Routing Interview Questions

Regularly placed Routing Protocols Questions: 

Hello friends, welcome back to my Networking Interview Questions Blog series. In the last blog from CCNA Classes in Pune, I covered Router and Routing Protocol RIP interview questions and Answers which was Part-I.  Now, in this blog, I am going to cover the next Part-II of the topic – Router and its Routing Protocols. In this part, I am going to cover Routing protocols EIGRP and OSPF. 

So let’s start with EIGRP Routing Protocol questions and answers from CCNA Training in Pune.

• In EIGRP, what are the requirements for forming neighborship?

to become neighbors in EIGRP, the following fields in a hello packet must match for routers: –

1. Autonomous System (AS) number.
2. Authentication.
3. K-values.

• Which tables do EIGRP routers create?

EIGRP router creates the following 3 tables to stores routing and topology information: –

1. Routing table – Stores the best paths to all the destination networks.
2. Neighbor table – Stores information about all EIGRP neighbor routers.
3. Topology table – Stores routing information which is collected from neighbor routers.

• What is the importance of the “#no auto-summary” command in EIGRP?

By default, EIGRP has the automatic route summarization feature enabled. Due to this, EIGRP aggregates the routes to Classful address at network boundaries during the routing updates.

The automatic route summarization feature will cause issues with discontiguous sub-networks. Hence, this feature is usually turned off. To turned off this feature, the “#no auto-summary” command is used.

• What are the EIGRP Hello and Hold timer?

Hello Timer – Routers will send a hello packet to its neighbor for every 5 seconds (Hello time).

Hold Timer – If a Router does not receive hello for 15 seconds i.e. continuous 3 hello packets are not received (Hold time), then it will assume that link to that particular router is down and it will drop the router neighborship.

• What is the Successor and Feasible successor?

A Successor is the best path to reach a destination network in the topology table.

The Feasible successor is the 2nd best path to reach a destination network after successor. It stands as a backup for the successor.

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• What is the Advertised Distance and Feasible distance?

Advertised distance (AD) is the metric distance (cost) of a neighbor router to reach the destination network.

Feasible Distance (FD) is the metric distance (cost) to reach the destination network. It is always AD + metric distance to the neighbor router.

• What Authentication method does EIGRP support?

EIGRP supports the Only MD5 authentication method.

• What is the EIGRP equation to calculate path cost?

• Which multicast address does EIGRP use?

A multicast address of 224.0.0.10 is used by EIGRP routers to communicate with neighbors.

• What are the algorithms used by routing protocols?

Following are the algorithms used by the dynamic routing protocols: –

1. RIP: Bellmen Ford
2. EIGRP: DUAL ( Diffusing Update Algorithm)
3. OSPF: Dijkstra

• What is the concept of areas in OSPF?

In OSPF – Open Shortest Path First, the autonomous system is divided into various areas to reduce the number of routing updates in the network as OSPF routing updates will be sent to only in one area.

The following are advantages of dividing the entire network i.e. Autonomous system into areas:-

1. Speed up the convergence of routing.
2. Decrease routing overhead.
3. Restrict network instability to single areas of the entire network.

• What is the Backbone Area in an AS?

While configuring OSPF, one area must be configured as area 0, referred to as the backbone area. This is the root of all other areas and they all must connect to the backbone area. Inter-area traffic is transmitted through this backbone area, area 0.

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• What is an Area Border Router(ABR) and Autonomous System Border Router (ASBR)?

ABR – Area Border Router is the router that connects other areas to the backbone area within an autonomous system (AS). Area Border Router can have its interfaces in multiple areas.

ASBR – Autonomous System Border Router is the router that connects different Autonomous Systems.

• In OSPF, what factors must match for two routers to become neighbors?

The following factors must be the same on both routers in order to form adjacency in OSPF: –

1. The subnet should be the same.
2. Area id must be the same for at least one common interface.
3. Hello, and Dead interval timer should be matched.
4. Authentication methods must be the same for both the routers.

• What is OSPF Router ID?

Router Id is configured to identify the router. We can configure it using command. If it is not configured, then the highest IP address of the router’s loopback interfaces is chosen as the Router ID. And if no loopback is also configured, then the highest IP address of the router’s physical interfaces will be chosen as the Router ID.

• What are DR and BDR in OSPF?

DR as Designated Router and BDR as Backup Designated Router. All other OSPF routers will form adjacencies only with the DR and BDR. DR and BDR serve as the central point for updating OSPF routing information. When link-state modifies, that will have sent only to the DR and BDR, instead of sending updates to every router on the network segment. Then DR will distribute updates to every other router inside the same area. BDR will work as a backup for DR. This will greatly reduce OSPF traffic and conserves the bandwidth.

