Fast Reroute (FRR) is the network’s ability to pre-compute backup paths and switch to them in under 50 milliseconds when a link or node fails — before routing protocols have had time to reconverge. TI-LFA (Topology-Independent Loop-Free Alternate) is the most advanced FRR mechanism available today, providing 100% failure coverage for any topology without the limitations of classic LFA or Remote LFA.
FRR Evolution: LFA, RLFA, and TI-LFA
| Mechanism | How it works | Coverage | Limitation |
|---|---|---|---|
| LFA | Pre-computed neighbor that is not affected by the failure | ~60–80% | Topology-dependent; many failures have no LFA |
| RLFA | LDP tunnel to remote repair node | ~95% | Requires LDP; repair node must be P-space |
| TI-LFA | SR segment stack to any post-convergence path node | 100% | Requires Segment Routing (IS-IS or OSPF) |
TI-LFA achieves 100% coverage by computing the post-convergence path (the path traffic would take after the network fully reconverges following the failure) and encoding it as an SR segment stack. This stack is pre-computed and pre-installed — when the failure occurs, the router immediately redirects traffic using the segment stack, with no need for signaling.
TI-LFA Network Topology
Configuring ISIS-SR with TI-LFA in OcNOS
! OcNOS -- IS-IS SR with TI-LFA enabled (all routers in domain)
!
router isis CORE
net 49.0001.0000.0000.0001.00
is-type level-2-only
segment-routing mpls
!
! Enable TI-LFA per-prefix (covers all prefixes in the domain)
fast-reroute per-prefix level-2 ti-lfa
!
address-family ipv4 unicast
segment-routing mpls
fast-reroute per-prefix ti-lfa
exit-address-family
!
! Assign Node SID to loopback
interface lo
ip address 10.0.0.1/32
ip router isis CORE
isis segment-routing prefix-sid index 1
!
! Configure IS-IS on all core interfaces
interface eth-0-1
ip router isis CORE
isis network point-to-point
isis metric 10
!
interface eth-0-2
ip router isis CORE
isis network point-to-point
isis metric 10Verification Commands
! OcNOS -- TI-LFA verification
!
! Step 1: Confirm TI-LFA is enabled and computing backup paths
show isis fast-reroute summary
! Expected output includes:
! Protected prefixes: N/N (100% coverage)
! TI-LFA paths computed: N
! Step 2: View backup path for a specific prefix
show isis fast-reroute 10.0.0.5/32 detail
! Shows: primary next-hop, backup next-hop, protecting segment list
! Step 3: Verify MPLS backup labels are installed in forwarding plane
show mpls forwarding-table backup
! Confirms labels are pre-installed and ready for immediate use
! Step 4: Check IS-IS SR prefix SID database
show isis segment-routing prefix-sids
show isis database detail | include Prefix-SID
! Step 5: Verify adjacency SIDs (used in TI-LFA segment stacks)
show isis adjacency detail | include Adj-SIDNode Protection vs. Link Protection
! OcNOS -- Configure node-protecting TI-LFA (protects against node failure)
!
router isis CORE
fast-reroute per-prefix level-2 ti-lfa
fast-reroute ti-lfa node-protection ! Protect against complete node loss
!
address-family ipv4 unicast
fast-reroute per-prefix ti-lfa
fast-reroute ti-lfa node-protection
exit-address-family
!
! Note: Node protection computes backup paths that avoid the entire
! downstream node, not just the failed link. Requires more complex
! segment stacks (typically 2-segment: repair + destination)- Segment Routing Explained: SR-MPLS Fundamentals
- LDP to SR Migration Guide
- Flex-Algo in OcNOS: Fundamentals
- OcNOS-SP Product Page
- Contact IP Infusion
IP Infusion Engineering Team