Introduction to IS-IS Routing

IS-IS Routing

History: Intermediate System to Intermediate System (IS-IS) is a routing protocol that was developed in the 1980s by the International Organization for Standardization (ISO) as a routing protocol for use in the OSI Reference Model’s Network Layer. It was designed for use in both LANs and WANs, and it was intended as a replacement for the aging Routing Information Protocol (RIP) and Open Shortest Path First (OSPF) protocols.

Why it is used: IS-IS is used primarily in service provider networks, particularly in the telecommunications industry. It is also used in large enterprise networks and in some government networks. IS-IS is well suited for large, complex networks because it can scale to handle a large number of routers and can handle different types of media, such as ATM, Frame Relay, and SONET.

Use Cases: IS-IS is used to route IP and IPv6 traffic, as well as non-IP traffic such as ISO CLNS (Connectionless Network Service). The primary use cases for IS-IS are in service provider networks, where it is used to route traffic across multiple autonomous systems, and in large enterprise networks, where it is used to route traffic within a single autonomous system. IS-IS is also used in some special-purpose networks, such as in the utilities and transportation industries, where it is used to route control plane traffic for SCADA (Supervisory Control and Data Acquisition) systems.

Pro and Con:

Pros:

• Scales well to large networks and can handle a large number of routers
• Can handle different types of media, such as ATM, Frame Relay, and SONET
• Can handle both IP and non-IP traffic
• Uses a link state routing (LSR) algorithm, which allows for faster convergence than distance-vector routing protocols like RIP and OSPF

Cons:

• May have a steeper learning curve than distance-vector protocols
• May require more memory and CPU resources to run
• May be less common in smaller networks, making it harder to find experts or troubleshoot issues

FAQ:

Q: What is IS-IS? A: IS-IS is a routing protocol that was developed in the 1980s by the International Organization for Standardization (ISO) as a routing protocol for use in the OSI Reference Model’s Network Layer. It is used primarily in service provider networks, particularly in the telecommunications industry, but also in some enterprise and government networks.

Q: How does IS-IS work? A: IS-IS uses a link state routing (LSR) algorithm. Routers in an IS-IS network exchange information about their directly connected links, and each router builds a database of all the links in the network. Each router then runs a shortest path first (SPF) algorithm on this database to calculate the best route to each destination.

Q: What are the main advantages of using IS-IS over other routing protocols? A: IS-IS scales well to large networks and can handle a large number of routers, it can handle different types of media, such as ATM, Frame Relay, and SONET, it can handle both IP and non-IP traffic, and it uses a link state routing (LSR) algorithm, which allows for faster convergence than distance-vector routing protocols like RIP and OSPF.

Comparing OSPF to IS-IS

OSPF and IS-IS are both Interior Gateway Protocols (IGPs) that are used to route traffic within a single autonomous system (AS). They are both link-state routing protocols, which means that routers in the network exchange information about the state of their directly connected links, and each router builds a database of all the links in the network. Both protocols then use this database to calculate the best routes to each destination.

Differences:

• OSPF is an Internet standard and is primarily used in IP networks, while IS-IS was developed by ISO and is primarily used in OSI networks, although it can also route IP traffic.
• OSPF uses a hierarchical topology, which divides the AS into smaller areas for better scalability, while IS-IS does not use areas and considers the entire AS as a single level.
• OSPF uses a 32-bit router ID, while IS-IS uses a 64-bit network entity title (NET)
• OSPF uses a multicast address of 224.0.0.5 for link-state updates, while IS-IS uses a multicast address of all IS-IS routers.

Similarities:

• Both OSPF and IS-IS are link-state protocols and use the shortest path first (SPF) algorithm to calculate the best routes.
• Both protocols have fast convergence times and can scale well to large networks with many routers.
• Both OSPF and IS-IS support multiple metrics for determining the best path, such as bandwidth, delay, reliability, and load.

In summary, OSPF and IS-IS are both link-state routing protocols that are used to route traffic within a single autonomous system. OSPF is geared towards IP networks and uses a hierarchical topology while IS-IS is geared towards OSI networks and doesn’t have the concept of areas. Both protocols have fast convergence times and can scale well to large networks with many routers, but they use different addressing schemes, and multicast addresses for link-state updates. Ultimately, the choice between OSPF and IS-IS will depend on the specific requirements and characteristics of your network.