IS-IS PDU Types

Common PDU Header

All IS-IS PDUs share a common header structure that provides basic protocol information and PDU identification.

PDU Type Values

Hello PDUs

Hello PDU Functions

Hello PDUs are used for neighbor discovery, adjacency establishment, and maintenance. IS-IS defines two types of Hello PDUs based on the network type.

LAN Hello PDU Structure

# LAN Hello PDU Structure
Common Header (8 bytes)
Circuit Type (1 byte)          # Level-1, Level-2, or both
Source ID (6 + 1 bytes)        # System ID + Pseudonode ID
Holding Time (2 bytes)         # Neighbor aging timer
PDU Length (2 bytes)           # Total PDU length
Priority (1 byte)              # DIS election priority
LAN ID (6 + 1 bytes)           # DIS System ID + Circuit ID
TLVs (variable)                # Type-Length-Value fields

Hello PDU TLVs

DIS Election Process

Link State PDUs (LSPs)

LSP Overview

Link State PDUs contain the actual topology information in IS-IS. They describe the router's local connectivity and are flooded throughout the appropriate level to build the link state database.

# LSP Header Structure
Common Header (8 bytes)
PDU Length (2 bytes)           # Total LSP length
Remaining Lifetime (2 bytes)   # LSP aging timer
LSP ID (8 bytes)              # System ID + Pseudonode ID + LSP Number
Sequence Number (4 bytes)      # LSP version identifier
Checksum (2 bytes)            # LSP integrity check
P/ATT/OL/IS Type (1 byte)     # Flags and router type
TLVs (variable)               # Type-Length-Value fields

LSP Flags

LSP TLVs

LSP Lifecycle

Sequence Number PDUs (SNPs)

SNP Functions

Sequence Number PDUs are used for database synchronization and LSP acknowledgment. IS-IS defines two types of SNPs for different synchronization scenarios.

CSNP Structure

# CSNP Structure
Common Header (8 bytes)
PDU Length (2 bytes)           # Total CSNP length
Source ID (6 + 1 bytes)        # Sending router ID
Start LSP ID (8 bytes)         # First LSP in range
End LSP ID (8 bytes)           # Last LSP in range
TLVs (variable)                # LSP entries TLV

Database Synchronization Process

TLV Structure

Type-Length-Value Format

IS-IS uses Type-Length-Value (TLV) encoding for variable-length fields in PDUs. This flexible format allows for protocol extensions and vendor-specific information.

Common TLV Types

TLV Example: Area Addresses

# Area Addresses TLV (Type 1)
Type: 01
Length: 04                     # 4 bytes of data
Value:
  Address Length: 03           # 3-byte area address
  Area Address: 49.0001        # Area 49.0001

# Hex representation: 01 04 03 49 00 01