OSPF Questions

Note: If you are not sure about OSPF, please read our OSPF Tutorial.

Quick OSPF review – OSPF is a link-state routing protocol, which runs Dijkstra algorithm to calculate the shortest path. – Link-state routing protocol like OSPF has a common view of entire topology – OSPF only uses bandwidth to calculate metric. – Area 0 is called the backbone area In order to become OSPF neighbor, the following values must be matched on both routers: + Area ID + Authentication + Hello and Dead Intervals + Stub area Flag + MTU Size – Hello packets of OSPF are multicast to 224.0.0.5

Question 1

Question 2

Question 3

Question 4

Explanation

In order to become OSPF neighbor following values must be match on both routers:

+ Area ID
+ Authentication
+ 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”.

Question 5

Explanation

D is 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

In this case Ethernet0 of R1 has Hello and Dead Intervals of 5 and 20 while R2 has Hello and Dead Intervals of 10 and 40 -> R1 and R2 cannot form OSPF neighbor relationship.

Question 6

Explanation

The information available to a distance vector router has been compared to the information available from a road sign. Link state routing protocols are like a road map. A link state router cannot be fooled as easily into making bad routing decisions, because it has a complete picture of the network. The reason is that unlike the routing-by-rumor approach of distance vector, link state routers have firsthand information from all their peer routers. Each router originates information about itself, its directly connected links, and the state of those links (hence the name). This information is passed around from router to router, each router making a copy of it, but never changing it. The ultimate objective is that every router has identical information about the internetwork, and each router will independently calculate its own best paths.

Reference: http://www.ciscopress.com/articles/article.asp?p=24090&seqNum=4

Question 7

Explanation

192.168.10.45 belongs to 192.168.10.32/27 subnet (range from 192.168.10.32 to 192.168.10.63) so the router will use FastEthernet0/1 as the exit interface.

Question 8

Explanation

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 224.0.0.5 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

Question 9

Explanation

The most obvious thing in this configuration is R1 forgot to run OSPF on interface Fa0/0 (with the “network 192.168.12.0 0.0.0.255 area …”) command so the computers behind 192.168.10.0/24 network does not know how to reach resources on a remote network.

Question 10

Explanation

The well-known formula to calculate OSPF cost is

Cost = 10⁸ / Bandwidth

so B is the correct answer.