SR based Micro-loop avoidance is a mechanism designed to prevent transient forwarding loops that can occur during IGP convergence events such as link flaps, metric changes, or other SPF-triggering events in IP networks, particularly within Segment Routing (SR) enabled domains. The primary goal is to ensure traffic continuity and minimize packet loss during convergence by computing loop-free SR tunnels for nodes at risk of micro-loops.

Micro-loops are temporary forwarding loops that can occur during IGP convergence, when routers update their forwarding paths at different times after a network change (like a link failures). These loops typically last for a few milliseconds to seconds, but can have serious consequences.

 

In the given topology, the link between R2 and R4 is initially down. During this state, R3 reaches R5 via R4. When the link between R2 and R4 comes up, the network begins to reconverge, and R3 recalculates its path to R5. Due to the lower cost via R1, R3 updates its next-hop to R1. As a result, R3 updates its forwarding table before R1. Since R1 has not yet converged, it still forwards packets towards R3 based on the old path, creating a transient micro-loop between R3 and R1. Once R1 completes its convergence and updates its path to forward traffic via R2, R2 still operating with the old path may continue forwarding back to R1, leading to a second micro-loop between R1 and R2. Asynchronous convergence in routers can lead to temporary micro-loops in fast-converging IGP networks, underscoring the need for effective micro-loop prevention strategies.

The IS-IS Segment Routing Micro-loop Avoidance feature provides a robust solution to mitigate these issues. It leverages SR-MPLS to ensure loop-freeness during IGP re-convergence following link-state changes, such as link or node failures, or metric updates. Micro-loops occur due to temporary inconsistencies in forwarding tables during these transitions. This feature coordinates synchronized updates to the FIB across routers, effectively reducing convergence-related packet loss. To ensure optimal protection against micro-loops, this feature should be enabled consistently on all routers within the network.

The topology illustrates an IS-IS Segment Routing network designed for micro-loop avoidance. It includes two Provider Edge routers (PE1 and PE2), two Provider routers (P1 and P2), and two traffic generators (Traffic Generator 1 and Traffic Generator 2). PE1 connects to Traffic Generator 1 via the customer-facing interface ce1, while PE2 connects to Traffic Generator 2 via ce20. The network-side interfaces (ce2, ce3, ce0, ce16, ce49, ce50) link PE1, P1, P2, and PE2, forming a multi-path topology with P1 and P2 interconnected.

The following steps configure IS-IS Segment Routing Micro-loop Avoidance on routers PE1, P1, P2, and PE2. These steps assume IS-IS SR and MPLS services are already partially configured. For example, only ELINE services are configured. To configure SR Micro-loop functionality on nodes with ISIS as IGP, follow the steps mentioned below:

Verify the configuration using the following commands on each router:

This section provides the commands required to configure SR micro-loop avoidance under ISIS mode.

Use this command to configure a SR micro-loop avoidance under IS-IS Segment Routing (SR) mode.

Use the no form of this command to disable micro-loop avoidance for under IS-IS Segment Routing (SR) mode.

When micro-loop avoidance is enabled under Segment Routing, the router ensures loop-free convergence by controlling FIB updates across the network during topology changes.

If fib-delay is not configured, default fib-delay of 1500ms(1.5sec) is configured.

ISIS router mode

This command is introduced in OcNOS/Version/6.6.1.

The following example shows the enabling of a SR microloop avoidance