On this page
IS-IS uses Type-Length-Value (TLV) objects to carry routing information. This extensible design makes it highly adaptable for IPv4, IPv6, MPLS, and beyond.
| TLV | Name | Purpose |
|---|---|---|
| 1 | Area Addresses | Identifies IS-IS areas for hierarchy. |
| 2 | IS Neighbors | Lists directly connected IS neighbors. |
| 6 | IS Neighbors (Extended) | Carries extended metrics for neighbor links. |
| 9 | LSP Entries | Summarizes received LSPs for flooding. |
| 10/11 | Authentication Info | Secures IS-IS exchanges with passwords/keys. |
| 22 | Extended IS Reachability | Supports larger metrics and TE attributes. |
| 128 | IP Internal Reachability | Advertises IPv4 internal routes. |
| 129 | Protocols Supported | Lists supported network protocols (IPv4, IPv6, etc.). |
| 130 | IP External Reachability | Advertises IPv4 external routes. |
| 135 | IPv6 Interface Address | Carries IPv6 interface addresses. |
| 236/237 | Multi-Topology Reachability | Enables multi-topology IS-IS (MT-ISIS). |
| 238 | IPv6 Reachability | Advertises IPv6 prefixes. |
| 242 | Extended IP Reachability | Provides more flexible IPv4 prefix advertisement. |
| 250 | Router CAPABILITY | Announces node capabilities (TE, SR, etc.). |
💡 Why it matters: TLVs let IS-IS evolve (IPv6, MPLS, Traffic Engineering, Segment Routing) without redesigning the core protocol.
📚 For a deeper dive into IS-IS adjacencies, LSDB synchronization, SPF, and design examples, check out my series: Mastering IS-IS
