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OSPF Interview Questions and Answers (Open Shortest Path First)

January 18, 2016

What is OSPF Routing protocol?
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.

Mention some characteristics of OSPF?
1.OSPF is a classless routing protocol that supports VLSM and CIDR.
2.It allows for creation of areas and autonomous system.
3.OSPF uses cost as its metric, which is computed based on the bandwidth of the link.
4.It has no hop-count limit. It supports unlimited Hop count.
5.OSPF supports both IPV4 & IPV6.
6.OSPF routes have an administrative distance of 110.

What is the need for dividing the autonomous system into various areas?
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.

What is the benefit of dividing the entire network into areas?
The following are benefits of dividing the entire network into areas -
1.Decrease routing overhead.
2.Speed up convergence.
3.Confine network instability to single areas of the network.

What is Backbone Area?
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.

Explain Area Border Router(ABR)?
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.

What is Autonomous System Border Router (ASBR)?
It is the Router that connects different Autonomous Systems.

What is OSPF Router ID?
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

What Parameters must match  for two routers to become neighbors?
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

How OSPF DR & BDR is elected?
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

Why DR and BDR are elected in OSPF?
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.

Explain the various OSPF states?
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.

Explain OSPF LSA, LSU and LSR?
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.

What are the steps required to change Neighborship into adjacency?
1.Two-way communication (using Hello Protocol)
2.Database Synchronization which means exchange of Database Description (DD) packets, Link State Request (LSR) packets, Link State Update (LSU) packets.
3.After Database synchronization is complete, the two routers are considered adjacent.

Explain OSPF timers?
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.

What is the default Hello Interval?
The default Hello Interval for OSPF is 10 seconds.

What is the default Dead Interval?
The Dead Interval is four times the Hello Interval. By default it is 40 seconds.

What multicast address does OSPF use?
OSPF use the multicast address of 224.0.0.5 & 224.0.0.6.

Tables maintained by OSPF?
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

What are different OSPF LSA types ?
1. 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.
2. 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.
3. 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.
4. Summary ASBR LSA (Type4) - It is generated by the ABR and contain routes to ASBRs.
5. External LSA (Type5) - External LSAs are generated by ASBRs and contain routes to networks that are external to current AS.
6. 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.  

How do we configure OSPF Routing Protocol?
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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