Ch 5 - Advanced EIGRP Concepts Flashcards
Which of the following are methods that EIGRP uses to initially populate (seed) its EIGRP topology table, before learning topology data from neighbors? (Choose two.)
a. By adding all subnets listed by the show ip route connected command
b. By adding the subnets of working interfaces over which static neighbors have
been defined
c. By adding subnets redistributed on the local router from another routing source
d. By adding all subnets listed by the show ip route static command
B and C. Other than the two listed correct answers, the local router also adds connected routes for which the network command matches the corresponding interfaces, so it might not add all connected routes. Also, EIGRP does not add static routes to the EIGRP topology table, unless those routes are redistributed.
Which of the following are both advertised by EIGRP in the Update message and included in the formula for calculating the integer EIGRP metric? (Choose two.)
a. Jitter
b. Delay
c. MTU
d. Reliability
B and D. EIGRP sends bandwidth, delay, reliability, load, MTU, and hop count in the message. The formula to calculate the metric includes bandwidth, delay, reliability, and load.
Router R1 uses S0/0 to connect through a T/1 to the Frame Relay service. Five PVCs terminate on the serial link. Three PVCs (101, 102, and 103) are configured on subinterface S0/0.1, and one each (104 and 105) are on S0/0.2 and S0/0.3. The configuration shows no configuration related to EIGRP WAN bandwidth control, and the bandwidth command is not configured. Which of the following is true about how Cisco IOS tries to limit EIGRP’s use of bandwidth on S0/0?
a. R1 limits EIGRP to around 250 kbps on DLCI 102.
b. R1 limits EIGRP to around 250 kbps on DLCI 104.
c. R1 limits EIGRP to around 150 kbps on every DLCI.
d. R1 does not limit EIGRP because no WAN bandwidth control has been configured.
A. EIGRP performs WAN bandwidth control without any explicit configuration, using default settings. Because no bandwidth commands have been configured, each subinterface uses the default 1544-kbps setting. For S0/0.1, WAN bandwidth control divides the 1544 by 3 (515 kbps) and then takes the (default) WAN bandwidth of 50 percent, meaning about 250 kbps for each of the three DLCIs. For the two subinterfaces with one PVC, the default 1544 is multiplied by the 50 percent default WAN bandwidth, meaning that each could use about 750 kbps.
The output of show ip eigrp topology on Router R1 shows the following output, which is all the output related to subnet 10.11.1.0/24. How many feasible successor routes does R1 have for 10.11.1.0/24?
P 10.11.1.0/24, 2 successors, FD is 2172423
via 10.1.1.2 (2172423/28167), Serial0/0/0.1
via 10.1.1.6 (2172423/28167), Serial0/0/0.2
a. 0
b. 1
c. 2
d. 3
A. This command lists all successor and feasible successor routes. The output states that two successors exist, and only two routes (listed with the “via…” text) exist. So, no feasible successor routes exist.
A network design shows that R1 has four different possible paths from itself to the data center subnets. Which of the following can influence which of those routes become feasible successor routes, assuming that you follow the Cisco recommended practice of not changing metric weights? (Choose two.)
a. The configuration of EIGRP offset lists
b. Current link loads
c. Changing interface delay settings
d. Configuration of variance
A and C. By default, the metric weights cause EIGRP to consider bandwidth and delay in the metric calculation, so changing either bandwidth or delay impacts the calculation of the feasible distance and reported distance, and impacts the choice of feasible successor routes. Offset lists also change the metric, which in turn can change whether a route is an FS route. Link loading would impact the metrics, but not without changing the metric weights to non-recommended values. Finally, variance impacts which routes end up in the IP routing table, but it is not considered by EIGRP when determining which routes are FS routes.
