ICND1 – OSPF Questions
Note: If you are not sure about OSPF, please read my OSPF tutorial first.
Answer A and C are obviously correct. For answer E, it allows extensive control of routing updates via Link-State Advertisement (LSA). Administrators can filter these LSAs to meet their requirements easily.
A is not correct because the backbone area of OSPF is always Area 0.
B is not correct because R1 or R3 must be the DR or BDR -> it has to establish neighbor adjacency with the other.
C is not correct because OSPF neighbor relationship is not established based on static routing. It uses multicast address 184.108.40.206 to establish OSPF neighbor relationship.
E is not correct because configure EIGRP on these routers (with a lower administrative distance) will force these routers to run EIGRP, not OSPF.
D and F are correct because these entries must match on neighboring routers:
– Hello and dead intervals
– Area ID (Area 0 in this case)
– Authentication password
– Stub area flag
The highest IP address of all loopback interfaces will be chosen -> Loopback 0 will be chosen as the router ID.
OSPF uses a metric referred to as cost. The cost of the entire path is the sum of the costs of the outgoing interfaces along the path. Cisco uses a simple formula to calculate OSPF cost:
OSPF cost = 108 / Bandwidth (bit)
Therefore, a 100 Mbps FastEthernet interface will have the cost of 108 / 100,000,000 (bps) = 1
Note: Cost for interfaces with bandwidth equal or larger than 10^8 bps is normalized to 1 so a 1Gbps interface will also have OSPF cost of 1.
For “O 192.168.12.240 /30 [110/128] via 192.168.12.233, 00:35:36, Serial0″ line, the first number in the brackets is the administrative distance of the information source; the second number is the metric for the route -> In this case the second number is the OSPF cost.
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.