Introduction to IS-IS
What You'll Learn
This chapter introduces IS-IS (Intermediate System to Intermediate System), a link-state routing protocol that operates at the Data Link Layer. You'll learn about its origins, key features, and how it compares to other routing protocols.
What is IS-IS?
IS-IS (Intermediate System to Intermediate System) is a link-state routing protocol designed for use within an autonomous system (intra-domain routing). Originally developed by Digital Equipment Corporation (DEC) for the OSI protocol suite, IS-IS has been adapted to support IP routing and is widely used in large-scale networks, particularly by Internet Service Providers (ISPs).
| Characteristic | IS-IS | Description |
|---|---|---|
| Protocol Type | Link-State | Maintains complete network topology database |
| Algorithm | Dijkstra SPF | Shortest Path First calculation |
| Layer | Data Link (Layer 2) | Operates directly over Layer 2 protocols |
| Metric | Cost-based | Supports narrow and wide metrics |
| Hierarchy | Two-level | Level-1 (intra-area) and Level-2 (inter-area) |
Key Features of IS-IS
Network Efficiency
- Fast convergence
- Efficient flooding mechanism
- Loop-free routing
- Support for ECMP
Scalability
- Hierarchical design
- Area-based organization
- Route summarization
- Supports large networks
History and Development
1980s - Origins
IS-IS was originally developed by Digital Equipment Corporation (DEC) as part of the OSI (Open Systems Interconnection) protocol suite. It was designed to route CLNP (Connectionless Network Protocol) packets.
1990 - RFC 1142
The original IS-IS specification was published as RFC 1142, defining the protocol for OSI environments.
1992 - RFC 1195 (Integrated IS-IS)
Integrated IS-IS was standardized, extending the protocol to support IP routing alongside CLNS. This made IS-IS dual-protocol capable.
2000s - Enhanced Features
Various RFCs added features like traffic engineering extensions, IPv6 support, and authentication mechanisms.
Present - Modern IS-IS
IS-IS continues to evolve with support for segment routing, multi-topology routing, and advanced traffic engineering capabilities.
IS-IS vs Other Protocols
| Feature | IS-IS | OSPF | EIGRP |
|---|---|---|---|
| Protocol Type | Link-State | Link-State | Hybrid/Advanced Distance Vector |
| Transport Layer | Data Link Layer | IP (Protocol 89) | IP (Protocol 88) |
| Areas | 2-level hierarchy | Multiple areas with backbone | Autonomous Systems |
| Metric | Cost (default 10) | Cost based on bandwidth | Composite metric |
| Convergence | Fast | Fast | Very Fast (DUAL) |
| CPU Usage | Moderate | Higher | Lower |
| ISP Usage | Very Common | Common | Less Common |
When to Choose IS-IS
- Large-scale networks: ISP backbone networks
- Stability: Networks requiring maximum stability
- Dual-stack: Networks running both IPv4 and IPv6
- Traffic Engineering: Networks using MPLS-TE
IS-IS Terminology
System Types
- ES (End System)
- A host or end device that sends and receives data but does not forward it to other systems.
- IS (Intermediate System)
- A router that forwards data between systems. Can be Level-1, Level-2, or Level-1-2.
- Level-1 IS
- Router that routes within a single area only.
- Level-2 IS
- Router that routes between areas (backbone router).
- Level-1-2 IS
- Router that performs both Level-1 and Level-2 routing.
Network Components
- Area
- A logical grouping of routers and links. Level-1 routing occurs within areas.
- Domain
- The entire IS-IS routing domain, consisting of all areas.
- NET (Network Entity Title)
- The IS-IS equivalent of a router ID, uniquely identifying each router.
- System ID
- 6-byte identifier that uniquely identifies a router within the domain.
- Circuit
- IS-IS term for a network interface or link.
PDU Types
| PDU Type | Name | Purpose | Level |
|---|---|---|---|
| Hello | Hello PDU | Neighbor discovery and adjacency maintenance | Level-1, Level-2 |
| LSP | Link State PDU | Advertise link state information and topology | Level-1, Level-2 |
| CSNP | Complete Sequence Number PDU | Database synchronization on broadcast networks | Level-1, Level-2 |
| PSNP | Partial Sequence Number PDU | Request missing LSPs and acknowledge LSPs | Level-1, Level-2 |