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.
Unsure how the routers would be able to determine that a ‘segment’ of of 10.1.40.X and 10.2.20.X would be the differentiating factor. Can someone explain why E and F are not the correct answers?
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There can only be one DR / BDR elected per LAN segment. Because the two routers are connected by a switch(switches are not capable of routing), this indicates that both routers are on the same LAN segment and subnet. There are two lan segments pictured and only one can be elected from each. The highest IP address in this area is branch 2 with 10.2.20.10. Branch 1 is in the same LAN segment and therefore is ineligible to become the BDR. The second highest IP address in the second LAN segment is 10.1.40.40, or Corp-4.
Keep at it, polish out what you don’t know, you’ll be fine !
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