Intermediate System to Intermediate System (IS-IS) is a dynamic routing protocol originally designed for the OSI (Open Systems Interconnection) protocol suite. However, due to its robustness and scalability, it was later adapted to support IP routing and became a popular Interior Gateway Protocol (IGP) alongside OSPF.

Here’s a breakdown of the IS-IS protocol:

Link-State Protocol: Similar to OSPF, IS-IS is a link-state protocol. Routers in an IS-IS domain exchange link-state information, which they then use to build a complete topological map of the network and calculate the best paths to destinations.

Areas: IS-IS divides a network into areas to optimize routing updates and reduce overhead. However, unlike OSPF, there isn’t a mandatory backbone area. The protocol inherently supports hierarchical routing using Level 1 (intra-area) and Level 2 (inter-area) routers.

Router Types:

  • Level 1 (L1) Routers: Handle routing within an area. They only know about routes within their own area.
  • Level 2 (L2) Routers: Responsible for inter-area routing. They maintain a view of the entire IS-IS domain and can also function as an L1 router.
  • L1/L2 Routers: Routers that operate both as L1 and L2. They can route packets within their own area and between different areas.

Network Types: IS-IS supports both broadcast (like Ethernet) and point-to-point networks.

NSAP Addresses: In its original form, IS-IS used NSAP (Network Service Access Point) addresses, which come from the OSI addressing scheme. However, for IP support, some adaptations were made.

Scalability: IS-IS is known for its scalability and is often used in large service provider networks.

Metric: Like OSPF, IS-IS uses a cost metric. The administrator can configure the cost on each link.

Authentication: IS-IS supports authentication mechanisms to secure routing updates.

IPv6 Support: IS-IS was one of the first IGPs to support IPv6, making it attractive for early IPv6 adopters.

ISO 10589 Standard: IS-IS is defined by the ISO 10589 standard. The extensions to support IP are defined in various RFCs.

Benefits:

  • Flexibility: IS-IS is less restrictive than OSPF in terms of network design and area structuring.
  • Scalability: Its hierarchical design, combined with the ability to support both IP and OSI, makes it highly scalable.
  • Robustness: As a link-state protocol, IS-IS can quickly adapt to network topology changes and recover from failures.

Drawbacks:

  • Less Common: While IS-IS is popular in ISP environments, it’s less commonly used in enterprise networks compared to OSPF.
  • Configuration Complexity: IS-IS can be complex to configure, especially for those unfamiliar with its terminology and the OSI model.

Overall, IS-IS is a powerful and flexible IGP, especially favored in large and complex network environments like those of service providers.