Refer to Exhibit:
The internetwork infrastructure of company XYZ consists of a single OSPF area as shown in the
graphic. There is concern that a lack of router resources is impeding internetwork performance. As
part of examining the router resources, the OSPF DRs need to be known. All the router OSPF
priorities are at the default and the router IDs are shown with each router. Which routers are likely
to have been elected as DR? (Choose two.)
A.
Corp-1
B.
Corp-2
C.
Corp-3
D.
Corp-4
E.
Branch-1
F.
Branch-2
Explanation:
There are 2 segments on the topology above which are separated by Corp-3 router. Each segment
will have a DR so we have 2 DRs. To select which router will become DR they will compare their
router-IDs. The router with highest (best) router-ID will become DR. The router-ID is chosen in the
order below: The highest IP address assigned to a loopback (logical) interface. If a loopback interface
is not defined, the highest IP address of all active router’s physical interfaces will be chosen. In this
question, the IP addresses of loopback interfaces are not mentioned so we will consider IP addresses
of all active router’s physical interfaces. Router Corp-4 (10.1.40.40) & Branch-2
(10.2.20.20) have highest “active” IP addresses so they will become DRs.
Is this question correct?
The DR / BDR election process first looks for the highest IP address. You can control which router will be the DR/ BDR if you configure a loopback IP address.
Since we can assume from the diagram that there are no loopback interfaces configured then the router with the highest LAN IP address will become the DR and the second highest will become the DR. 10.2.20.20 is the highest IP address of a router so therefore “Branch-2” should become the DR. 10.2.10.10 is the second highest IP address and therefore “brankch-1” should become the BDR. so I should it should be E and F
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Okay, I believe I have found the answer to my own question. The election process does have to do in this case with the highest IP address since we are assuming that all OSPF priorities are the same (default).
There can only be one OSPF DR / BDR per lan segment.
From the Cisco website ;
There can be only one DR and one BDR per multiaccess segment (like Ethernet LAN)
Since Branch-1 and Branch-2 are on the same LAN segment, only one can be elected a DR or BDR.
Please read the following thread if you still have questions ;
https://learningnetwork.cisco.com/thread/15136
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It’s correct, there are 2 lan segments, so, 2 DR will be elected.
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