352-001 · Question #376
A customer is running OSPF on a broadcast network type in a DMVPN single-hub, single-cloud topology and dynamically routes from the data center to the branch networks. The customer wants to speed up c
The correct answer is C. The OSPF hello and dead timers should be adjusted at the data center router to match the timers. When mixing OSPF network types across DMVPN hub and spoke routers, hello and dead timer mismatches between point-to-multipoint and point-to-point interfaces must be resolved to maintain neighbor adjacencies.
Question
A customer is running OSPF on a broadcast network type in a DMVPN single-hub, single-cloud topology and dynamically routes from the data center to the branch networks. The customer wants to speed up convergence and to avoid having to elect DRs by changing the network type to point- to-multipoint at the data center and to point-to-point at the branches. Which option will meet the fast convergence requirement and not break any OSPF neighbor adjacencies when this redesign is implemented?
Options
- AThe OSPF area containing the branch office routers should be set to stubby.
- BThe OSPF neighbors should be set manually at the data center router.
- CThe OSPF hello and dead timers should be adjusted at the data center router to match the timers
- DThe OSPF database should be cleared manually for the new network types to take effect.
- EThe interface MTU sizes should be increased on all routers.
How the community answered
(40 responses)- A10% (4)
- B5% (2)
- C63% (25)
- D3% (1)
- E20% (8)
Why each option
When mixing OSPF network types across DMVPN hub and spoke routers, hello and dead timer mismatches between point-to-multipoint and point-to-point interfaces must be resolved to maintain neighbor adjacencies.
Configuring a stub area restricts LSA flooding but does not resolve the hello and dead timer mismatch introduced by using different OSPF network types.
Manually defining OSPF neighbors is required on non-broadcast interfaces but does not address the timer incompatibility that prevents adjacency formation between point-to-multipoint and point-to-point interfaces.
OSPF requires matching hello and dead timers between adjacent peers to form and maintain adjacency. Point-to-multipoint interfaces default to hello=30s/dead=120s, while point-to-point interfaces default to hello=10s/dead=40s. Manually adjusting the data center router timers to match the branch routers resolves the mismatch, preserves all adjacencies, and achieves faster convergence without DR election overhead.
Clearing the OSPF database resets learned topology state but does not fix the underlying timer mismatch that would still prevent new adjacencies from forming.
MTU mismatches affect OSPF DBD packet exchange but are unrelated to the hello and dead timer incompatibility caused by different network types.
Concept tested: OSPF network type timer compatibility in DMVPN
Source: https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/13703-8.html
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