IGRP - [Part 2] - INTERVIEW QUESTIONS

Question - 1 What is category of IGRP ?

IGRP is a cisco proprietary distance vector protocol. It is class-full and do not support VLSM/CIDR.

Question - 2 What is administrative distance of IGRP ?

Administrative distance of IGRP is 100.

Question - 3 What is limit of hop count in IGRP ?

Limit of hop count in IGRP is 255 and by default is 100.

Question - 4 How can we change hop count in IGRP ?

We can change the hop-count via command "metric maximum-hops 255"
Router(config)#router igrp 10
Router(config-router)#metric maximum-hops 255

Question - 5 Which protocol is more scalable among RIP and IGRP ? and why ?

We can use RIP for small organization and  IGRP for medium to larger size organization, so IGRP is more scalable than RIP. Because Limit of hop count in RIP is 15, means if anything require 16 hop, it will be deemed unreachable, so it is not suitable for large network. But Limit of hop count in IGRP is 255, so we can use it for large network.

Question - 6 Explain IGRP working process ?

In a IGRP network, each router broadcast its entire routing table to its all neighboring routers or on all active interfaces after every 90 second. After that, when a router receives a neighbor's routing table, it uses these provided information to update its own routing table and then sends own updated routing table to its all neighbors.

Question - 7 Explain IGRP and its metric. What is composite metric does it use? How it’s different than of RIP metric ?

IGRP is also a distance vector protocol and depends upon hop count. Its default limit is 100 and max is 255. But, Its metric is 24 bit composite which includes Bandwidth, Delay, Load, Reliability, MTU. By default it uses Bandwidth and Delay.  IGRP formula is (10000000/Bandwidth in kbps)+ (Sum of Delay in micro second)/10 . RIP uses only hop count.

Question - 8 Explain Bandwidth, Delay, Reliability and MTU  in IGRP.

• Bandwidth- Bandwidth is measured as the amount of data, which can be transferred from one end to another within a network in a particular amount of time. Basically, bandwidth is represented as a bit rate and measured in bits per second. Bandwidth is a static value, it can be changed by using the bandwidth command but it will not change the actual bandwidth of the interface, this command will only used to influence routing protocols.

• Load- It is a dynamic value because it can be change over time. Load will show you how busy the interface is based on the packet rate and the bandwidth on the interface.

• Delay- Delay is the amount of time, which is consumed by a packet to cross the link. It is a static value. As per Cisco IOS - A FastEthernet interface has a default delay of 100 usec. It can be changed by using the delay command but it will not change the actual delay for this interface, this command will only used to influence routing protocols.

• Reliability- Reliability at 255/255 means 100% reliable (you do not have any issues on the physical or data-link layer). This value will decrease If you have any issues. It is a dynamic value because it can be change over time.

• MTU (Maximum Transmission Unit)- MTU is the largest packet or frame size, specified in octets that can be sent in a packet or frame based network such as the internet. MTU is not used for the metric calculation. 

Note - Only bandwidth and delay will be used in the metric calculation.

Question - 9 How can we change bandwidth of an interface in IGRP ?

CORP(config)#interface F0/0
CORP(config-if)#Bandwidth 500
CORP#sh int F0/0
FastEthernet0/0 is up, line protocol is up
  Hardware is AmdFE, address is cc02.58a9.0000 (bia cc02.58a9.0000)
  Internet address is 192.168.12.1/24
  MTU 1500 bytes, BW 500 Kbit, DLY 100 usec,

Question - 10 How can we change delay for an interface in IGRP ?

CORP(config)#interface F0/0
CORP(config-if)#Delay 50
CORP#sh int F0/0
FastEthernet0/0 is up, line protocol is up
  Hardware is AmdFE, address is cc02.58a9.0000 (bia cc02.58a9.0000)
  Internet address is 192.168.12.1/24
  MTU 1500 bytes, BW 500 Kbit, DLY 500 usec,

Question - 11 Why IGRP uses only Bandwidth and delay in metric calculation ?

• IGRP uses only Bandwidth and Delay in metric calculation because loading and reliability are dynamic values and these can be change over time. 
• So if we will consider Loading and Reliability for metric calculation, then routers will calculate 24/7 and sending updates to each other just because the load or reliability of an interface has changed, so this situation will consume high CPU and increase the complexity. 
• If you will use only two static values (Bandwidth and Delay), then routers will not have to do any recalculation unless an interface goes down or a router died.

Question - 12 What are the differences in metric calculation of EIGRP and IGRP ?

• EIGRP metric = (IGRP metric) x 256. 
• The IGRP update packet have 24 bits for the metric field, but EIGRP update packet have 32 for the metric field.

