Top 35 OSPF Interview Questions and Answers
1. What is OSPF ?
OSPF stands for open shortest Path First . It is a link state routing Protocol .It uses shortend path first algorithom in order to build and calculate the shortest path to all known destinations .It is an Interier Gateway Protocol (IGP) that is used mainly in larger TCP/IP interworks and within autonomous systems of the Internet
2. What is OSPFv3 ?
OSPFv3 is a link state routing protocol for IPv6 as its predessor OSPFv2 in IPv4 .OSPFv3 still uses the autonomous areas to separate networks into areas .OSPFv3 uses an IPv6 multicast address range of FF02::5 for OSPF routes and FF02::6 for OSPF designated routers when sending updates and acknwoledgment .
3. What are the most important features of OSPF ?
It is fast convergence and scalability
OSPF has no hop count limit
It provides routing information to the internet protocol of the TCP /IP protocol suite (most used alternative to RIP Protocol )
It supports VLSM , CICR supernating , and non contiguous network segments .
It supports the logical grouping of network segments into areas .
It can load balancer network traffic different multiple paths of the same metric value and less bandwidth.
The uses multicast addresses 22.214.171.124 and 126.96.36.199 for routing updates.
4. What is Hop count ?
Hop count is the number of intermediate networks devices such as router through which a given data must pass between source and destination . Administrative count is the choosing best path to destination router.
Different routing protocol their ADs are
External EIGRP- 170
5. What multicast address does OSPF use ?
OSPF use the multicast address of 188.8.131.52 and 184.108.40.206
6. How to generate default route in OSPF ?
There are two ways to inject a default route into a normal area . If the ASBR already has the default route in its routing table , you can advertise the existing 0.0.0.0/0 into the OSPF domain with the default-information originate router configuration command .
7. What is OSPF Router ID ?
Router id is 32 bit number similar to IP address .It is elected as highest IP of the active physical interfaces if loopback is not present . It can also be set manually but it will take effect when OSPF process is reset completely or device is reloaded . There should be only one router-id per device running OSPF to avoid database issues .
8. What is Routing Protocol ?
A Routing protocol can be analyzed as a process . A protocol is formalized through the Request for comments (RFC) process .This process involves open written commentary on a proposed technology in an effort to bring about standardization of that techonology. An example of a routing protocol standard would be OSPF as defined in RFC 2328.
9. What is RIP ?
RIP stands for Routing Information Protocol . It is one of a family of IP Routing protocols , and it is an Interier Gateway Protocol (IGP) designed to distribute routing information within an Autonomous System (AS).
10. What is EIGRP ?
EIGRP stands for Enhanced Interier Gateway Routing Protocol . It is a Cisco propretary enhanced Distance Vector routing protocol . EIGRP is based on IGRP, hence the configuration is similar . Enhanced Interier Gateway Routing Protocol (EIGRP) is considered as a Hybrid Routing Protocol because EIGRP has characteristics of both Distance Vector and Link State Routing Protocols .
11. What is Linked state protocol ?
Link state protocols is also known as shortest routing protocols or shortest path first algorithms.Link states routing protocols dont view networks in terms of adjacent routers and hop counts, but they build a comprehensive view of the overall network which fully describes the all possible routes along with their costs . Link states protocols are based on Shortest Path First (SPF) algorithm to find the best path to a destination. Shortest Path First (SPF) algorithm is also known as Dijkstra algorithm.
12. What are the OSPF Packet Types ?
OSPF uses five packet types in exchanging messages about up to date routing information to maintain accurate information about the network with neighbours.
# Hello Packet
# DBD or DDP Packet (Database Description Packets )
LSR Packet (Link State Request)
LSU Packet (Link State Update)
LSack Packet (Link state Acknowledgement )
13. What is Hello Packet in OSPF ?
Hello Packet are OSPF packet Type 1 . These packets are multicast periodically to 220.127.116.11 multicast address on all interfaces (Unicast on virtual links) enabling dynamic discovery of neighbours and maintain neighbour relationship . On broadcast and NBMA networks . Hello packets are used to elect DR and BDR .
14. What is LSA in OSPF ?
LSA stands for Link State Advertisement . It is a message that communicates the depends on routers and sequence number .
15. What is LSDB in OSPF ?
LSDB stands for link state Database . LSDB is the database that OSPF builds and it based on the information that is has found in LSAs (Link State Advertisements) . The LSDB is synchronized between routers within the same area .Each router stores the received LSA packets in the link-states database (LSDB). After LSDBs are synced between the routes , OSPF uses the shortest path first (SPF) algorithmto calculate the best routes .
16. What is DBD in OSPF ?
DBD stands for Database Description Packets (also refferd as DDPs ). It is used to check if the database is synchronized between the sending and receiving router. These packets are exchanged when an adjacency is being initialized. They describe the contents of the topological database . It doesnot include full LSAs but would include LSA headers in the Link state database of the sender .
17. What is LSR in OSPF ?
LSR stands for Link State Request . After exchanging Database Description Packets with a neighbouring router, a router may find that parts of its topological database are out of date. The Link State Request packet is used to request the pieces of the neighbours database that are more up to date . The sending of Link State Request packets is the last step in bringing up an adjacency what other have (DBDs)-What i have (LADB)=What I need to order(LSR).
