Refer to the exhibit. Which two statements are true about the loopback address that is configured
on RouterB? (Choose two.)
A.
It indicates that RouterB should be elected the DR for the LAN.
B.
It decreases the metric for routes that are advertised from RouterB.
C.
It provides stability for the OSPF process on RouterB.
D.
It specifies that the router ID for RouterB should be 10.0.0.1.
E.
It ensures that data will be forwarded by RouterB.
Explanation:
A loopback interface is virtual in nature, and thus will never go down as long as the router is
powered on. It doesn’t rely on any physical network or cable to be plugged in. This makes it a
prime choice for any good reference point. That brings us to the “why” about using it. When OSPF
routers talk to one another, they all identify themselves. That is done by a RID, or Router ID value.
An OSPF router may talk to many neighbors out multiple interfaces, but it only has one Router ID
it uses for all conversations. How does a router choose its identifier? Well, there are a couple
ways. Typically, the router chooses its highest IP address of all physical interfaces. However, if
there’s a loopback interface (seen as a manual intervention), the OSPF process will always use
the loopback address as its RID value.
In this network, stability is ensured for Router-B as it will not become the DR or the BDR because
the other routers will have a higher router ID since the have a higher loopback IP address. The
DR/BDR election process is as follows:
A designated router (DR) is the router elected by the network by elections. The DR is elected
based on the following default criteria:
* If the priority setting on a OSPF router is set to 0, that means it can NEVER become a DR or
BDR.
* When a DR fails and the BDR takes over, there is another election to see who becomes the
replacement BDR.
* The router sending the Hello packets with the highest priority.
* If two or more routers tie with the highest priority setting, the router sending the Hello with the
highest RID (Router ID) wins.
* (NOTE) A RID is the highest logical (loopback) IP address configured on a router, if no
logical/loopback IP address is set then the Router uses the highest IP address configured on its
interfaces. (e.g. 192.168.0.1 would be higher than 10.1.1.2)
* Usually the router with the second highest priority number becomes the BDR (Backup
Designated Router)
* The range of priority values range from 1 – 255, with a higher value increasing its chances of
becoming DR or BDR.
* IF a HIGHER priority OSPF router comes online AFTER the election has taken place, it will not
become DR or BDR until (at least) the DR and BDR fail.