Question - 13 Which algorithm is used by IGRP to determine the best path ?

IGRP uses Bellman Ford algorithm to determine the best path.

Question - 14 How can we configure IGRP ?

Router(config)#router IGRP 10
Router(config-router)network 172.19.0.0
Router(config-router)network 172.20.0.0

Note :- 10 is an Autonomous System number and only routers in the same autonomous system will send updates between each other for different Autonomous systems to communicate redistribution should be done.

Question - 15 Explain IGRP timers.

IGRP uses four different type of timers.

• Route update timer (90 Second) -  It is amount of time between periodic routing updates in which each router broadcast its entire routing table to its all neighboring routers or on all active interfaces.

• Route invalid timer (270 Second) - If router did not get any updates about a particular route for that period, then it will declare that route has become invalid.  When it will be happen, the router will broadcast the updates to all its neighbors to tell them that route is invalid.

• Hold down timer (280 Second) During this amount of time routing information will be suppressed.  If an update packet is received that indicates the route is unreachable then this particular route will enter into Hold Down State . It will be continues either until an update packet is received with a better metric, the original route comes back up, or the hold down timer expires.

• Route flush timer (630 Second) It is amount of time between a route becoming invalid and its removal from the routing table . 

Route Invalid Timer < route flush timer. 

Question - 16 Explain IGRP loop avoidance mechanism.

• Split Horizon -  As per this rule, routing information cannot be sent back in the direction from which it was received. So it reduces the incorrect routing information and routing overhead in a distance vector network

• Route Poisoning - Triggering an automatic update for the failed network without waiting for the update timer to expire. This update is sent out all interface with an identify metric for that network.

• Hold downs - A hold down prevents regular update messages from reinstating a route that is going up and down (called flapping).

Question - 17 Explain passive interface in IGRP.

You can set the single or all interface as passive, if you configured a interface as passive, means the routing updates will not be sent out this interface. "Passive interface [interface_name]" command will prevent the interface from sending out routing updates but will allow the interface to receive or learn updates.

Question -18 How can we configure passive interface in IGRP on particular interface ?

Router#config t
Router(config)#router igrp 10
Router(config-router)#network 192.168.30.0
Router(config-router)#passive-interface serial 0/0

Question - 19 How can we configure passive interface in RIP on all interface ?

We can configure all interfaces by using "passive-interface default" command and then individually ues the "no passive-interface" command on the interfaces we want updates to be sent out

Router#config t
Router(config)#router igrp 10
Router(config-router)#network 192.168.30.0
Router(config-router)#network 192.168.40.0
Router(config-router)#passive-interface default
Router(config-router)#no passive-interface F0/0

Question - 20 How can we configure passive interface in RIP when we used the neighbor command under the RIP process ?

If you used the neighbor command under the RIP process, the router will send uni-cast updates as well as multicast updates. The passive interface command must be used disable Multicast/broadcast updates and allowing only uni-cast.

Router#config t
Router(config)#router igrp 10
Router(config-router)#passive-interface S0/0/0
Router(config-router)#passive-interface S0/1/0
Router(config-router)#neighbor 192.168.30.1
Router(config-router)#neighbor 192.168.40.1

Question - 21 What are differences and similarities between RIP and IGRP ?

Routing Information Protocol (RIP)  :-

• It is a multi vendor Protocol.
• It is not a more scalable protocol because hop count limit in RIP is 16, everything is required 16 hop deemed unreachable.
• Basically it is used for smaller size organization.
• Hop count limit in RIP is 16.
• Administrative Distance of RIP is 120.
• In RIP we can not create a separate administrative boundary (AS) in the network.
• RIP uses the hop count to determine the best path.
• It works on Bellman Ford algorithm.
• RIPv1 do not support VLSM.
• RIPv2 support VLSM.

Interior Gateway Routing Protocol (IGRP) :- 

• It is a Cisco -proprietary protocol.
• It is more scalable protocol than RIP, because hop count limit is 255 in IGRP.
• Basically it is used for medium to larger size organization in the network.
• Hop count limit in IGRP is 255. 
• Administrative Distance of IGRP is 100.
• In IGRP we can create a separate administrative boundary in the network with the help of AS number.
• IGRP uses the metric (Bandwidth, delay, Reliability, MTU and Load) to determine the best path.
• It works on Bellman Ford algorithm.
• IGRP do not support VLSM.

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Anubhav Upadhyay

Hello and welcome to networktopic Blog. My name is Anubhav. I am a Senior Network Egineer. I have created this blog specially to serve interview questions and answer on Network Routing and Switching, I will try my best to serve correct and updated networking knowledge for you as per my corporate experience.

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