OSPF INTERVIEW QUESTIONS
Question 1. What Is OSPF Routing Protocol?
Question 2. Mention Some Characteristics Of Ospf?
Question 1. What Is OSPF Routing Protocol?
Answer
:
Open
shortest path first is an Open Standard Link State routing protocol which works
by using Dijkastra algorithm to initially construct the shortest paths and
follows that by populating the routing table with resulting best paths.
Answer :
- OSPF is a classless routing protocol that supports VLSM and CIDR.
- It allows for creation of areas and autonomous system.
- OSPF uses cost as its metric, which is computed based on the bandwidth of the link.
- It has no hop-count limit. It supports unlimited Hop count.
- OSPF supports both IPV4 & IPV6.
- OSPF routes have an administrative distance of 110.
Question 3. What Is The Need For Dividing The Autonomous System Into
Various Areas?
Answer
:
we would
divide the autonomous system into various areas to keep route updates to a
minimum to conserve resources and to keep problems from propagating throughout
the network.
Question 4. What Is The Benefit Of Dividing The Entire Network Into
Areas?
Answer
:
The
following are benefits of dividing the entire network into areas -
- Decrease routing overhead.
- Speed up convergence.
- Confine network instability to single areas of the network.
Question 5. What Is Backbone Area?
Answer
:
While
configuring multi-area OSPF, one area must be called area 0, referred to as
backbone area. All other areas must connect to backbone area as inter-area
traffic is send through the backbone area.
Question 6. Explain Area Border Router(abr)?
Answer
:
It is the
router that connects other areas to the backbone area within an autonomous
system. ABR can have its interfaces in more than one area.
Question 7. What Is Autonomous System Border Router (asbr)?
Answer
:
It is the
Router that connects different Autonomous Systems.
Question 8. What Is Ospf Router Id?
Answer
:
Router Id
is used to identify the Router. Highest IP address of the router's loopback
interfaces is chosen as the Router ID, If no loopback is present than highest
IP address of the router's physical interfaces will be chosen as Router ID.
Question 9. What Parameters Must Match For Two Routers To Become
Neighbors?
Answer
:
The
following parameters must be the same on both routers in order for routers to
become neighbors:-
- 1.Subnet
- 2.Area id
- 3.Hello and Dead interval time
- 4.Authentication
Question 10. How Ospf Dr & Bdr Is Elected?
Answer
:
- The router with the highest priority becomes the DR and router with second highest priority becomes the BDR. If there is a tie in priority, router with the highest Router ID will become DR.
- By default priority on Cisco routers is 1. We can manually change it.
- If the Router priority is set to 0 (Zero), that router will not participate in DR/BDR election.
- DR election process is not preemptive. If a router with a higher priority is added to the network, it will not become DR untill we clear OSPF process and DR/BDR election takes place again.
Command to
change the priority on an interface
router(config)#
interface fa0/0
router(config-if)#
ip ospf priority 100
Question 11. Why Dr And Bdr Are Elected In Ospf?
Answer
:
All OSPF
routers will form adjacencies with the DR and BDR. If link-state changes, the
update will be sent only to the DR, which then forwards it to all other
routers. This greatly reduces the flooding of LSAs therefore conserving the
bandwidth.
Question 12. Explain The Various Ospf States?
Answer
:
OSPF
routers need to go through several states before establishing a neighbor
relationship:-
1.Down - No Hello packets have been received on the interface.
2.Attempt - In Attempt state neighbors must be configured manually. It
applies only to nonbroadcast multi-access (NBMA) networks.
3.Init
state -
Router has received a Hello message from the other OSFP router.
4. 2way state
- The neighbor has received the Hello
message and replied with a Hello message of his own. Bidirectional
Communication has been established. In Broadcast network DR-BDR election can
occur after this point.
5.Exstart state – DR & BDR establish adjacencies with each router in the
network. Master-slave election will takes place (Master will send its DBD
first).
6.Exchange state – Routing information is exchanged using DBD (Database
Descriptor) packets, Link-State Request (LSR). Link-State Update packets may
also be sent.