Router R1 is three router hops away from subnet 10.1.1.0/24. According to various show interfaces commands, all three links between R1 and 10.1.1.0/24 use the following settings: bandwidth (in kbps): 1000, 500, 100000 and delay (in microseconds): 12000, 8000, 100. Which of the following answers correctly identify a value
that feeds into the EIGRP metric calculation? (Choose two.)
a. Bandwidth of 101,500 kilobits per second
b. Bandwidth of about 34,000 kilobits per second
c. Bandwidth of 500 kilobits per second
d. Delay of 1200 tens-of-microseconds
e. Delay of 2010 tens-of-microseconds
f. Delay of 20100 tens microseconds
C and E. The EIGRP metric calculation treats bandwidth and delay differently. For bandwidth, EIGRP takes the lowest bandwidth, in kbps, which is in this case 500 kbps. For delay, EIGRP takes the cumulative delay, which is 20100 per the various show interfaces commands. However, the show interfaces command uses a unit of microseconds, and the interface delay command and the EIGRP metric formula use a unit of tens-of-microseconds, making the delay that feeds into the formula be 2010.
Routers R1 and R2 are EIGRP neighbors. R1 has been configured with the eigrp stub connected command. Which of the following are true as a result? (Choose two.)
a. R1 can learn EIGRP routes from R2, but R2 cannot learn EIGRP routes from R1.
b. R1 can send IP packets to R2, but R2 cannot send IP packets to R1.
c. R2 no longer learns EIGRP routes from R1 for routes not connected to R1.
d. R1 no longer replies to R2’s Query messages.
e. R2 no longer sends Query messages to R1.
C and E. R1, as a stub router with the connected option, still advertises routes, but only routes for connected subnets. R1 announces its stub attribute to R2, so R2 chooses to not send Query messages to R1, knowing that R1 cannot be a transit router for other subnets anyway.
Router R1 lists four routes for subnet 10.1.1.0/24 in the output of the show ip eigrp topology all-links command. The variance 100 command is configured, but no other related commands are configured. Which of the following rules is true regarding R1’s decision of what routes to add to the IP routing table? Note that RD refers to reported distance and FD to feasible distance.
a. Adds all routes for which the metric is <= 100 * the best metric among all routes
b. Adds all routes because of the ridiculously high variance setting
c. Adds all successor and feasible successor routes
d. Adds all successor and feasible successor routes for which the metric is <= 100 * the best metric among all routes
Chapter 5: Advanced EIGRP Concepts 157
D. EIGRP considers only successor and feasible successor routes. Each of those routes must have metrics such that variance * metric is less than the best route’s metric; the best route’s metric is called the feasible distance (FD).
A network design shows that R1 has four possible paths from itself to the data center subnets. Which of the following commands is most likely to show you all the possible next-hop IP addresses for these four possible routes?
a. show ip eigrp topology
b. show ip eigrp topology all-links
c. show ip route eigrp
d. show ip route eigrp all-links
e. show ip eigrp topology all-learned
B. Of the five options, show ip route eigrp all-links and show ip eigrp topology all-learned are not valid commands. Both show ip eigrp topology and show ip route eigrp can show at most successor and feasible successor routes. However, show ip eigrp topology all-inks shows also nonfeasible successor routes, making it more likely to show all possible neighbors.
Router R1 has been configured for EIGRP. The configuration also includes an ACL with one line—access-list 1 permit 10.10.32.0 0.0.15.255—and the EIGRP configuration includes the distribute-list 1 in command. Which of the following routes could not be displayed in the output of the show ip eigrp topology command as a result? (Choose two.)
a. 10.10.32.0/29
b. 10.10.44.0 /22
c. 10.10.40.96 /27
d. 10.10.48.0 /23
e. 10.10.60.0 /30
D and E. The two listed commands correctly configure EIGRP route filtering such that prefixes matched by the ACL’s permit clause will be allowed. All other prefixes will be filtered because of the implied deny all at the end of the ACL. The ACL permits numbers in the range 10.10.32.0–10.10.47.255, which leaves 10.10.48.0 and 10.10.60.0 unmatched by the permit clause.
The command output that follows was gathered from Router R1. If correctly referenced by an EIGRP distribution list that filters outbound Updates, which of the following statements are true about the filtering of various prefixes by this prefix list? (Choose three.)
