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) RoutersHandle routing within an area. They only know about routes within their own area.
- Level 2 (L2) RoutersResponsible for inter-area routing. They maintain a view of the entire IS-IS domain and can also function as an L1 router.
- L1/L2 RoutersRouters 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:
- FlexibilityIS-IS is less restrictive than OSPF in terms of network design and area structuring.
- ScalabilityIts hierarchical design, combined with the ability to support both IP and OSI, makes it highly scalable.
- RobustnessAs 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.
Key terms in plain language
Open a term for a concise explanation of language used on this page.
Fiber Internet
Internet delivered through strands of glass using light. Fiber commonly supports high capacity, low latency, and strong upload performance, but availability must be confirmed for the exact address.
Broadband
A general term for always-on, high-speed Internet access. Broadband can be delivered over fiber, cable, DSL, fixed wireless, cellular, or satellite networks.
Bandwidth
The amount of data a connection can carry in a given time, usually measured in Mbps or Gbps. More bandwidth supports more users, devices, and simultaneous applications.
Latency
The time it takes data to travel between two points. Lower latency improves voice, video meetings, cloud applications, gaming, and other real-time services.
Dedicated Internet Access (DIA)
A business-grade Internet connection with capacity dedicated to the customer rather than shared in the same way as typical consumer broadband. It often includes symmetrical speeds and an SLA.
SD-WAN
Software-defined wide area networking. It manages multiple connections and chooses paths based on application needs, performance, and policy to improve resilience and control.