352-001 · Question #553
You are hired to assist an enterprise customer to design their global WAN network. A protected DWDM circuit with disjoint fiber routes and guaranteed restoration times is ordered to connect two hub…
The correct answer is A. BFD failure detection must be faster than DWDM restoration time. When BFD is deployed alongside a protected DWDM circuit, its failure detection timers must be tuned to fire before the DWDM protection switching completes, or BFD becomes functionally useless as a fast detection mechanism at the IP routing layer.
Question
You are hired to assist an enterprise customer to design their global WAN network. A protected DWDM circuit with disjoint fiber routes and guaranteed restoration times is ordered to connect two hub sites. Which option is a BFD design consideration in relation to protected DWDM?
Options
- ABFD failure detection must be faster than DWDM restoration time
- BThe BFD hello timer must match the DWDM circuit restoration time
- CBFD failure detection must be longer than DWDM restoration time
- DBFD cannot be used with protected DWDM
How the community answered
(38 responses)- A63% (24)
- B5% (2)
- C21% (8)
- D11% (4)
Why each option
When BFD is deployed alongside a protected DWDM circuit, its failure detection timers must be tuned to fire before the DWDM protection switching completes, or BFD becomes functionally useless as a fast detection mechanism at the IP routing layer.
Protected DWDM circuits perform optical-layer restoration within a guaranteed time window. BFD failure detection must be configured faster than that restoration window so the IP routing plane can respond to failures that DWDM cannot transparently cover - if BFD timers are slower, DWDM always restores before BFD fires, rendering BFD non-functional as a routing-layer detection tool. This timer relationship is the critical design constraint when combining BFD with DWDM protection.
Matching the BFD hello timer exactly to the DWDM restoration time is not a valid design target - BFD must be tuned to be faster than restoration, not equal to it, for BFD to contribute any detection value.
Setting BFD detection longer than DWDM restoration time means DWDM always restores before BFD triggers, making BFD irrelevant and defeating its purpose as a fast IP-layer failure detection mechanism.
BFD is fully compatible with protected DWDM circuits; the engineering challenge is proper timer tuning relative to the DWDM restoration guarantee, not a fundamental incompatibility.
Concept tested: BFD timer tuning relative to DWDM protection restoration
Source: https://www.cisco.com/c/en/us/support/docs/ip/bidirectional-forwarding-detection-bfd/117127-configure-bfd-00.html
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