Ch 7: EIGRP

Question 1

EIGRP uses the protocol number _______ to identify its packets.

a. 87
b. 88
c. 89
d. 90

Answer 1

B. EIGRP uses protocol number 88.

Question 2

EIGRP uses _______ packet types for inter-router communication.

a. three
b. four
c. five
d. six
e. seven

Answer 2

C. EIGRP uses the hello, request, reply, update, and query packet types

Question 3

What is an EIGRP successor?

1. The next-hop router for the path with the lowest path metric for a destination prefix
2. The path with the lowest metric for a destination prefix
3. The router selected to maintain the EIGRP adjacencies for a broadcast network
4. A route that satisfies the feasibility condition where the reported distance is less than the feasible distance

Answer 3

1. An EIGRP successor is the next-hop router for the successor route (which is a loop-free route with the lowest path metric).

Question 4

Which of the following attributes does the EIGRP topology table contain? (Choose all that apply.)

1. destination network prefix
2. hop count
3. total path delay
4. maximum path bandwidth
5. list of EIGRP neighbors

Answer 4

All except #4 are contained.

The EIGRP topology table contains the destination network prefix, path attributes (hop count, minimum path bandwidth, and total path delay), and a list of nearby EIGRP neighbors.

Question 5

Which of the following destination addresses does EIGRP use when feasible? (Choose two.)

1. IP address 224.0.0.9
2. IP address 224.0.0.10
3. IP address 224.0.0.8
4. MAC address 01:00:5E:00:00:0A
5. MAC address 0C:15:C0:00:00:01

Answer 5

2 and 4. EIGRP uses the multicast IP address 224.0.0.10 or MAC address 01:005E:00:00:0A when feasible.

Question 6

Which value can be modified on a router to manipulate the path taken by EIGRP but avoid having impacts on other routing protocols, such as OSPF?

1. interface bandwidth
2. interface MTU
3. interface delay
4. interface priority

Answer 6

3. The interface delay can be modified to change the EIGRP path calculations without modifying the path calculation of OSPF.

Question 7

EIGRP uses a reference bandwidth of ________ with the default metrics.

1. 100 Mbps
2. 1 Gbps
3. 10 Gbps
4. 40 Gbps

Answer 7

3. EIGRP uses a reference bandwidth of 10 Gbps with the default metrics.

Question 8

The default EIGRP hello timer for a high-speed interfaces is ______.

1. 1 second
2. 5 seconds
3. 10 seconds
4. 20 seconds
5. 30 seconds
6. 60 seconds

Answer 8

2. EIGRP uses a default hello timer of 5 seconds for high-speed interfaces.

Question 9

When a path has been identified using EIGRP and in a stable fashion, the route is considered ______.

a. passive
b. dead
c. active
d. alive

Answer 9

A. EIGRP considers stable paths to be passive.

Question 10

How does an EIGRP router indicate that a path computation is required for a specific route?

1. EIGRP sends out an EIGRP update packet with the topology change notification flag set.
2. EIGRP sends out an EIGRP update packet with a metric value of zero.
3. EIGRP sends out an EIGRP query with the delay set to infinity.
4. EIGRP sends a route withdrawal, notifying other neighbors to remove the route from the topology table.

Answer 10

3. EIGRP sends out a query packet with the delay set to infinity to indicate that a route has gone active.

Question 11

True or false: EIGRP summarization occurs for network prefixes as it crosses all network interfaces.

a. True
b. False

Answer 11

B. False. Summarization of prefixes occurs as traffic is advertised out an interface with summarization configured.

Question 12

T/F: A router can run multiple EIGRP processes.

Answer 12

True.

A router can run multiple EIGRP processes. Each process operates under the context of an autonomous system, which represents a common routing domain.

Question 13

T/F: Routers within the same domain use the same metric calculation formula and exchange routes only with members of the same autonomous system.

Answer 13

True.

Note: An EIGRP autonomous system should not be confused with a Border Gateway Protocol (BGP) autonomous system.

Question 14

What is a Successor route?

Answer 14

The route with the lowest path metric to reach a destination.

The successor route for R1 to reach 10.4.4.0/24 on R4 is R1→R3→R4.

Question 15

What is a “Successor”?

Answer 15

The first next-hop router for the successor route.

The successor for 10.4.4.0/24 is R3.

Question 16

What is FD (Feasible Distance)?

Answer 16

The metric value for the lowest-metric path to reach a destination. The feasible distance is calculated locally using the formula shown in the “Path Metric Calculation” section, later in this chapter.

The FD calculated by R1 for the 10.4.4.0/24 network is 3328 (that is, 256+256+2816).

Question 17

What is Reported Distance?

Answer 17

The distance reported by a router to reach a prefix. The reported distance value is the feasible distance for the advertising router.

R3 advertises the 10.4.4.0/24 prefix with an RD of 3072.
R4 advertises the 10.4.4.0/24 to R1 and R2 with an RD of 2816.

Question 18

What is a feasibility condition?

Answer 18

A condition under which, for a route to be considered a backup route, the reported distance received for that route must be less than the feasible distance calculated locally. This logic guarantees a loop-free path.

Question 19

What is a Feasible Successor?

