IS-IS (Intermediate System to Intermediate System)
A Link-State protocol similar to OSPF, but highly favored in Service Provider networks. It runs directly over Layer 2, making it protocol agnostic (it can route IPv4, IPv6, or anything else easily).
1. OSPF vs IS-IS Comparison
While both use Dijkstra's SPF algorithm, their architecture differs:
| Feature | OSPF | IS-IS |
|---|---|---|
| Transport | IP (Protocol 89) | CLNS (Layer 2) - No IP needed |
| Area Boundary | Inside a Router (ABR) | On the Link (L1/L2 adjacency) |
| Extensibility | Rigid LSA Types | TLV (Type-Length-Value) - Very Flexible |
| Broadcast Network | DR / BDR | DIS (Designated IS) - No Backup |
Why Service Providers Love IS-IS
Because IS-IS uses TLVs (Type-Length-Value), adding support for new features (like IPv6 or MPLS-TE) is just adding a new TLV. OSPF often requires a completely new LSA type or protocol version (OSPFv2 vs v3).
2. Addressing (NSAP / NET)
IS-IS routers are identified by an ISO address called a NET (Network Entity Title), not an IP address.
Example: 49.0001.1921.6800.1001.00
- Area ID:
49.0001(Variable length). - System ID:
1921.6800.1001(Fixed 6 bytes, usually derived from MAC or IP). - N-Selector:
00(Always 00 for a router).
3. Hierarchical Levels
Instead of Area 0, IS-IS uses Levels:
- Level 1 (L1): Intra-area routing (like OSPF non-backbone). Knows internal topology only.
- Level 2 (L2): Backbone routing (like OSPF Area 0). Connects areas.
- L1/L2 Router: Acts as the border. Maintains separate databases for L1 and L2.
4. Adjacency Types
- L1 Adjacency: Between routers in the same area.
- L2 Adjacency: Between routers in different areas (Backbone).
- L1/L2 Adjacency: Can form both types if configured.
References
- RFC 1195: Use of OSI IS-IS for Routing in TCP/IP - Adapting IS-IS for IP networks (Integrated IS-IS).
- ISO/IEC 10589:2002 - The original ISO standard for Intermediate System to Intermediate System.