CCNA Interview Questions and Answers -9


1. How do I stop individual interfaces from developing adjacencies in an OSPF network?

To stop routers from becoming OSPF neighbors on a particular interface, issue the passive-interface command at the interface.

2. When I have two type 5 link-state advertisements (LSAs) for the same external network in the OSPF database, which path should be installed in the routing table?

When you have two type 5 LSAs for the same external network in the OSPF database, prefer the external LSA that has the shortest path to the Autonomous System Boundary Router (ASBR) and install that into the IP routing table. Use the show ip ospf border-routers command to check the cost to the ASBR.

3. Should I use the same process number while configuring OSPF on multiple routers within the same network?

OSPF, unlike Border Gateway Protocol (BGP) or Enhanced Interior Gateway Routing Protocol (EIGRP) does not check the process number (or autonomous system number) when adjacencies are formed between neighboring routers and routing information is exchanged.
Can we have OSPF run over a GRE tunnel?
Yes we can have OSPF run over a GRE tunnel.


4. What is an OSPF adjacency?

An OSPF adjacency is a conceptual link to a neighbor over which LSAs can be sent.

5. What are the five OSPF packet types? What is the purpose of each type?

The five OSPF packet types, and their purposes, are:
Hellos – which are used to discover neighbors, and to establish and maintain adjacencies
Updates – which are used to send LSAs between neighbors
Database Description packets – which a router uses to describe its link state database to a neighbor during database synchronization
Link State Requests – which a router uses to request one or more LSAs from a neighbor’s
link state database
Link State Acknowledgments – used to ensure reliable delivery of LSAs

6. What is a link state database? What is link state database synchronization?

The link state database is where a router stores all the OSPF LSAs it knows of, including its own. Database synchronization is the process of ensuring that all routers within an area have identical link state databases.

7. What is the default Hello Interval?

The default OSPF HelloInterval is 10 seconds.

8. What is the default Router Dead Interval?

The default Router DeadInterval is four times the HelloInterval.

9. What is a Router ID? How is a Router ID determined?

A Router ID is an address by which an OSPF router identifies itself. It is either the numerically highest IP address of all the router’s loopback interfaces, or if no loopback interfaces are configured, it is the numerically highest IP address of all the router’s LAN interfaces.

10. What is an area?

An area is an OSPF sub-domain, within which all routers have an identical link state database.

11. What is the significance of area 0?

Area 0 is the backbone area. All other areas must send their inter-area traffic through the backbone.

12. What is Max-Age?

MaxAge, 1 hour, is the age at which an LSA is considered to be obsolete.

13. Are OSPF routing protocol exchanges authenticated?

Yes, OSPF can authenticate all packets exchanged between neighbors. Authentication may be through simple passwords or through MD5 cryptographic checksums. To configure simple password authentication for an area, use the command ip ospf authentication-key to assign a password of up to eight octets to each interface attached to the area. Then, issue the area x authentication command to the OSPF router configuration to enable authentication. (In the command, x is the area number.)
Cisco IOS Software Release 12.x also supports the enabling of authentication on a per-interface basis. If you want to enable authentication on some interfaces only, or if you want different authentication methods on different interfaces that belong to the same area, use the ip ospf authenticationinterface mode command.

14. What is the link-state retransmit interval, and what is the command to set it?

OSPF must send acknowledgment of each newly received link-state advertisement (LSA). It does this by sending LSA packets. LSAs are retransmitted until they are acknowledged. The link-state retransmit interval defines the time between retransmissions. You can use the command ip ospf retransmit-interval to set the retransmit interval. The default value is 5 seconds.

15. What are the four OSPF router types?

The four OSPF router types are:
Internal Routers = whose OSPF interfaces all belong to the same area
Backbone Routers = which are Internal Routers in Area 0
Area Border Routers = which have OSPF interfaces in more than one area
Autonomous System Boundary Routers = which advertise external routes into the OSPF Domain

16. What are the four OSPF path types?

The four OSPF path types are:
Intra-area paths
Inter-area paths
Type 1 external paths
Type 2 external paths

17. What is the purpose of the subnets keyword when redistributing OSPF?

Without the Subnets keyword, only major network addresses that are not directly connected to the router will be redistributed.

