OSPF Interview Questions & Answers - Networkwalks Academy

What is OSPF?

Open Shortest Path First is an open standard link state routing protocol which works by using Dijkastra’s SPF algorithm to find the shortest paths.

What is OSPF Router ID?

OSPF Router ID is an identifier which is used to identify the Router. It is a 32-bit number.

How does a Router select its OSPF Router ID?

Most of Routers (including Cisco) derive the Router ID in the following precedence order:

Manually configured Router ID
Highest Loopback Address
Highest Physical IP Address on the Router

What are OSPF Timer (Intervals)?

Below are OSPF Intervals:

OSPF Timers (OSPF Intervals)
OSPF Network Type	Default Hello Interval	Default Dead Interval
Broadcast	10 seconds	40 seconds
Non-broadcast	30 second	120 seconds
Point-to-Point	10 seconds	40 seconds
Point-to-Multipoint	30 seconds	120 seconds
Point-to-Multipoint Non-broadcast	30 seconds	120 seconds
Loopback	N/A	N/A

How can you configure OSPF Router ID on Cisco Routers?

Below commands are used to manually configure the Router ID on a Cisco Router:

networkwalks_R1(config)# router ospf 5 
networkwalks_R1(config-router)# router-id 5.5.5.5

What are the OSPF network/Interface types?

Below are the OSPF network/Interface types:

Can two Routers in OSPF domain have same Router ID?

No, OSPF does not allow neighborship between Routers with duplicate RIDs. All OSPF RIDs in a domain should be unique.

Reference: https://networkwalks.com/networkwalks-summary-cheatsheets/

Can we change the Router ID in OSPF domain?

OSPF Router ID should not be changed after the OSPF process is started and the OSPF neighborships are established. If you change the OSPF Router ID, we need to either reload the IOS or use “#clear ip ospf process” command (restart the OSPF process) for changed RID to take effect.

Is it possible to configure OSPF without backbone area?

Yes, but then only intra-area communication is possible. Inter-area communication is not possible without backbone area.

What are key attributes of OSPF?

Below are the key attributes (important properties) of OSPF:

Protocol Type of OSPF: Link State
Algorithm: SPF (Dijkstra’s)
Type: IGP (Interior Gateway Protocol)
Metric of OSPF: Cost (Bandwidth)
Equal Cost Routes handling: CEF Load Balancing
No. of packet types: 5
Admin Distance: 110
Reference BW: 108
Standard: RFC2328 (OSPFv2), RFC2740 (OSPFv3/IPv6)
Multicast Address: 224.0.0.5, 224.0.0.6
Transport: IP (Port89) … refer to networkwalks website
V-Link Support: YES

What is the difference between OSPF neighborship & OSPF Adjacency?

Neighbor & Adjacency are two different terms in OSPF.

Two Routers in OSPF are called NEIGHBORS if they are exchanging Hello Packets.

Two Routers in OSPF are called an ADJACENCY if they are exchanging Hello Packets as well as Database

What are the steps required to change Neighborship into adjacency?

There are the steps required for a neighborship to change into adjacency:

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 as adjacent

What are the four OSPF Router types?

The OSPF Router types are:

Internal Routers (IR) = An OSPF Router whose all interfaces belong to the same area
Backbone Routers (BR) = An OSPF Router which is Internal Router in Area 0
Area Border Routers (ABR) = An OSPF Router which has interfaces in more than one area
Autonomous System Boundary Routers (ASBR) = An OSPF Router which advertises external routes into the OSPF Domain

What is LSA, LSU and LSR in OSPF?

The OSPF 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 LSAs in their respective topology database. Each Router then checks its topology database and sends LSR message requesting all LSAs that was not found in its topology table. Other Router responds with the LSU that contains all LSAs requested by the neighbor.

LSA = Link State Advertisement
LSU = Link State Update
LSR = Link State Request

What is a LSA OSPF? How does an LSA differ from an OSPF Update packet?

A Router originates link state advertisements (LSA) to describe one or more destinations. An OSPF Update packet transports LSAs from one neighbor to another. Although LSAs are flooded throughout an area or OSPF domain, Update packets never leave a data link.

Is it possible to have one numbered side and leave the other side unnumbered in OSPF?

