400-007 · Question #385
Company XYZ has two offices connected to each other over unequal redundant paths and they are running OSPF as the routing protocol. An external network architect recommends BFD for OSPF. Which effect
The correct answer is D. It would detect that the neighbor is down in a subsecond manner. BFD (Bidirectional Forwarding Detection) is a lightweight protocol that supplements OSPF by providing subsecond neighbor failure detection, far faster than OSPF's native hello and dead timer mechanism.
Question
Company XYZ has two offices connected to each other over unequal redundant paths and they are running OSPF as the routing protocol. An external network architect recommends BFD for OSPF. Which effect would BFD have in the case of a link failure?
Options
- AIt would drop the dead per detection time to a single hello
- BIt would keep an alternate path ready in case of a link failure
- CIt would optimize the route summarization feature of OSPF
- DIt would detect that the neighbor is down in a subsecond manner
How the community answered
(21 responses)- A5% (1)
- B5% (1)
- D90% (19)
Why each option
BFD (Bidirectional Forwarding Detection) is a lightweight protocol that supplements OSPF by providing subsecond neighbor failure detection, far faster than OSPF's native hello and dead timer mechanism.
BFD does not modify or replace OSPF hello or dead timers - it is a separate protocol that works alongside OSPF and signals failures through its own detection mechanism, not through OSPF hello packets.
Maintaining an alternate path in the routing table is a function of OSPF's SPF algorithm and equal-cost multipath routing, not BFD - BFD only accelerates the detection of failures, not path precomputation.
BFD has no relationship to route summarization, which is an OSPF configuration feature used to aggregate prefixes and reduce routing table size.
BFD operates independently of OSPF hello timers, using its own lightweight control packets to detect forwarding path failures in milliseconds rather than relying on OSPF's default 40-second dead interval. When BFD detects a failure, it immediately signals OSPF to trigger reconvergence. This subsecond detection dramatically reduces traffic loss during a link failure event.
Concept tested: BFD subsecond failure detection with OSPF
Source: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bfd/configuration/xe-16/irb-xe-16-book/irb-bfd.html
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