IS-IS Micro-Loop Avoidance in OcNOS 6.3: Preventing Transient Forwarding Loops

In Layer 3 networks, IS-IS and OSPF prevent permanent loops by computing loop-free shortest paths. However, during network convergence — the brief period after a topology change while all routers are updating their forwarding tables — transient micro-loops can form. A micro-loop occurs when two adjacent routers have inconsistent forwarding tables: Router A has already updated its path to a destination, but Router B has not yet, causing packets to bounce between them until B converges.

While micro-loops are short-lived (typically milliseconds to seconds), they can saturate links and cause significant packet loss during high-traffic periods. OcNOS 6.3 introduces IS-IS micro-loop avoidance using Segment Routing to eliminate this problem.

How Micro-Loops Form

IS-IS Micro-Loop Avoidance in OcNOS 6.3

OcNOS 6.3 implements RFC 8333 micro-loop avoidance using Segment Routing. When a topology change is detected, instead of immediately installing the new shortest-path forwarding entries, the router temporarily uses an SR explicit path that follows the old topology — the path that was loop-free before the change. This holds traffic on a known-good path during the convergence window, then switches to the new shortest path once all routers have converged.

The convergence delay is configurable (typically 1–5 seconds) and gives the slowest router in the domain time to update before any router uses the new path.

! OcNOS 6.3 -- IS-IS micro-loop avoidance configuration
!
router isis CORE
  net 49.0001.0000.0000.0001.00
  is-type level-2-only
  segment-routing mpls
  !
  ! Enable micro-loop avoidance with 2-second hold-down
  microloop avoidance segment-routing
  microloop avoidance rib-update-delay 2000   ! 2000ms = 2 seconds
  !
  address-family ipv4 unicast
    segment-routing mpls
    fast-reroute per-prefix ti-lfa             ! TI-LFA handles actual failures
  exit-address-family
!
! Verification:
show isis microloop avoidance
! Output shows: micro-loop avoidance state, pending path updates,
! and the SR explicit paths being used during convergence hold-down

Micro-Loop Avoidance vs. TI-LFA

These two mechanisms complement each other. TI-LFA handles the fast failover when a failure is detected. Micro-loop avoidance handles the convergence window — the period after all routers have detected the failure but before all have updated their forwarding tables. Together they provide complete protection against both permanent failures and transient convergence artifacts.

• ISIS-SR with TI-LFA: Fast Reroute Guide
• OcNOS 6.3 Release Overview
• OcNOS-SP Product Page
• Contact IP Infusion

IP Infusion Engineering Team