18. What are the OSPF network types?

The four OSPF network types are:
– Point-to-point networks
– Broadcast networks
– Non-broadcast
– Non-broadcast multi-access (NBMA) networks
Point-to-multipoint networks configurationNote- To change network type we use “Router(config-if)# ip ospf network point-to-multipoint [non-broadcast]” command

19. What is a Designated Router?

A Designated Router is a router that represents a multiaccess network, and the routers connected to the network, to the rest of the OSFP domain.

20. How does a Cisco router calculate the outgoing cost of an interface?

Cisco IOS calculates the outgoing cost of an interface as 100/BW, where BW is the configured bandwidth of the interface.

21. What is the purpose of the variable IP-OSPF-Transmit-Delay?

This variable adds a specified time to the age field of an update. If the delay is not added before transmission over a link, the time in which the link-state advertisement (LSA) propagates over the link is not considered. The default value is 1 second. This parameter has more significance on very low-speed links.

22. What is a partitioned area?

An area is partitioned if one or more of its routers cannot send a packet to the area’s other routers without sending the packet out of the area.

23. What is a virtual link?

A virtual link is a tunnel that extends an OSPF backbone connection through a non-backbone area.

24. What is the difference between OSPF network entries and OSPF router entries?

OSPF network entries are entries in the route table, describing IP destinations. OSPF router entries are entries in a separate route table that record only routes to ABRs and ASBRs.

25. Which three fields in the LSA header distinguish different LSAs? Which three fields in the LSA header distinguish different instances of the same LSA?

The three fields in the LSA header that distinguish different LSAs are the Type, Advertising Router, and the Link State ID fields. The three fields in the LSA header that distinguish different instances of the same LSA are the Sequence Number, Age, and Checksum fields

26. Is it true that only the static option of the virtual link in OSPF allows discontiguous networks, regardless of the mask propagation properties?

No, virtual links in OSPF maintain connectivity to the backbone from nonbackbone areas, but they are unnecessary for discontiguous addressing. OSPF provides support for discontiguous networks because every area has a collection of networks, and OSPF attaches a mask to each advertisement.

27. What does the clear ip ospf redistribution command do?

The clear ip ospf redistribution command flushes all the type 5 and type 7 link-state advertisements (LSAs) and scans the routing table for the redistributed routes. This causes a partial shortest path first algorithm (SPF) in all the routers on the network that receive the flushed/renewed LSAs. When the expected redistributed route is not in OSPF, this command may help to renew the LSA and get the route into OSPF.

28. Does OSPF form adjacencies with neighbors that are not on the same subnet?

The only time that OSPF forms adjacencies between neighbors that are not on the same subnet is when the neighbors are connected through point-to-point links. This may be desired when using the ip unnumbered command, but in all other cases, the neighbors must be on the same subnet.

29. How often does OSPF send out link-state advertisements (LSAs)?

OSPF sends out its self-originated LSAs when the LSA age reaches the link-state refresh time, which is 1800 seconds. For more information, refer toLink-State Advertisements.

30. How do I stop individual interfaces from developing adjacencies in an OSPF network?

To stop routers from becoming OSPF neighbors on a particular interface, issue the passive-interface command at the interface.

In Internet service provider (ISP) and large enterprise networks, many of the distribution routers have more than 200 interfaces. Configuring passive-interface on each of the 200 interfaces can be difficult. The solution in such situations is to configure all the interfaces as passive by default using a single passive-interface default command. Then, configure individual interfaces where adjacencies are desired using the no passive-interfacecommand. For more information, refer to Default Passive Interface Feature.

There are some known problems with the passive-interface default command. Workarounds are listed in Cisco bug ID CSCdr09263 ( registeredcustomers only) .