EIGRP Theory and Commands

Question 1

What is EIGRP?

Answer 1

Enhanced Interior Gateway Routing Protocol - a Cisco proprietary routing protocol, known as a hybrid protocol because EIGRP’s routing metrics use attributes used by both distance vector and link state routing protocols.

Question 2

What is the biggest difference between how OSPF and EIGRP advertise routes?

Answer 2

• OSPF knows all the routes from all routers in an Area, however…
• EIGRP only knows the routes from its directly connected neighbors

Question 3

What is the command to enable EIGRP on a Cisco router?

Answer 3

R1(config)# router eigrp {AS-number}

Example: router eigrp 1

Question 4

What is the command to advertise a network prefix into EIGRP?

Answer 4

R1(config-router)# network {prefix} {wildcard-mask}

Example: network 10.1.1.0 0.0.0.255

Question 5

What is the Successor in EIGRP?

Answer 5

The route from EIGRP that was the best route and installed to the routing table (RIB)

Question 6

What is the Feasible Distance in EIGRP?

Answer 6

The value of the metric of an EIGRP route in the routing table (the Successor)

Question 7

What symbol in the routing table (RIB) denotes an EIGRP learned route?

Answer 7

The letter “D”

Question 8

What is the Administrative Distance of EIGRP?

Answer 8

90

Question 9

What are the five different packet types used by EIGRP?

Answer 9

1.) Hello
2.) Request
3.) Update
4.) Query
5.) Reply

Question 10

What are the default hello and hold down timers on a standard Ethernet connection between two EIGRP routers?

Answer 10

Hello is 5 seconds, Hold-down is 15 seconds

Question 11

What are the default hello and hold down timers on a slow connection (like a T1) between two EIGRP routers?

Answer 11

Hello is 60 seconds, Hold-down is 180 seconds

Question 12

What are the two elements that influence the calculation of the EIGRP metric?

Answer 12

1.) Worst bandwidth through the path
2.) Sum of delays

Question 13

What is the newer method for calculating EIGRP called and what makes it different than the Classic Metrics?

Answer 13

Wide Metrics - it addresses scalability issues that come with higher speed interfaces by adding an additional K-value to the calculation.

Question 14

What is the command that will display the EIGRP database information on a Cisco router?

Answer 14

show ip eigrp topology

Question 15

What is the Reported Distance in EIGRP?

Answer 15

The metric that was advertised from a neighbor regarding the cost of a specific route. In other words, the advertised metric by the perspective of the neighbor device.

Question 16

What is a Feasible Successor?

Answer 16

If more than one route exists in the EIGRP topology database, the less desirable route could be installed to the routing table if the Successor Route were to go down.

Question 17

What is the Feasibility Condition for a Feasible Successor to exist?

Answer 17

The reported distance for the Feasible Successor must be less than the Feasible Distance of the Successor Route.

NOTE: this is to prevent loops. Keep in mind the Feasible Distance will be the advertised distance plus link delay.

Question 18

What is the command to enable unequal cost load balancing when a Feasible Successor exists?

Answer 18

R1(config-router)# variance {multiplier}

Question 19

How is the Variance Multiplier calculated against a Feasible Successor?

Answer 19

If the Feasible Successor’s Feasible Distance is divided by the Successor Route’s Feasible Distance and it is less than or equal to the Variance Multiplier, then the Feasible Successor will be installed into the RIB as an unequal cost load balanced path.

Example where the FS metric is 5000, the Successor metric is 3000, and the multiplier is 2.
5000 / 3000 = 1.66; Less than the variable multiplier of 2 so it will be installed in the RIB.

Question 20

What is the command to specify a passive interface in EIGRP?

Answer 20

R1(config-router)# passive-interface {interface-id}

Question 21

What are the commands to create a summary route for EIGRP?

Answer 21

R1(config)# interface {int-id}
R1(config-if)# ip summary-address eigrp {AS-number} {prefix}/{bits}

Example:

R1(config)# int Fa1/1
R1(config-if)# ip summary-address eigrp 1 10.0.0.0/8