R1# sh ip prefix-list
ip prefix-list question: 3 entries
seq 5 deny 10.1.2.0/24 ge 25 le 27
seq 15 deny 10.2.0.0/16 ge 30 le 30
seq 20 permit 0.0.0.0/0
a. Prefix 10.1.2.0/24 will be filtered because of clause 5.
b. Prefix 10.1.2.224/26 will be filtered because of clause 5.
c. Prefix 10.2.2.4/30 will be filtered because of clause 15.
d. Prefix 10.0.0.0/8 will be permitted.
e. Prefix 0.0.0.0/0 will be permitted.
B, C, and E. Sequence number 5 matches prefixes 10.1.2.0–10.1.2.255, with prefix lengths between 25 and 27, and denies (filters) those prefixes. This results in answer A being incorrect, because the prefix length (/24) is not in the correct range. Clause 15 matches prefixes 10.2.0.0–10.2.255.255, with prefix length exactly 30, matching answer C. Clause 20 matches only prefix 0.0.0.0 with length /0, so only a default route would match this entry. As a result, 10.0.0.0/8 does not match any of the three clauses.
R1 has correctly configured EIGRP to filter routes using a route map named question. The configuration that follows shows the entire route map and related configuration. Which of the following is true regarding the filtering action on prefix 10.10.10.0/24 in this case?
route-map question deny 10
match ip address 1
route-map question permit 20
match ip address prefix-list fred
!
access-list 1 deny 10.10.10.0 0.0.0.255
ip prefix-list fred permit 10.10.10.0/23 le 25
a. It will be filtered because of the deny action in route map clause 10.
b. It will be allowed because of the double negative (two deny references) in clause
10.
c. It will be permitted because of matching clause 20’s reference to prefix-list fred.
d. It will be filtered because of matching the implied deny all route map clause at the end of the route map.
C. When used for route filtering, the route map action (permit or deny) defines the filtering action, and any referenced match commands’ permit or deny action just defines whether the prefix is matched. By not matching ACL 1 with a permit action, EIGRP does not consider a match to have occurred with clause 10, so it moves to clause 20. The prefix list referenced in clause 20 has a permit action, matching prefixes 10.10.10.0–10.10.11.255, with prefix lengths from 23 to 25. Both criteria match the prefix in question, making answer C correct.
An engineer has typed four different single-line prefix lists in a word processor. The four answers show the four different single-line prefix lists. The engineer then does a copy/paste of the configuration into a router. Which of the lists could match a subnet whose prefix length is 27? (Choose two.)
a. ip prefix-list fred permit 10.0.0.0/24 ge 16 le 28
b. ip prefix-list barney permit 10.0.0.0/24 le 28
c. ip prefix-list wilma permit 10.0.0.0/24 ge 25
d. ip prefix-list betty permit 10.0.0.0/24 ge 28
B, C and D. Answer A is invalid. The ge value must be larger than /24 in this case, so the command is rejected. Answer B implies a prefix length range of 24–28, inclusive. Answer C implies a range of 25–32 inclusive, because no le parameter exists to limit the prefix length lower than the full length of an IPv4 subnet mask. The same logic applies with answer D, but with a range of 28–32, so this final list could not match prefix lengths of /27.
An engineer plans to configure summary routes with the ip summary-address eigrp asn prefix mask command. Which of the following, when added to such a command, would create a summary that includes all four of the following subnets: 10.1.100.0/25, 10.1.101.96/27, 10.1.101.224/28, and 10.1.100.128 /25?
a. 10.1.0.0 255.255.192.0
b. 10.1.64.0 255.255.192.0
c. 10.1.100.0 255.255.255.0
d. 10.1.98.0 255.255.252.0
B. 10.1.0.0/18 implies a range of 10.1.0.0–10.1.63.255, which includes none of the four subnets. 10.1.64.0/18 implies a range of 10.1.64.0–10.1.127.255, which includes all subnets. 10.1.100.0/24 implies a range of 10.1.100.0–10.1.100.255, which leaves out two of the subnets. Finally, 10.1.98.0/22 does not actually represent a summary. Instead, 10.1.96.0/22 represents a range of 10.1.96.0–10.1.99.255, with 10.1.98.0 as listed in answer D being an IP address in that range. As such, Cisco IOS would actually accept the command, would change the parameter from 10.1.98.0 to 10.1.96.0, and would not include the four listed subnets.