• How OSPF DR & BDR are elected?
• The router with the Highest Priority elected as the Designated Router (DR) and the second-highest priority router becomes the Backup Designated Router (BDR).
• If there is a tie in priority, the router with the highest Router ID will become DR and the second-highest Router ID will become BDR.
• Cisco routers have by default priority as 1. We can change it using the following command:

router(config)# interface  fa0/1

router(config-if)# ip ospf priority 10

• If the Router priority is set to zero (0), that router will not participate in the DR/BDR election.
• DR election process is not preemptive means if a router with a higher priority is added to the network, it will not become DR. We need to manually clear the OSPF process and DR/BDR election starts again from scratch.

Command to change the priority on an interface

router(config)# interface fa0/0

router(config-if)# ip ospf priority 100

• Describe OSPF LSA, LSU, LSR and LSAck?

The Link-State Advertisements (LSAs) are used by OSPF routers to exchange routing and topology information. When two neighbors decide to interchange routes, they send each other a list of all LSA in their individual topology database.

After getting LSAs, each router then checks its topology database. Then if some LSAs are missing or some are out of date i.e. not updated, the router sends Link State Request (LSR) message requesting all updated or missing LSAs that were not found in the topology table.

For this LSR, other router replies with the Link State Update (LSU) that contains all LSAs requested by the neighbor router. After getting LSUs, Link-State Acknowledgment (LSAck) will be sent to confirm receipt of LSU messages.

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• What are OSPF timers and their values?

Hello Timer – This specifies how frequently OSPF router will send the hello packet to other OSPF routers. By default, it is 10 secs for OSPF.

Dead Timer – This specifies how much time a router will wait for hello packets before it announces that the neighbor router is dead. By default, it is 40 secs for OSPF.

• What multicast addresses does OSPF use?

OSPF uses the multicast addresses of 224.0.0.5 & 224.0.0.6.

Every non-DR or non-BDR router will be sent routing information to a DR and BDR using the multicast address of 224.0.0.6.

Then DR will send these routing updates to all other router using multicast address 224.0.0.5. If DR fails, BDR will take over its role and redistribute the updates.

• Which Tables are maintained by OSPF?

OSPF routers maintain routing and topology information using three OSPF tables as mentioned below: –

1. Routing table – Stores the best paths to all the destination networks.
2. Neighbor table – Stores information about all EIGRP neighbor routers.
3. Topology table – Stores routing information which is collected from neighbor routers.

• What are the three basic LSA types of OSPF?

1. Router LSA (Type1) – Type 1 or Router LSA is generated by Each router that contains its active interfaces, IP addresses, neighbors and the link-cost. LSA Type 1 is flooded to all the routers only within an area.

2. Network LSA (Type2) – Type 2 or Network LSA is generated by the Designated Router (DR) that lists all the routers on the segment it is adjoining to. LSA Type 2 is flooded only within an area.

3. Summary LSA (Type3) – Type 3 or Summary LSAs are produced by Area Border Routers (ABRs) to advertise networks from an area to the other areas in an Autonomous System. It contains information about inter-area routes.

• Compare OSPF with EIGRP?

1. EIGRP – Enhanced Interior Gateway Routing Protocol

Enhanced Interior Gateway Routing Protocol (EIGRP) is highly prized for its fast convergence and ease of deployment. It is commonly used in many large Enterprise networks. EIGRP is a protocol that is very simple to understand and deploy. It’s IPv6 ready support, scales efficiently in a well-designed network, and provides exceptionally quick convergence times.

When network topology changes occur, EIGRP does not exchange the whole routing table, instead only the updates are exchanged. Hence, it also has very fast convergence times when there are changes in the network topology.

EIGRP has one more important feature –  It can provide equal-cost multipath (ECMP) and unequal cost multipath load sharing.   That’s why it does more efficient use of links,

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2. OSPF – Open Shortest Path First

While configuring Routing in a multi-protocol enterprise network with hundreds of possible routes, we need a robust, efficient, and scalable link-state routing protocol Open Shortest Path First protocol (OSPF) to keep an accurate, up-to-date routing table.

OSPF is an open standard routing protocol and can run on routers from a variety of vendors, including Cisco, Nortel, and Lucent. As a result, OSPF protocol allows admins running large enterprise networks with products from multiple vendors to use a single routing protocol.

Here CCNA Training in Pune is concluding both the parts of questions on Routing Protocols. We will meet soon with interview questions on NAT, ACL, VPN, and IPv6. Till then, All the Best !!

Author:- Sumaiyya Suhail Bagwan
Department Name:- Networking
Designation:- Technical Trainer

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