No, OSPF does not work if you have one side numbered and the other side unnumbered. This creates a discrepancy in the OSPF database that prevents routes from being installed in the routing table.

Why do you receive the error message “cannot allocate router id” during configuration of OSPF?

OSPF picks up the highest IP address as a Router ID. If there are no interfaces in up mode with an IP address, it returns the error message “To correct the problem, configure a loopback interface”.

What are the advantages of using OSPF Summarization?

OSPF Summarization reduces the amount of information stored in routing tables
OSPF Summarization allocates an existing pool of addresses more economically
OSPF Summarization reduces the load on Router processor and memory resources
OSPF Summarization reduces number of update messages
OSPF Summarization reduces the bandwidth usage

Which issues do DR and BDR solve in OSPF?

DR and BDR solve below two problems in OSPF:

High No. of Adjacencies
Excessive LSA flooding

What is the election criteria to elect DR and BDR in OSPF?

Highest OSPF Interface Priority
Highest Router ID (if OSPF interface priorities are equal)

BDR

Router with the Second Highest OSPF Interface Priority

What is the range of OSPF priority values?

OSPF Priority Range is from 0 to 255

1 = Default Priority

0 = Means the Router will never become DR/BDR

What is OSPF Virtual Link?

OSPF requires the use of a backbone area (area 0) with each area connecting to area 0 through an Area Boundary Router (ABR). However, in some cases, regular area might not have a convenient point of connection to the backbone area. In this case, OSPF uses virtual link to connect that regular area to backbone area virtually. An OSPF virtual link allows two ABRs that connect to the same non-backbone area to form a neighbor relationship through that non-backbone area, even when separated by many other Routers and subnets. This virtual link acts like a virtual point-to-point connection between the two Routers, with that link inside area 0. The Routers form a neighbor relationship, inside area 0, and flood LSAs over that link.

Can we change the reference bandwidth in OSPF?

Yes, we can change the reference bandwidth using the OSPF auto-cost reference-bandwidth command.

What is the value of OSPF reference Bandwidth?

By default, reference bandwidth is 100 Mbps. The OSPF link-cost is a 16-bit number. Therefore, the maximum value supported is 65,535.

What is the Link-State Retransmit (LSR) interval?

OSPF must send acknowledgment of each newly received LSA. LSAs are retransmitted until they are acknowledged. The link-state retransmit-interval defines the time between retransmissions. We can use the command IP OSPF retransmit-interval to set the retransmit-interval. The default value is 5 seconds.

How do you 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. For example:

networkwalks_R1(config)# router ospf 1 
networkwalks_R1(config-router)# passive-interface fa0/0

Is it possible to have OSPF over a GRE tunnel?

Yes, it is possible to have OSPF run over a GRE tunnel.

What is an OSPF adjacency?

An OSPF adjacency is a conceptual link to a neighbor over which Link State Advertisements (LSAs) can be sent.

What are the five OSPF packet types?

These are the five types of OSPF packets:

HELLO
DBD (Database Descriptor)
LSR (Link State Request)
LSU (Link State Update)
LSAck (Link State Acknowledge)

What is a OSPF LSDB (Link State Database)?

The link state database is where a Router stores all the OSPF LSAs it knows of, including its own.

What is link state database synchronization?

Link State Database synchronization is the process of ensuring that all Routers within an area have identical link state databases.

What is an area in OSPF?

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

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.

What is Max-Age?

Max-Age 1 hour is the age at which a Link-State Advertisement (LSA) is considered to be obsolete.

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.

What are the steps that two OSPF routers need to go through to build OSPF neighborship?

OSPF Routers need to go through several state before establishing a neighbor relationship as in below:

What are different OSPF Link-State Advertisement (LSA) Types?

Below are the OSPF LSA Types:

What is the order of preference for OSPF Route?

OSPF routes are selected in below preference order:

Intra-Area OSPF routes (O)
Inter-Area OSPF routes (OIA)
External OSPF routes type-1 (OE1)
External OSPF routes type-2 (OE2)
NSSA1 (N1)
NSSA-2 (N2)

This rule of preference cannot be changed & it applies only within a single OSPF process.

*If a Router is running more than one OSPF process, route comparison occurs. With route comparison, the metrics and administrative distances (if they have been changed) of the OSPF processes are compared. Route types are disregarded when routes supplied by two different OSPF processes are compared.

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