R1 has five working interfaces, with EIGRP neighbors existing off each interface. R1 has routes for subnets 10.1.1.0/24, 10.1.2.0/24, and 10.1.3.0/24, with EIGRP integer metrics of roughly 1 million, 2 million, and 3 million, respectively. An engineer then adds the ip summary-address eigrp 1 10.1.0.0 255.255.0.0 command to interface Fa0/0. Which of the following is true?
a. R1 loses and then reestablishes neighborships with all neighbors.
b. R1 no longer advertises 10.1.1.0/24 to neighbors connected to Fa0/0.
c. R1 advertises a 10.1.0.0/16 route out Fa0/0, with metric of around 3 million (largest metric of component subnets).
d. R1 advertises a 10.1.0.0/16 route out Fa0/0, with metric of around 2 million (median metric of component subnets).
B. The ip summary-address command does reset neighborships, but only on the interface under which it is configured. After those neighborships come up, R1 will advertise the summary route, but none of the subordinate routes inside that summary. The summary route will use a metric equal to the metric of the lowest metric subordinate route, approximately 1,000,000 in this case.
In a lab, R1 connects to R2, which connects to R3. R1 and R2 each have several working interfaces, all assigned addresses in Class A network 10.0.0.0. Router R3 has some working interfaces in Class A network 10.0.0.0, and others in Class B network 172.16.0.0. The engineer experiments with the auto-summary command on R2 and R3, enabling and disabling the command in various combinations. Which of the following combinations will result in R1 seeing a route for 172.16.0.0 /16, instead of the individual subnets of Class B network 172.16.0.0? (Choose two.)
a. auto-summary on R2 and no auto-summary on R3
b. auto-summary on R2 and auto-summary on R3
c. no auto-summary on R2 and no auto-summary on R3
d. no auto-summary on R2 and auto-summary on R3
B and D. R2 has interfaces only in Class A network 10.0.0.0, so the auto-summary setting has no effect. R3 has interfaces in both Class A network 10.0.0.0 and Class B network 172.16.0.0, so auto-summary causes R3 to summarize all subnets of 172.16.0.0/16 as a summary route when advertising to R2.
Router R1 exists in an enterprise that uses EIGRP as its routing protocol. The show ip route command output on Router R1 lists the following phrase: “Gateway of last resort is 1.1.1.1 to network 2.0.0.0.” Which of the following is most likely to have caused this output to occur on R1?
a. R1 has been configured with an ip default-network 2.0.0.0 command.
b. R1 has been configured with an ip route 0.0.0.0 0.0.0.0 1.1.1.1 command.
c. R1 has been configured with an ip route 2.0.0.0 255.0.0.0 1.1.1.1 command.
d. Another router has been configured with an ip default-network 2.0.0.0 command.
D. The phrase quoted in the question means that R1 is using its route for Class A network 2.0.0.0 to decide where to send packets by default. R1’s route for network 2.0.0.0 must have 1.1.1.1 as its next-hop router. This phrase occurs when EIGRP has learned
a route for Class A network 2.0.0.0 that has been flagged as a candidate default route by another router. The router flagging a route as a candidate default route, using the ip default-network command, does not actually use the route as its default route.
Enterprise Router R1 connects an enterprise to the Internet. R1 needs to create and advertise a default route into the enterprise using EIGRP. The engineer creating the implementation plan has chosen to base this default route on the ip route command, rather than using ip default-network. Which of the following are not useful steps with this style of default route configuration? (Choose two.)
a. Create the default route on R1 using the ip route 0.0.0.0 0.0.0.0 outgoing-interface command.
b. Redistribute the statically configured default route.
c. Disable auto-summary.
d. Configure the network 0.0.0.0 command.
e. Ensure that R1 has no manually configured summary routes using the ip summary-address eigrp command.
