What is the default OSPF dead interval on a broadcast multi-access network (e.g., Ethernet) when the hello interval is 10 seconds?
OSPF derives the dead interval as four times the hello interval. With a hello interval of 10 seconds on a broadcast multi-access Ethernet segment, the default dead interval is 40 seconds, matching the stated scenario parameters exactly.
Why this answer
On broadcast multi-access networks like Ethernet, OSPF defaults to a hello interval of 10 seconds. The dead interval is calculated as 4 times the hello interval, resulting in a default dead interval of 40 seconds. This ensures that a router has multiple missed hello opportunities before being declared dead, providing stability against transient network issues.
Exam trap
The trap here is that candidates often confuse the default dead interval multiplier (thinking it is 3 instead of 4) or mistakenly apply the NBMA dead interval logic to broadcast networks, leading them to select 30 or 20 seconds.
How to eliminate wrong answers
Option B (30 seconds) is wrong because it incorrectly assumes a multiplier of 3, but the OSPF standard (RFC 2328) specifies a multiplier of 4 for broadcast networks. Option C (20 seconds) is wrong because it suggests a multiplier of 2, which is used for NBMA networks (e.g., Frame Relay) where the hello interval is 30 seconds and the dead interval is 120 seconds, not for Ethernet. Option D (10 seconds) is wrong because it confuses the hello interval with the dead interval; the dead interval must be longer to allow for missed hellos.