9 - EIGRP Concepts

Question 1

What is the Feasibility Condition?

Answer 1

• If a non-successor route’s Reported Distance is less than the Feasible Distance then the route is a Feasible Successor Route
• Basically “is my neighbor closer to that subnet than I am?”

Question 2

What is the Feasible Distance?

Answer 2

The local routers Composite Metric of its best route to reach a subnet - in other words, the LOCAL ROUTER’s Metric

Question 3

What is the Reported Distance?

Answer 3

The next-hop router’s metric for the same route.

Question 4

Successor

Answer 4

• The local router’s route with the best metric, or best Feasible Distance.
• ‘show ip route’ shows the successor routes.

Question 5

What is a Feasible Successor?

Answer 5

A route who’s Reported Distance is less than the local router’s Feasible Distance, if the best route fails EIGRP switches to the Feasible Successor.

Question 6

What does DUAL stand for?

Answer 6

Diffusing Update Algorithm.

Question 7

If a successor route fails what does EIGRP do?

Answer 7

Switches over to the Feasible Successor route, if no Feasible Successor route then DUAL takes over to find a new route.

Question 8

What does DUAL do?

Answer 8

1.) The local router sends EIGRP QUERY messages to its remaining neighbors looking for a route 2.) the neighbor replys with an EIGRP REPLY message confirming a viable route 3.) The route is then entered in the local routers table.

Question 9

What 3 steps does EIGRP do when first joining the network?

Answer 9

1.) Neighbor Discovery 2.) Topology Exchange 3.) Chooses Routes

Question 10

What type of messages are used to discover neighbors?

Answer 10

Hello Messages

Question 11

What type of messages are used to exchange topology?

Answer 11

EIGRP Update Messages

Question 12

What address are EIGRP Hello messages sent to ?

Answer 12

224.0.0.10

Question 13

What address are EIGRP Update Messages sent to ?

Answer 13

The Unicast address of the neighboring router. If multiple neighbors are on the subnet then to 224.0.0.10

Question 14

What protocol does EIGRP use?

Answer 14

Reliable Transport Protocol (RTP)

Question 15

What advantage does RTP have?

Answer 15

provides mechanism to resend any messages that were not received

Question 16

What is the default Hello interval?

Answer 16

5 seconds

Question 17

What is the Hold Interval

Answer 17

The amount of time a router will wait to hear a Hello from its neighbor before declaring it down.

Question 18

What is the default Hold Interval

Answer 18

3 times the Hello time or 15 seconds.

Question 19

How does EIGRP notify its neighbors of a topology change?

Answer 19

EIGRP Partial Update

Question 20

What are the checks that EIGRP performs to determine if it can be a neighbor?

Answer 20

• passes authentication
• Same Autonomous System number
• Source IP address used by the neighbor to send Hello is on same subnet
• EIGRP K-values must match

Question 21

What is the formula EIGRP uses to calculate the Composite Metric?

Answer 21

Metric = ((10⁷ / least-bandwidth in path in Kbps) + cumulative delay) * 256

Question 22

For RIP, what is considered an infinite route?

Answer 22

16

Question 23

In EIGRP, what is considered an infinite route?

Answer 23

2³² - 1

Question 24

What does OSPF consider to be an infinite route?

Answer 24

2²⁴ - 1

Question 25

In which 2 situations does an EIGRP router send Multicast Updates?

Answer 25

• When the router needs to update multiple routers on the same subnet
• When the router is sending Hello messages

Question 26

What transport protocol does EIGRP use?

Answer 26

• Reliable Transport Protocol (RTP)
• which is also IP port 88

Question 27

What is the multicast address used by EIGRP?

Answer 27

224.0.0.10

Question 28

What is the variance command for?

Answer 28

To allow for load-balancing across multiple paths by making routes who’s feasible distances aren’t exactly equal to the successor’s appear to be equal.

Question 29

How does the ‘variance’ command work?

Answer 29

1. Multiply the successor’s metric by the variance.
2. If a feasible successor’s RD is less than the successor’s FD times the variance then treat the feasible successor’s route as though it were an equal path.

Question 30

Can a route that is not a FS be added to the routing table by using the ‘variance’ command?

Answer 30

No.