OSPF is a link-state routing protocol so it converges more quickly than distance-vector protocol. OSPF uses cost to determine the best route. The popular formula to calculate OSPF cost is: cost = 108 / Bandwidth [ in kbps] (in fact the formal formula is: cost = reference bandwidth / configured bandwidth of interface in kbps. On Cisco routers, the reference bandwidth defaults to 100000 kbps)
From the output of the “show ip ospf database”:
We can see OSPF Router ID will be used as source of Type 1 LSA (220.127.116.11 & 18.104.22.168). Also the router will chose the highest loopback interface as its OSPF router ID (if available).
Unlike OSPFv2 which form adjacencies using their IPv4 interface addresses, OSPFv3 use the link-local addresses (FE80::/10) to form adjacencies. Therefore it can support multiple IPv6 subnets on a single link as an interface can have multiple IPv6 addresses (and two nodes can communicate with each other even if they do not share a common IP subnet.) -> B is correct.
IPv6 uses the term “link” instead of “subnet” or “network” to define a medium used to communicate between nodes at the link layer -> D is correct.
OSPFv3 can support more than 1 instance over a common link. For example you can run instance 1 on an interface with the command:
Router(config-if)# ipv6 ospf 100 area 0 instance 1
-> C is not correct.
The “show ipv6 route” displays the current contents of the IPv6 routing table. This device is running OSPF so we can deduce it is running OSPFv3 (OSPF for IPv6). An example of the “show ip v6 route” is shown below:
By default, OSPF uses a 10-second hello timer and 40-second hold (dead) timer on broadcast and point-to-point links, and a 30-second hello timer and 120-second hold timer for all other network types.
In order to become OSPF neighbor following values must be match on both routers:
+ Area ID
+ Hello and Dead Intervals
+ Stub Flag
+ MTU Size
Therefore we need to adjust the MTU size on one of the router so that they are the same. Or we can tell OSPF to ignore the MTU size check with the command “ip ospf mtu-ignore”.
In the router ospf command, the ranges from 1 to 65535 so o is an invalid number -> B is correct but A is not correct.
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.