IS-IS, loop and convergence time

_ 20/01/2025_ Yoann Jonard

IS-IS, loop and convergence time

How IS-IS Routing Protocol Prevents Network Loops and Achieves Sub-Second Convergence ?

Introduction

IS-IS is a link-state, interior gateway protocol that was developed for the International Organization for Standardization (ISO). ISO terminology refers to routers as Intermediate Systems (IS), hence the name Intermediate System-to-Intermediate System (IS-IS). Its efficient handling of network loops and ability to provide sub-second convergence times make it an essential component of modern networking. Below, SIER explores with you how IS-IS prevents loops, allowing you to increase your network redundancy, and achieves rapid convergence.

Loop prevention

Any-to-Any

To provide a loop-free network and to learn and distribute network information, SPBM uses the Intermediate-System-to-Intermediate-System (IS-IS) link state routing protocol. IS-IS is designed to find the shortest path from any one destination to any other in a dynamic fashion. IS-IS creates any-to-any connectivity in a network in an optimized, loop-free manner, without the long convergence delay experienced with the Spanning Tree Protocol.

Active ports

IS-IS does not block ports from use, but rather employs a specific path. As such, all links are available for use. IS-IS dynamically learns the topology of a network and constructs unicast and multicast mesh connectivity. IS-IS parallel adjacency support allows you to configure multiple IS-IS links between the two nodes. Each node in the network calculates a shortest-path tree to every other network node based on System-IDs (BMAC addresses). Only one adjacency with the shortest path is selected as an active adjacency.
Only an active interface with an active adjacency is added into local SPF calculations. This mechanism ensures the local node selects the shortest path and has the same view as the rest of the SPB network.

By relying on a consistent and synchronized view of the network, IS-IS ensures that packets take the shortest and most efficient path without the risk of looping, even in complex topologies. Loop-free operation ensures that redundant links can be utilized without causing instability, allowing traffic to be dynamically rerouted during failures. This increases the overall uptime and efficiency of the network.

Convergence time

Fast convergence is critical for maintaining high availability and resilience in modern networks. IS-IS is renowned for its ability to achieve sub-second convergence through the following mechanisms.

Link State Packet (LSP)

IS-IS floods LSPs (Link State Packets) with minimal delay when a topology change is detected. The protocol uses efficient mechanisms to prioritize and propagate critical updates, ensuring that all routers are updated swiftly.

Bidirectional Forwarding Detection (BFD)

While not part of IS-IS itself, BFD provides low-overhead, short-duration failure detection between two systems. BFD also provides a single mechanism for connectivity detection over any media, at any protocol layer. Because BFD sends rapid failure-detection notifications to the routing protocols that run on the local system, which initiates routing table recalculations, BFD helps reduce network convergence time.

Equal Cost Multi Path (ECMP)

With Equal Cost Multipath (ECMP), the switch can determine up to eight equal-cost paths to the same destination prefix. You can use multiple paths for load sharing of traffic. These multiple paths provide faster convergence to other active paths in case of network failure. By maximizing load sharing among equal-cost paths, you can use links between routers more efficiently when sending IP traffic.

PSNP and CSNP intervals

The PSNP (Partial Sequence Number PDU) interval is the time between transmissions of PSNP packets in an IS-IS network. PSNPs are used to request missing LSPs (Link State PDUs) and acknowledge received LSPs. A shorter PSNP interval speeds up convergence but increases overhead, while a longer interval reduces overhead at the cost of slower convergence. The default PSNP interval is 2 seconds on Extreme Networks switches and can range from 1 to 120 seconds.
The CSNP (Complete Sequence Number PDU) interval in the IS-IS routing protocol determines the frequency at which a designated router (DIS) sends CSNP packets to maintain synchronization of the link-state databases among routers in a network. CSNPs contain a complete list of all Link-State PDUs (LSPs) in the IS-IS database, ensuring that all routers have consistent and up-to-date routing information. The default CSNP interval is 10 seconds on Extreme Networks switches and can range from 1 to 600 seconds.

Conclusion

The IS-IS routing protocol’s ability to handle network loops and achieve sub-second convergence makes it a cornerstone of modern networking. By leveraging link-state principles, the SPF algorithm, and efficient update mechanisms, IS-IS ensures both stability and agility in dynamic environments. This enables networks to provide high availability and resilience, even in the face of failures, making IS-IS, and thus the SPB Fabric, an indispensable tool for large-scale and mission-critical deployments.