CCNP Route FAQ: EIGRP Route Summarization and Filtering

CCNP Route FAQ: EIGRP Route Summarization and Filtering


Figure: Choosing Locations for Route Summarization

Q1. Router R1 has been configured for EIGRP. The configuration also includes an ACL with one line–access-list 1 permit 10.100.32.0 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?
a. 10.10.32.0/19
b. 10.10.44.0/22
c. 10.10.40.96/27
d. 10.10.48.0/23
e. 10.10.60.0/30

Answer: 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 due to 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.

Q2. 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 is true about the filtering of various prefixes by this Prefix list?

a. Prefix 10.1.2.0/24 will be filtered due to clause 5.
b. Prefix 10.1.2.224/26 will be filtered due to clause 5.
c. Prefix 10.2.2.4/30 will be filtered due to clause 15.
d. Prefix 10.0.0.0/8 will be permitted.
e. Prefix 0.0.0.0/0 will be permitted.

Answer: B, C, and E. Sequence number 5 matches prefixes from 10.1.2.0–10.1.2.255, with prefix lengths between 25–27, 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 from 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.

Q3. 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?

a. It will be filtered due to 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 due to matching clause 20’s reference to prefix-list fred.
d. It will be filtered due to matching the implied deny all route map clause at the end of the route map.

Answer: 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 from 10.10.10.0–10.10.11.255, with prefix lengths from 23–25. Both criteria match the prefix in question, making answer C correct.

Q4. 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?
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

Answer: B and C. 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 from 24–28, inclusive. Answer C implies a range from 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 from 28–32, so this final list could not match prefix lengths of /27.

Q5. 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

Answer: B. 10.1.0.0/18 implies a range from 10.1.0.0–10.1.63.255, which includes none of the four subnets. 10.1.64.0/18 implies a range from 10.1.64.0–10.1.127.255, which includes all subnets. 10.1.100.0/24 implies range 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 range 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, IOS would actually accept the command, and change the parameter from 10.1.98.0 to 10.1.96.0, and would not include the four listed subnets.

Q6. R1 has 5 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. 1 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).

Answer: 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 us a metric equal to the metric of the lowest metric subordinate route, approximately 1,000,000 in this case.

Q7. 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?
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

Answer: 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.

Q8. 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.
e. Another router has been configured with an ip route 2.0.0.0 255.0.0.0 1.1.1.1 command.

Answer: 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.

Q9. 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 is not a useful step with this style of default route configuration?
a. Create the default route on R1 using the ip route 0.0.0.0 0.0.0.0 outgoinginterface command.
b. Redistribute the statically configured default route.
c. Disable auto-summary.
d. Configure the network 0.0.0.0 command.
e. Ensure R1 has no manually configured summary routes using the ip summary-address eigrp command.

Answer: 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.

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James Palmer

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