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EIGRP Interview Questions and Answers

January 18, 2016

Explain EIGRP Routing Protocol?
Enhanced Interior Gateway Routing Protocol (EIGRP Protocol) is an enhanced distance vector routing protocol which Uses Diffused Update Algorithm (DUAL) to calculate the shortest path. It is also considered as a Hybrid Routing Protocol because it has characteristics of both Distance Vector and Link State Routing Protocols.
EIGRP supports classless routing and VLSM, route summarization, incremental updates, load balacing and other features.

What are the requirements for neighborship in EIGRP?
The following fields in a hello packet must match for routers to become neighbors -
1.Autonomous System number.
2.K-values.
3.Authentication.
4.Primary address should be used.
5.If static neighborship than should be define on both sides.

What tables does EIGRP routers maintain?
EIGRP router stores routing and topology information in three tables:
1. Neighbor table - Stores information about EIGRP neighbors.
2. Topology table - Stores routing information which is learned from neighbor routers.
3. Routing table  - Stores the best paths to all networks.

Why no auto-summary command is used in EIGRP?
By default, EIGRP behaves like a classfull routing protocol which means it does not advertises the subnet mask information along with the routing information. No auto-summary command will ensure that EIGRP sends the subnet mask information along with the routing information.

What metric does EIGRP use?
EIGRP calculates its metric by using Bandwidth, Load, Delay, Reliability and MTU.

What are the EIGRP Hello and Hold timer?
Hello Time - Router will send a hello to its neighbor every 5 seconds (Hello time).
Hold Time - If a Router does not receive hello for 15 seconds (Hold time) than it will assume that link is down and it will drop the neighborship.

What are the default values EIGRP Hello and Hold timer?
Hello Time - 5 seconds
Hold Time - 15 seconds

What is Successor?
Successor is the best path to reach to a destination in the topology table.

What is Feasible successor?
Feasible successor is the second best path to reach a destination after Successor. It acts as backup for the Successor.

What is Feasible distance?
Feasible distance is the distance (metric) to reach destination network.

What is Advertised Distance/Reported Distance?
Advertised distance is the distance (Metric) of a neighbor router to destination network. This is the metric of a destination network as reported by a neighbor.

What Authentication does EIGRP supports?
EIGRP supports Only MD5.

Give the Formula EIGRP uses to calculate Metric?
((10^7/least bandwidth of link) + cumulative delay)*256

What is the Different Administrative Distance that EIGRP use?
1.Internal - 90
2.External - 170
3.Summary - 5

What multicast address does EIGRP use?
EIGRP routers use the multicast address of 224.0.0.10

How we configure EIGRP?
Router(config)# router eigrp 100
Router(config-router)# network 172.16.1.0 0.0.0.255
Router(config-router)# network 10.16.1.0 0.0.0.255
Router(config-router)# no auto-summary

Give some commands to troubleshoot EIGRP?
#show ip route - It shows full Routing Table.
#show ip route eigrp - It shows only EIGRP routes (routes learned through EIGRP protocol) in the routing table.
#show ip eigrp neighbors - It shows  EIGRP Neighbor Table.
#show ip eigrp topology - It shows EIGRP Topology Table.

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