18. What is LSU in OSPF ?
LSU stands for Link State Update. A Packet that contains fully detailed LSAa typically sent in response to an LSR message.Or (LSU packets are used to reply to LSRs and also to announce new information’s changes to the current topology )
19. What is LSAck in OSPF ?
LSAck stands for Link State Acknowledgment . It is send by the receiving router to confirm receipt of an LSU message .
20.What are the OSPF neighbour states ?
Before establishing a neighbor relationship. OSPF routers need to go through several state changes . These states are explain below.
Init State : A router has received a Hello Message from other OSPF router .
2-Way State : The neighbor has received a Hello Message and replied with a Hello message of his own.
Exstart State : It is begining of the LSDB exchanging between both routers . Routers are starting to exchange link state information .
Exchange State : DBD packet are exchanged .DBDs contain LSAs headers. Routers will use this information to see what LSAs need to be exchanged.
Loading State : One neighbor send LSRs (Link State Requests) for every network it doesn’t know about. The other neighbor replies with the LSUs (Link State Updates ) which contain information about requested networks . After all the requested information have been received , other neighbor goes through the same process.
Full State : The both routers have the synchronized database and are fully adjacent with each other .
21. What are the multicast IPs used by OSPF? What multicast IP does DR/BDR router and non-DR use ?
OSPF Routers use 18.104.22.168 multicast IP address . However in DR/BDR it uses 22.214.171.124 and 126.96.36.199 DR/BDR will use 188.8.131.52 while replying to OSPF routers in Broadcast Multi access network .
22. What are the different LSA types in OSPF ?
LSA Type-1 Router
LSA Type-2 Network
LSA Type-3 Summary
LSA Type-4 Summary ASBR
LSA Type-5 Autonomous System External
LSA Type-6 Multicast OSPF
LSA Type-7 Not-so-Stubby area
LSA Type-8 External attributes LSA for BGP
LSA Type-9, 10-11 Opaque LSAs for future purpose
23. What are the OSPF tables ?
There are three types of OSPF tables
Neighbor table : It stores information about OSPF neighbors
Topology table : It stores the topology structure of a network
Routing table : The best route for each known network
24. What are the OSPF Router Types ?
There are four router types used in OSPF .
ABR-Area Border Router
ASBR-Autonomous System Backbone Router
25. What are internal router in OSPF ?
Routing device that connect to only one OSPF area are called Internal Routers. All interfaces on internal routers are directly connected to network within a single area. Routers with only interfaces in the backbone area also belong to this category and have a single link state database .
26. What is ABR in OSPF ?
ABR stands for Area border router .This router that has directly connected network belonging to multiple areas .ABR have multiple link-state database , one for each area , including the backbone. ABRs are attached to multiple OSPF areas , so there can be multiple ABRs within a network.
27. What is ASBR in OSPF ?
ASBR stands for Autonomous System Backbone Router . ASBRs are connected to more than one AS and exchange routing information with routers in another AS. ASBRs advertise the exchanged external routing information throughout their AS. Every router within an AS knows how to get to each ASBR with its AS.
28. What is BR in OSPF ?
BR stands for Backbone Router . Routers whose interfaces connect them only to the backbone area are consider backbone routers (BRs). BRs do not have an interface to the other OSPF areas , because if they did , they would be considered ABRs .
29. What is OSPF super backbone area ?
The idea is that super backbone can override the default behaviour of OSPF and provide connectivity between customers without the need for crazy virtual links or tunnelling (cant use VLS across area 0 anyway )
30. What problems can cause OSPF neighborship in down state ?
Steps in forming neighborship in OSPF are almost similar to EIGRP’s. Two OSPF devices should be connected back to back with cables connected on correct interfaces. IP and mask should be correct . Then run the ospf process it not run already and advertise the interface segments on both routers . Hello dead timers and MTU size should be same at both ends . There should not be any access-list or transparent firewall blocking the OSPF Packets or IP addresses . Neighborship should established now. Area number and authentication parameter.
31. What are the OSPF Time intervals ?
Hello intervals : How often OSPF router will send the hello packet to other OSPF
Dead intervals : How long a router will wait for hello packets before it declares the neighbour dead , Router
32. What is the default hello interval ?
The default Hello Interval for OSPF is 10 second .
33. What is the default dead interval ?
By default it is 40 seconds.
34. What is Process id ?
Process id is used similarly to Autonomous system but it has local significance only. It can be different on neighbors running OSPF .
35. What is the concept of auto-cost in OSPF ?
The default reference bandwidth for OSPF is 10^8 bps or 100 Mbit. Increasing the reference-bandwidth allows a more granular OSPF design. If changed it should be changed on all routers in the OSPF domain . The reason you would change the referance-bandwidth is that you may have link faster than 100M in your network . If you have Gigabit networks but are using the default referance-bandwidth , then Gigabit links are equal in cost to fast Ethernet .
device(config)# router ospf
device(config-ospf-router)# auto-cost referance-bandwidth 500