7.Loading state – LSRs (Link State Requests) are send to neighbors for every
network it doesn't know about. The Neighbor replies with the LSUs (Link State
Updates) which contain information about requested networks. The requested
information have been received, other neighbor goes through the same process
8.Full state - All neighbor routers have the synchronized database and
adjacencies has been established.
Question 13. Explain Ospf Lsa, Lsu And Lsr?
Answer
:
The LSAs
(Link-State Advertisements) are used by OSPF routers to exchange routing and
topology information. When two neighbors decide to exchange routes, they send
each other a list of all LSA in their respective topology database. Each router
then checks its topology database and sends Link State Request (LSR) message
requesting all LSAs that was not found in its topology table. Other router
responds with the Link State Update (LSU) that contains all LSAs requested by
the neighbor.
Question 14. What Are The Steps Required To Change Neighborship Into
Adjacency?
Answer
:
- Two-way communication (using Hello Protocol)
- Database Synchronization which means exchange of Database Description (DD) packets, Link State Request (LSR) packets, Link State Update (LSU) packets.
- After Database synchronization is complete, the two routers are considered adjacent.
Question 15. Explain Ospf Timers?
Answer
:
Hello
interval - This defines how often OSPF router
will send the hello packet to other OSPF router.
Dead
interval - This defines how long a router will
wait for hello packets before it declares the neighbor dead.
Question 16. What Is The Default Hello Interval?
Answer
:
The
default Hello Interval for OSPF is 10 seconds.
Question 17. What Is The Default Dead Interval?
Answer
:
The Dead
Interval is four times the Hello Interval. By default it is 40 seconds.
Question 18. What Multicast Address Does Ospf Use?
Answer
:
OSPF use
the multicast address of 224.0.0.5 & 224.0.0.6.
Question 19. Tables Maintained By Ospf?
Answer
:
Router
participating in OSPF routing protocol maintains three OSPF tables:-
1.Neighbor table - Stores information about OSPF
neighbors.
command
to see # sh ip ospf neighbor
2.Topology table - Stores the topology structure of a network.
command
to see # sh ip ospf topology
3.Routing table - Stores the best routes to all known networks.
command to
see # sh ip route ospf
Question 20. What Are Different Ospf Lsa Types ?
Answer
:
Router LSA (Type1) - Each router generates a Type 1 LSA
that lists its active interfaces, IP addresses, neighbors and the cost. LSA
Type 1 is flooded only within an area.
Network LSA (Type2)
- Type 2 LSA is sent out by the
designated router (DR) and lists all the routers on the segment it is adjacent
to. Type 2 LSA are flooded only within an area. It contains the information
about DR's.
Summary LSA (Type3)
- Type 3 LSAs are generated by Area
Border Routers (ABRs) to advertise networks from one area to the rest of the
areas in Autonomous System. It contains the information about inter-area
routes.
Summary ASBR LSA (Type4) - It is generated by the ABR and contain routes to ASBRs.
External LSA (Type5) - External LSAs are generated by
ASBRs and contain routes to networks that are external to current AS.
Not-So-Stubby Area LSA (Type7)
- Stub areas do not allow Type 5
LSAs. A Not So Stubby Area (NSSA) allows advertisement of Type 5 LSA as
Type 7 LSAs. Type LSA is generated by an ASBR inside a Not So Stubby Area
(NSSA) to describe routes redistributed into the NSSA.
Question 21. How Do We Configure Ospf Routing Protocol?
Answer
:
router(config)# router ospf 10
router(config-router)# network 12.1.1.0 0.0.0.255
area 0
router(config-router)# network 23.1.0.0
0.0.255.255 area 1
router(config-router)# exit
- Router ospf 10 command enables the OSPF process. Here “10” indicates the OSPF process ID and can be different on neighbor routers. Process ID allows multiple OSPF processes to run on the same router.
- Second command configures 12.1.1.0/24 network in area 0.
- Third command configures 23.1.0.0/16 network in area 1.
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