352-001 · Question #550
Which two options are Loop-Free Alternate design considerations? (Choose two)
The correct answer is B. Backup coverage and effectiveness is dependent on the network topology E. It can impact SLA-sensitive appliance by routing traffic to low bandwidth links while IGP. Loop-Free Alternate (LFA) provides pre-computed IP Fast Reroute backup paths, but its coverage is topology-dependent and it can cause SLA issues by sending traffic over suboptimal links during convergence.
Question
Which two options are Loop-Free Alternate design considerations? (Choose two)
Options
- AMPLS TE must be enabled because it is used for building the backup paths
- BBackup coverage and effectiveness is dependent on the network topology
- CIt can simplify the capacity planning by matching the backup path with the post-convergence path
- DIt provides an optional backup path by avoiding low bandwidth and edge links
- EIt can impact SLA-sensitive appliance by routing traffic to low bandwidth links while IGP
How the community answered
(30 responses)- A10% (3)
- B57% (17)
- C27% (8)
- D7% (2)
Why each option
Loop-Free Alternate (LFA) provides pre-computed IP Fast Reroute backup paths, but its coverage is topology-dependent and it can cause SLA issues by sending traffic over suboptimal links during convergence.
MPLS TE is not required for LFA - LFA is an IGP-native IP Fast Reroute mechanism that uses SPF calculations and does not rely on MPLS TE tunnels for building backup paths.
LFA backup coverage is entirely dependent on whether a neighboring node offers a loop-free alternate path per the inequality check defined in RFC 5286. In topologies with few alternate paths, many prefixes will have no LFA coverage, making topology a primary design constraint.
LFA does not simplify capacity planning - because the backup path often differs from the post-convergence path, capacity planning must account for two separate traffic distributions, increasing complexity.
LFA does not inherently avoid low-bandwidth or edge links - the backup next-hop is selected purely based on the loop-free inequality check, with no automatic exclusion of low-bandwidth interfaces.
During IGP convergence, LFA reroutes traffic to a pre-computed backup path that may traverse low-bandwidth or congested links. This can violate latency, jitter, or bandwidth SLAs for real-time or SLA-sensitive traffic flows until the primary path is restored.
Concept tested: Loop-Free Alternate IP Fast Reroute design tradeoffs
Source: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_ospf/configuration/xe-16/iro-xe-16-book/iro-ip-fast-reroute.html
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