-IS routers form adjacencies with neighboring routers to exchange routing information. The adjacency formation process involves several steps and depends on the network type and router levels.

Adjacency Formation Steps

1

Hello Exchange

Routers send Hello PDUs to discover neighbors

→
2

Parameter Check

Verify area ID, authentication, MTU

→
3

Adjacency State

Move to Up state if parameters match

→
4

Database Sync

Exchange and synchronize LSP databases

Network Types and Adjacencies

Network Type Hello Type DIS Election LSP Flooding
Broadcast LAN Hello Yes (Designated IS) Via DIS using CSNP
Point-to-Point P2P Hello No Direct flooding with PSNP ack

Adjacency Requirements

Adjacency Formation Requirements

  • Area Match: Level-1 routers must be in same area
  • Level Compatibility: At least one common level
  • Authentication: Matching authentication parameters
  • MTU: Compatible maximum transmission unit
  • Hello Intervals: Compatible timing parameters

IS-IS Databases

Link State Database (LSDB)

IS-IS maintains separate Link State Databases for Level-1 and Level-2 routing. Each database contains Link State PDUs (LSPs) that describe the network topology.

Database Components

Level-1 LSDB
  • Contains intra-area topology
  • All routers in area have identical L1 LSDB
  • Used for intra-area routing decisions
  • Includes attached bit information
Level-2 LSDB
  • Contains inter-area topology
  • All L2 routers have identical L2 LSDB
  • Used for inter-area routing decisions
  • Includes area reachability information

LSP Structure and Content

LSP Identifier Format

# LSP ID Structure: System-ID.Pseudonode-ID.LSP-Number
1921.6800.0001.00-00    # Router LSP from system 1921.6800.0001
1921.6800.0001.01-00    # Pseudonode LSP for LAN segment
1921.6800.0001.00-01    # Fragment 1 of router LSP (if large)

Database Synchronization

LSDB Synchronization Process

1. LSP Generation

Router generates LSPs describing its local topology

2. LSP Flooding

LSPs are flooded to all routers in the appropriate level

3. Database Consistency

All routers maintain identical LSDB for each level

4. SPF Calculation

Dijkstra algorithm computes shortest paths

Previous: Chapter 1 - Introduction Next: Chapter 3 - PDUs and Packets