352-001 · Question #98
A company is planning to connect its 30 sites with a VPLS WAN backbone. A router at each site should establish neighborships with all other routers using the OSPF routing protocol. Which three points
The correct answer is A. It is a best practice that the routers with the most powerful hardware should take the role of the DR B. If the IP OSPF priority is the same for all routers, the highest loopback IP address and router ID will F. To force two routers to become a DR and a BDR, the IP OSPF priority can be set to zero at all other. In an OSPF network over VPLS with mixed hardware, proper DR/BDR selection requires understanding priority rules, router-ID tiebreakers, and how to make specific routers ineligible.
Question
A company is planning to connect its 30 sites with a VPLS WAN backbone. A router at each site should establish neighborships with all other routers using the OSPF routing protocol. Which three points must be considered regarding DR and BDR when different router platforms are used? (Choose three.)
Options
- AIt is a best practice that the routers with the most powerful hardware should take the role of the DR
- BIf the IP OSPF priority is the same for all routers, the highest loopback IP address and router ID will
- CTo select the DR and BDR, the IP OSPF priority must be set to a higher value than the default value
- DTo select the DR and BDR, the IP OSPF priority must be set to a lower value than the default value
- EThe role for the DR and BDR will be selected when a new OSPF router comes up.
- FTo force two routers to become a DR and a BDR, the IP OSPF priority can be set to zero at all other
How the community answered
(35 responses)- A71% (25)
- C9% (3)
- D14% (5)
- E6% (2)
Why each option
In an OSPF network over VPLS with mixed hardware, proper DR/BDR selection requires understanding priority rules, router-ID tiebreakers, and how to make specific routers ineligible.
Best practice dictates that the most powerful hardware platforms should serve as DR and BDR because they must handle the processing load of generating and flooding LSAs and maintaining full adjacencies with every router on the segment.
When all routers share the same OSPF priority, the router with the highest router ID - typically derived from the highest loopback IP address - wins the DR/BDR election as the tiebreaker.
OSPF priority does not need to exceed the default value of 1 to participate in DR/BDR election; only relative priority among routers on the segment matters for determining the winner.
Setting a lower priority value reduces a router's likelihood of becoming DR or BDR rather than forcing election - this is the opposite of the correct mechanism.
OSPF DR/BDR election is non-preemptive by default, meaning a new router joining the segment does not trigger a new election unless the existing DR or BDR goes offline.
Setting ip ospf priority 0 on all non-DR/BDR routers makes them ineligible for election entirely, guaranteeing that only the two designated routers can ever become DR and BDR.
Concept tested: OSPF DR/BDR election rules and priority configuration
Source: https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/13699-10.html
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