DR and BDR Considerations with Different Router Platforms
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.)
Answers
Explanations
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A. B. C. D. E. F.ABF.
In a multi-access network, such as Ethernet, OSPF elects a designated router (DR) and a backup designated router (BDR) to reduce the number of adjacencies and flooding, and increase network stability. OSPF DR and BDR are elected based on their priority value, and in case of a tie, based on their router ID.
When different router platforms are used, the following three points should be considered regarding DR and BDR:
A. It is a best practice that the routers with the most powerful hardware should take the role of the DR and BDR.
This statement is not entirely true. Although powerful hardware can help the router to process more OSPF packets and maintain a stable network, the OSPF DR and BDR election process is based on the priority value, not the hardware specifications. Therefore, a router with a lower hardware specification can be the DR or BDR if its priority value is higher than the others.
B. If the IP OSPF priority is the same for all routers, the highest loopback IP address and router ID will decide which routers will take the DR and BDR role during the selection process.
This statement is partially true. If the OSPF priority is the same for all routers, the router with the highest loopback IP address will become the DR, and the router with the second-highest loopback IP address will become the BDR. In case there are no loopback interfaces, the router ID will be used instead of the loopback IP address.
C. To select the DR and BDR, the IP OSPF priority must be set to a higher value than the default value at the DR and BDR during the selection process.
This statement is true. By default, the OSPF priority value is 1, which means that the router is not eligible to become the DR or BDR. Therefore, to become the DR or BDR, the router's OSPF priority value must be set to a higher value than the default value of 1 at the DR and BDR.
D. To select the DR and BDR, the IP OSPF priority must be set to a lower value than the default value at the DR and BDR during the selection process.
This statement is false. To become the DR or BDR, the router's OSPF priority value must be set to a higher value than the default value of 1 at the DR and BDR, as mentioned in statement C.
E. The role for the DR and BDR will be selected when a new OSPF router comes up.
This statement is true. When a new OSPF router joins the network, it will send a Hello packet to its neighbors, which will trigger the DR and BDR election process. The new router can become the DR or BDR if its priority value is higher than the others.
F. To force two routers to become a DR and a BDR, the IP OSPF priority can be set to zero at all other OSPF routers.
This statement is false. Setting the OSPF priority value to zero will make the router ineligible to become the DR or BDR, even if its hardware specification is higher than the others. Therefore, setting the priority value to zero at all other OSPF routers will prevent any router from becoming the DR or BDR, which is not a desired outcome.