Answer 19

A route that satisfies the feasibility condition and is maintained as a backup route. The feasibility condition ensures that the backup route is loop free. (i.e. the RD < FD for the route)

The route R1→R4 is the feasible successor because the RD 2816 is lower than the FD 3328 for the R1→R3→R4 path.

Question 20

T/F:

EIGRP’s topology table is a vital component to DUAL and contains information to identify loop-free backup routes. The topology table contains all the network prefixes advertised within an EIGRP autonomous system. Each entry in the table contains the following:

• Network prefix
• EIGRP neighbors that have advertised that prefix
• Metrics from each neighbor (for example, reported distance, hop count)
• Values used for calculating the metric (including load, reliability, total delay, maximum bandwidth)

Answer 20

False.

Almost…

EIGRP’s topology table is a vital component to DUAL and contains information to identify loop-free backup routes. The topology table contains all the network prefixes advertised within an EIGRP autonomous system. Each entry in the table contains the following:

• Network prefix
• EIGRP neighbors that have advertised that prefix
• Metrics from each neighbor (for example, reported distance, hop count)
• Values used for calculating the metric (for example, load, reliability, total delay, minimum bandwidth)

The minimum bandwidth is important for eliminating slow links.

Question 21

With the command sh ip eigrp topology, what are the meanings of P and A?

Answer 21

The 10.4.4.0/24 route is passive (P), which means the topology is stable. During a topology change, routes go into an active (A) state when computing a new path.

Question 22

T/F: EIGRP neighbors never exchange full routing tables, only incremental updates.

Answer 22

False.

EIGRP neighbors exchange the entire routing table when forming an adjacency, and they advertise only incremental updates as topology changes occur within a network.

The neighbor adjacency table is vital for tracking neighbor status and the updates sent to each neighbor.

Question 23

What are the 5 packet types that EIGRP uses to communicate among routers?

Answer 23

EIGRP uses five different packet types to communicate with other routers, as shown in Table 7-3. EIGRP uses its own IP number (88); it uses multicast packets where possible and unicast packets when necessary. Communication between routers is done with multicast, using the group address 224.0.0.10 when possible.

Question 24

In the EIGRP metric calcuation what is delay, exactly?

Answer 24

Delay is the total measure of delay in the path, measured in tens of microseconds (μs).

Question 25

What is the “variance multiplier” and “variance value” in EIGRP.

Answer 25

EIGRP supports unequal-cost load balancing, which allows installation of both successor routes and feasible successors into the EIGRP RIB. EIGRP supports unequal-cost load balancing by changing EIGRP’s variance multiplier.

Dividing the feasible successor metric by the successor route metric provides the variance multiplier. The variance multiplier is a whole number, so any remainders should always round up.

The EIGRP variance value is the feasible distance (FD) for a route multiplied by the EIGRP variance multiplier.

Any feasible successor’s FD with a metric below the EIGRP variance value is installed into the RIB. EIGRP installs multiple routes where the FD for the routes is less than the EIGRP multiplier value up to the maximum number of ECMP routes, as discussed earlier.

Question 26

T/F: A secondary function for the EIGRP hello packets is to ensure that EIGRP neighbors are still healthy and available.

Answer 26

True.

EIGRP hello packets are sent out in intervals determined by the hello timer. The default EIGRP hello timer is 5 seconds, but it is 60 seconds on slow-speed inter- faces (T1 or lower).

Question 27

What is the EIGRP hold timer? What is the default timer setting?

Answer 27

EIGRP uses a second timer for the hold time, which is the amount of time EIGRP deems the router reachable and functioning.

The hold time value defaults to 3 times the hello interval. The default value is 15 seconds, and it is 180 seconds for slow-speed interfaces.

The hold time decrements, and upon receipt of a hello packet, the hold time resets and restarts the countdown. If the hold time reaches 0, EIGRP declares the neighbor unreachable and notifies DUAL of a topology change.

Question 28

Why would a router send out a query packet with the delay set to infinite for a prefix?

Answer 28

If a feasible successor is not available for a prefix, DUAL must perform a new route calcula- tion. The route state changes from passive (P) to active (A) in the EIGRP topology table.

The router detecting the topology change sends out query packets to EIGRP neighbors for the route. The query packet includes the network prefix with the delay set to infinity so that other routers are aware that it has gone active. When the router sends the EIGRP query packets, it sets the reply status flag set for each neighbor on a prefix basis.

Question 29

T/F: Scalability of an EIGRP autonomous system does not depend on summarization.

Answer 29

False.

EIGRP works well with minimal optimizations. Scalability of an EIGRP autonomous system depends on summarization. As the size of an EIGRP autonomous system increases, convergence may take longer. Scaling an EIGRP topology requires summarizing routes in a hierar- chical fashion.

Question 30

T/F: EIGRP summarizes network prefixes on an prefix basis.

Answer 30

False.

EIGRP summarizes network prefixes on an interface basis.

A summary aggregate is configured for the EIGRP interface. Prefixes within the summary aggregate are suppressed, and the summary aggregate prefix is advertised in lieu of the original prefixes. The summary aggregate prefix is not advertised until a prefix matches it. Interface-specific summarization can be performed in any portion of the network topology. In addition to shrinking the rout- ing tables of all the routers, summarization creates a query boundary and shrinks the query domain when a route goes active during convergence.