C and E. With the suggested configuration style, the static route must first be configured statically, as shown in answer A. Then, either this route must be redistributed as a static route into EIGRP (answer B) or pulled into EIGRP by virtue of the network 0.0.0.0 EIGRP subcommand (answer D). The other two options have no effect on default route creation and advertisement.
Routers seed their local topo databases with which of the following local information:
a. Prefixes of connected subnets on EIGRP enabled interfaces
b. Prefixes of connected subnets for interfaces referenced in an EIGRP neighbor command.
c. Prefixes learned by the redistribution of routes, from other routing protocols or other routing sources.
d. IP address of nearest ABR
a, b, c
Name the 5 basic EIGRP protocol messages.
Hello Update Query Reply Ack.
Updates contain prefix, length, metrics(bandwidth, delay, reliability, load) and non-metric items like MTU size and Hopcount.
Updates and Acks are the only two message types involved in the topology database exchange process.
What number is used to represent bandwidth in the formula used to calculate a metric for a route in the topology table if the interface is a Fast Ethernet interface?
10^5Kbps = 100,000Kbps=10Mbps
10 ‘tens of microseconds’ = 100 microseconds would be delay
What number is used to represent delay in the formula used to calculate a metric for a route in the topology table if the serial interface is a DS1 consisting of 24 multiplexed 64Kbps DS0s? (DS1 = T1)
2000 ‘tens of microseconds’ = 20,000 microseconds
1544Kbps = 1.544Mbps= 24*(64Kbps) would be bandwidth
What are the default settings for the EIGRP composite metrics, load and reliability?
1 and 255
If the values are 1 and 255 of load and reliability, what is the impact on the EIGRP metric calculation?
None. These are not used by default. Bandwidth and delay are used by default because they have non-zero K-values.
In the EIGRP topo database what is the hop count of a connected route?
- This is different than RIP which lists connected routes as 1 hop away.
R1 advertises 10.1.11.0/24 to R2 using an EIGRP update message. The message includes four metric components plus two more components that are not used in the formula to calculate metrics from the info in the topo table. Name these, all.
The four metric components are bandwidth, delay, load and reliability. MTU and hop count are the other two and are not used directly in the metric calculation.
R2 receives an update from R1, in an EIGRP update message. R2 adds the info received to it’s topo table, with which of these changes:
a. R2 considers the interface the route was received on to be the outgoing interface for potential routes to reach that network.
b. R2 adds the outgoing delay (e.g. 2000 tens of microseconds for a T1) to the delay listed in the message.
c. R2 compares the bandwidth in the update message received with it’s own outgoing bandwidth for that interface and chooses the lower one for the metric calc.
d. R2 also updates load (with the highest value), reliability (lowest value), and MTU (lowest value) and adds one to the Hop count.
All four are true. These are the steps by which EIGRP propagates topology information from router to router.
R2 considers the interface the update was received on to be the outgoing interface for routes to that network.
R1 has a Fast Ethernet interface and a T1 serial connection to R2. R2 has a Gigabit ethernet connection to R3. EIGRP is active on all interfaces. What bandwidth and delay go into the top table of R3 for the Fast Ethernet interface on R1? Show your work. ;-)
For this question you need to know several things, including basic scientific notation aka ‘standard form’ in the UK, default bandwidth/metrics and the EIGRP metric formula.
Fast Ethernet bandwidth = 100Mbps = 100,000,000bps 100,000Kbps
Fast Ethernet delay = 100 microseconds = 10 tens of microseconds
T1 bandwidth = 1.544Mbps = 1544Kbps
T1 delay = 20,000 micro seconds = 2000 tens of microseconds
GigE bandwidth = 1Gbps = 1,000,000,000bps = 1,000,000Kbps
GigE delay = 10 microseconds = 1 tens of microseconds
We know that EIGRP will use the ‘least bandwidth’ and the cumulative delay, so let’s find these first.
least bandwidth = 1544 = lowest cost bandwidth in Kbps
cumulative delay = 10 + 2000 + 1 = 2011 (tens of microseconds)
Armed with the above facts you can plug the numbers into the EIGRP metric formula:
metric = (10^7 / least bandwidth + cumulative delay) * 256
metric = (10^7 / 1544 + 2011) * 256 = 2172847
T/F: The output from the show interfaces and show eigrp topology 10.1.11.0/24 commands will show delay in microseconds not tens of microseconds.
