EIGRP Interview

Question 1

What is EIGRP?

Answer 1

Enhanced Interior Gateway Routing Protocol (EIGRP) is an enhanced distance vector routing protocol which uses Diffused Update Algorithm (DUAL) to calculate the shortest path.

EIGRP supports classless routing and VLSM, route summarization, incremental updates, load balancing and other features.

Question 2

What are the requirements for neighborship in EIGRP?

Answer 2

The following fields in a hello packet must match for routers to become neighbors:-
1. Autonomous System number.
2. K-values.
3. Authentication.
4. The primary address should be used.
5. If static neighborship then should be defined on both sides.

Question 3

What is the metric of the EIGRP protocol & its default values?

Answer 3

1. Bandwidth (K1=1)
2. Load (K2=0)
3. Delay (K3=1)
4. Reliability (K4=0)
5. Maximum Transmission Unit (K5=0)

By default, EIGRP only uses Bandwidth (K1) and Delay (K3) to calculate metrics.

Question 4

What are the four basic components of EIGRP?

Answer 4

1. It supports IPv4, and IPv6.
2. The Reliable Transport Protocol - RTP is used in EIGRP for detecting packet loss.
3. The Neighbor Discovery and Recovery Module - Hello messages are used for Neighbor Discovery and Recovery.
4. The Diffusing Update Algorithm - It is an algorithm used by EIGRP for selecting the lowest cost loop-free path for each possible destination.

Question 5

What are the different packet types used by EIGRP?

Answer 5

1. Hello - Neighborship is discovered and maintained by Hello Packets.
2. Acknowledgement - ACK packets are used to acknowledge the receipt of an update, query and reply packets. Acknowledgement packets are Unicast.
3. Update - EIGRP uses Update messages to send routing information to neighbors. Update packets can be sent to a single neighbor using unicast or to a group of neighbors using multicast.
4. Query - Query packets are used when EIGRP router has lost path (Successor) to a certain network and does not have any backup paths (Feasible Successor). The router sends query packets to its neighbors asking them if they have information about this particular network. Query packets are multicast.
5. Reply - Reply packets are used in response to the query packets. Reply packets are unicast to the originator of the query.

Question 6

Explain EIGRP Router ID?

Answer 6

In EIGRP, duplicate RIDs do not prevent routers from becoming neighbors and two EIGRP routers with the same router ID will still form a neighbor relationship.

The only time the value of EIGRP RIDs consider is when injecting external (redistributed) routes into EIGRP. In this case, the routers injecting the external routes should have unique RIDs to avoid confusion.

To manually configure the router ID
R1(config)# router eigrp 10
R1(config-router)# eigrp router-id 1.1.1.1

Question 7

Explain Unequal Cost Load Balancing in EIGRP?

Answer 7

Unequal cost load balancing in EIGRP is the concept by which load sharing can take place on paths that do not have an equal metric.

In EIGRP variance is used for Unequal cost load balancing.

By default, EIGRP will automatically load balance across equal-metric routes.

Question 8

What is an Active State?

Answer 8

Active state within EIGRP is when a router sends a query message to its neighbor in order to find a feasible successor for the destination, but is has not received a reply from its neighbor.

Question 9

What are Active State and Passive State?

Answer 9

Active State: Routes for which the successor route fails and no feasible successor routes exist to move to an active state forcing the EIGRP to send out query packets and reconverge.

A route is in a passive state for which the router has a successor route, and no failure has yet occurred. A stable EIGRP network will have all routes in a Passive state.

Question 10

How Passive Interface command works in EIGRP?

Answer 10

With EIGRP running on a network, the passive-interface command stops sending outgoing hello packets, hence the router cannot form any neighbor relationship via the passive interface. This behavior stops both outgoing and incoming routing updates.

Question 11

What is the Feasibility Condition in EIGRP?

Answer 11

For any route to be a feasible successor it has to fulfil feasibility condition which is as follows:- The advertised distance of Feasible successor should be less than Feasible distance of Successor
AD of feasible successor < FD of successor.

Question 12

What is the Multicast IP address used by EIGRP?

Answer 12

EIGRP uses the multicast address 224.0.0.10

Question 13

What are internal AD?

Answer 13

90

Question 14

What are external AD?

Answer 14

170

Question 15

What are EIGRP K values?

Answer 15

K1 (Bandwidth) - This value represents the bandwidth of the path in kilobits per second (kbps).

K2 (Delay) - This value represents the delay of the path in tens of microseconds (10^-6 seconds).

K3 (Reliability) - This value represents the reliability of the path as a fraction between 0 and 1.

K4 (Load) - This value represents the load on the path as a fraction between 0 and 1.

K5 (MTU) - This value represents the maximum transmission unit (MTU) of the path in bytes.

Question 16

What is Reported Distance?

Answer 16

Reported Distance is The metric for a route advertised by a neighbor.

Question 17

What is Feasible Distance?

Answer 17

Feasible Distance is The distance advertised by a neighbor plus the cost to get to that neighbor.

Question 18

What is Stuck In Active (SIA)?

Answer 18

Stuck In Active (SIA) is The condition when a route becomes unreachable and not all queries for it are answered; adjacencies with unresponsive neighbors are reset.

Question 19

What is Passive Interface?

Answer 19

Passive Interface is An interface which does not participate in EIGRP but whose network is advertised.

Question 20

What is Passive Interface?

Answer 20

Passive Interface is An interface which does not participate in EIGRP but whose network is advertised.

Question 21

What is Stub Router?

Answer 21

In EIGRP, a stub router can be configured to advertise only its directly connected networks to its neighboring routers, without advertising any routes learned from other routers.

Stub routers can also be used to reduce the amount of EIGRP traffic in the network and improve network performance.

Question 22

What is Passive state?

Answer 22

In EIGRP, the Passive state refers to a phase in the EIGRP neighbor relationship process where a router has received a reply to its query message from its neighbor and has determined a feasible successor for a destination network.