Cisco IOS requires explicit configuration to enable DPD.
Why this answer
Dead Peer Detection (DPD) is disabled by default in Cisco IOS for IPsec site-to-site VPNs. This means that without explicit configuration using the 'crypto isakmp keepalive' command, the router will not send DPD messages to verify the liveness of the remote peer. DPD must be manually enabled to detect peer failures and trigger failover or tunnel teardown.
Exam trap
Cisco often tests the misconception that DPD is enabled by default or that it has a fixed interval, when in fact it must be manually configured and the default state is disabled.
How to eliminate wrong answers
Option A is wrong because DPD is not enabled by default with a 10-second interval; DPD is disabled by default, and if enabled, the interval is configurable but not a fixed 10 seconds. Option B is wrong because DPD is not enabled by default with a 30-second interval; again, DPD is disabled by default, and the interval is not preset to 30 seconds. Option D is wrong because DPD is not enabled only for IKEv2 by default; DPD is disabled by default for both IKEv1 and IKEv2, and it must be explicitly configured for either version.