IGRP - [Part 1] - QUICK REVIEW

Protocol Type -  Distance Vector Protocol

Cisco Proprietary -  Yes

IGP/EGP - IGP (Interior Gateway Protocol)

Administrative Distance -100

Routed Protocol Support -  IPv4

Transport Protocol - IP protocol 9

Updates - Full route table update

VLSM/CIDR Support - Does not support

Classful/Classless Protocol  - Classful Protocol

Dis-contiguous Network Support - Support

Hop Count Limit - 255 (by default is 100)

Algorithm - Bellman Ford

Broadcast/Multicast Based -  Multicast Based

Multicast Address - 224.0.0.10

Authentication - There is no authentication

Hierarchical Network Requirement -  In IGRP we can create a separate administrative boundary in the network with the help of Autonomous System number

Metrics - IGRP uses 24 bit composite matrices which includes -

• Bandwidth
• Delay
• Load
• Reliability
• MTU. 

By default it uses Bandwidth and Delay to calculate its distance metric. Reliability, Load, and MTU are optional attributes, these can be used to calculate the distance metric. 
IGRP formula is (10000000/Bandwidth in kbps)+ (Sum of Delay in micro second)/10.

Manual Summarization Support - Does not Support

Auto Summarization Support - Support

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.

Loop Avoidance Mechanism -  

• Split Horizon
• Route Poisoning
• Hold-downs

Best Path Selection - The below formula is used to calculate the composite metric of IGRP for the best path selection.
Metric = [K1 * Bandwidth + (K2 * Bandwidth)/ (256-Load) + K3*Delay] * [K5/(Reliability + K4)] 

The default values are 

K1 = 1

K3 = 1

K2 = 0

K4 = 0

K5 = 0

If K5 = 0, then [K5/ (reliability + K4)] = 0

So default Metric = Bandwidth + Delay

Where Bandwidth = 10000000/Bandwidth in kbps

and Delay = Sum of Delay in micro second/10.

so finally IGRP formula is (10000000/Bandwidth in kbps)+ (Sum of Delay in micro second)/10

Load Balancing -  IGRP perform equal-cost load balancing by default. And unequal-cost load-balancing with the variance command. Load Balancing Across 6 Equal or Unequal Cost Paths

Passive Interface -  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.

Convergence - Slow convergence due to periodic update.

Timer -

• Route update timer (90 Second)
• Route invalid timer (270 Second)
• Hold down timer (280 Second)
• Route flush timer (630 Second)

Strength - 

• Easy to use and configure.
• Uses the matrices delay, bandwidth, reliability, and load. So these are very helpful to select the best path

Weakness - 

• Converges is slower than RIP
• IGRP does not support a VLSM, means it can send routing updates based only on a fixed-length subnet mask (FLSM) or routes that belongs to classful boundaries. 
• Prone to routing loops

How to enable

Router#config t 
Router(config)#router IGRP 10 ----------- 10 is AS number

Router(config-router)network 172.18.0.0

SHARE

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.

• 
• 
• 
• 
•