True. 10s of microseconds is used in the EIGRP metric calculation, update messages and in the delay command that allows you to set the delay on an EIGRP interface..
The unit actually stored in the topo table is tens of microseconds, but the show topo command list the output in microseconds.
Thanks Cisco! (I would draw a big middle finger in text art right here bu that will take to long.)
T/F: Full EIGRP updates are sent for a failed neighbor recovery or when new adjacencies are formed.
True. Unless something changes there are no more topo updates, and then it is a partial update for changes in topo.
T/F: Split Horizon is enabled by default in EIGRP.
True.
What protocol is used for EIGRP update messages.
RTP. Reliable Transport Protocol.
T/F: RTP, Reliable Transport Protocol, is configurable by engineers.
False. There are no settings that can be changed for this protocol.
Why is split horizon a problem with EIGRP and multi-point Frame Relay interfaces?
S.H. will not send routing updates from the same interface they are learned from. In a hub and spoke model the hellos and update messages are blocked by S.H. This prevents spokes from learning routes or forming neighborships with each other.
What is the solution to EIGRP problems with multipoint Frame Relay interfaces in a hub-spoke model?
no ip split-horizon eigrp
This allows spoke routers to learn routes from each other but they will never become neighbors.
Update messages will be forwarded with this enabled.
They never become neighbors because in a hub-spoke model there is no PVC between the spoke routers, only to the hub. No hellos = no neighborship.
What command will show that split horizon is disabled?
sh ip eigrp interfaces detail
What is the default bandwidth allowed for EIGRP on an interface?
50% of what is defined by the bandwidth command
What is the command to define bandwidth for EIGRP?
ip bandwidth-percent eigrp
Cisco recommends setting the bandwidth on pt-pt links to the speed of teh CIR of the single PVC on the interface. The CIR is the Committed Information Rate that is guaranteed by the ISP.
What is the default bandwidth for serial interfaces for EIGRP?
1544Kbps
T/F: For multi-pint interfaces, IOS first divides the sub-if bandwidth by the number of configure PVCs, then determines the EIGRP percentage based on that number.
True. This is a feature of Cisco WAN Bandwidth Control.
With a Frame Relay medium, a WAN router has S0/0/0.20 configured as a multi-pt with 3 PVCs configured on it. Given this interface config, what is the bandwidth for each PVC as allotted by Cisco WAN BW control?
int ser 0/0/0.20 ip address 168.32.34.87 255.255.255.248 frame-relay interface-dlci 201 frame-relay interface-dlci 202 frame-relay interface-dlci 203 bandwidth 300 ip bandwidth-percent eigrp 1 20
20Kbps each = 20% of 300kbps /3 PVCs
The ip bandwidth-percent eigrp command combined with the bandwidth command set the allowed bandwidth on each PVC.
With a Frame Relay medium, a WAN router has S0/0/0.30 configured as a pt-pt with 1 PVCs configured on it. Given this interface config, what is the bandwidth for the PVC as allotted by Cisco WAN BW control?
int ser 0/0/0.20 ip address 172.16.1.17 255.255.255.252 frame-relay interface-dlci 221 bandwidth 100 ip bandwidth-percent eigrp 1 30
30Kbps = 30% of 100Kbps
T/F: EIGRP builds the route table from info in the topo table.
True. The metrics are stored in the topo table.
What is the difference between Feasible Distance and Reported Distance?
FD is the metric for a route from the local router’s perspective. RD is the metric for a route from the neighboring router’s perspective.
FD is used by the local router to choose the best route for that prefix. The FD with the lowest metric is added to the route table.
RD is used by the local router when converging to a new route.
T/F: Reported distance is sometimes referred to as Advertised Distance.
True.
T/F: The topo table for each prefix/length lists one or more possible routes.
True.
T/F: For connected subnets the topo table does not list the outgoing interface.
False.
