CCNP Route FAQ: OSPF Route Summarization, Filtering, and Default Routing

Q1. Router B1, an internal router in area 1, displays the following output. The only two ABRs connected to area 1 are performing Type 3 LSA filtering. Which of the following answers is true based on the information in the output from B1?

R1# show ip route 10.1.0.0 255.255.0.0 longer-prefixes
! Legend lines omitted for brevity
10.0.0.0/8 is variably subnetted, 17 subnets, 3 masks
O    10.1.2.0/24 [110/658] via 10.10.13.1, 00:00:32, Serial0/0/0.1
O IA 10.1.1.0/24 [110/658] via 10.10.23.2, 00:41:39, Serial0/0/0.2
O IA 10.1.3.0/24 [110/658] via 10.10.23.2, 00:41:39, Serial0/0/0.2

a. A Type 3 LSA for 10.2.2.0/24 was filtered by both ABRs.
b. A Type 3 LSA for 10.1.2.0/24 was not filtered by both ABRs.
c. A Type 3 LSA for 10.1.3.0/24 was not filtered by at least one ABR.
d. A Type 3 LSA for 10.1.1.0/24 filtered by both ABRs.

Figure: Type 3 LSA Filtering Example

Answer: C. The output lists all R1’s routes for subnets within the range of 10.1.0.0–10.1.255.255, whose prefix lengths are longer than /16. One answer lists subnet 10.2.2.0/24, which is not in this range, so the output cannot be used to confirm nor deny whether the subnet was filtered. R1’s route for 10.1.2.0/24 is an intra-area route by virtue of not listing an IA code by the route; Type 3 LSA filtering only filters Type 3 LSAs, which routers use to calculate interarea routes, so the output tells us nothing about any filtering of 10.1.2.0/24. The output shows a single interarea route for 10.1.3.0/24, so at least one ABR has flooded a Type 3 LSA for this route. Additionally, the output confirms that at least one ABR flooded a type 3 LSA for 10.1.3.0/24, or the output would not show an IA route for 10.1.3.0/24. So, the type 3 LSA for 10.1.3.0/24 was not filtered by both ABRs.

Q2. The following command output was gathered from Router R1, an ABR between areas 0 (backbone) and area 1. In this internetwork, area 0 contains all the subnets of class A network 10.0.0.0. R1’s OSPF process has a distribute-list prefix question in command configured. Assuming the subnets listed in the answers actually exist in area 0, which of the following occurs on router R1?

R1#sh ip prefix-list
ip prefix-list question: 4 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 le 32

a. R1 will not create/flood a type 3 LSA for subnet 10.1.2.0/26 into area 1.
b. R1 will not create/flood a Type 3 LSA for subnet 10.1.2.0/24 into area 1.
c. R1 will not have an OSPF route for subnet 10.1.2.0/26 in its IP routing table.
d. R1 will not have an OSPF route for subnet 10.1.2.0/24 in its IP routing table.

Answer: C. When referenced from a distribute list, OSPF filters routes from being added to that router’s IP routing table but has no impact on the flow of LSAs. As such, neither A nor B is correct. An OSPF distribute-list command does attempt to filter routes from being added to the IP routing table by OSPF, so the two answers that mention the IP routing table might be correct. 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. So, the prefix list will match 10.1.2.0/26 with the first line, with a deny action. The 10.1.2.0/24 subnet does not match the first line of the prefix list, but it does match the third line, the match all line, with a permit action. Because 10.1.2.0/26 is matched by a deny clause, this route is indeed filtered, so it is not added to R1’s IP routing table. 10.1.2.0/24, matched with a permit clause, is allowed and would be in the IP routing table.

Q3. Use the same scenario as the previous question, with one change. Instead of the distribute-list prefix question in command configured on R1, R1’s OSPF process has an area 1 filter-list prefix question in command configured. Again assuming that the subnets listed in the answers actually exist in area 0, which of the following occurs on router R1?

R1#sh ip prefix-list
ip prefix-list question: 4 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 le 32

Answer: A. When referenced from an area filter-list command, OSPF filters Type 3 LSAs created on that router, preventing them from being flooded into area 1 (per the configuration command). As an ABR, R1 would calculate intra-area routes to these area 0 subnets, so this filtering will have no effect on R1’s routes. 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. So, the prefix list will match 10.1.2.0/26 with the first line, with a deny action. The 10.1.2.0/24 subnet does not match the first line of the prefix list because the prefix length does not match, but it does match the third line, the match all line, with a permit action. By matching subnet 10.1.2.0/26 with a deny action, the filter-list does prevent R1 from flooding a Type 3 LSA for that subnet. By matching 10.1.2.0/24 with a permit action, R1 does not filter the Type 3 LSA for that subnet.

Q4. R1, an ABR between backbone area 0 and area 1, has intra-area routes in area 0 for 10.1.1.0/24, 10.1.2.0/24, and 10.1.3.0/24. These routes have metrics of 21, 22, and 23, respectively. An engineer then adds the area 0 range 10.1.0.0 255.255.0.0 command under the OSPF process of R1. Which of the following is true? (Choose two.)
a. R1 loses and then re-establishes neighborships with all neighbors.
b. R1 no longer advertises 10.1.1.0/24 to neighbors into area 1.
c. R1 advertises a 10.1.0.0/16 route into area 1 with a metric of 23 (largest metric).
d. R1 advertises a 10.1.0.0/16 route into area 1 with metric of 21 (lowest metric).

Answer: B and D. The area range command does not cause a failure in neighborships. Because at least one intra-area subordinate subnet of 10.1.0.0/16 exists in R1, R1 both creates a summary route for 10.1.0.0/16 and stops advertising LSAs for the (three) subordinate subnets. By default, the metric of the summary is the metric of the lowest-metric component subnet.

Q5. The following output exists on Router R1, a router internal to area 1. What can you determine as true from the output of the show ip ospf database summary command?

Routing Bit Set on this LSA
LS age: 124
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links (Network)
Link State ID: 10.1.0.0 (summary Network Number)
Advertising Router: 1.1.1.1
LS Seq Number: 80000001
Checksum: 0x878F
Length: 28
Network Mask: /22
TOS: 0 Metric: 11

a. The LSA was created by an ABR due to an area range command.
b. The LSA was created by an ASBR due to a summary-address command.
c. If created by an area range command, the best metric for a subordinate subnet on that ABR must have been 11.
d. None of the other answers is correct.

Answer: D. The show ip ospf database summary command lists only Type 3 LSAs. The summary-address command creates Type 5 LSAs on ASBRs, ruling out one answer. The output does not specify whether the LSA was created as a summary route; all references to the word “summary” refer to Type 3 Summary LSAs. If created by an area range command, the metric defaults to be the best metric of all subordinate subnets, but it may also be explicitly set, ruling out another of the possible answers. In short, this LSA may represent a route summarized by the area range command, but that fact cannot be proved or disproved by the output as shown..

Q6. Router R1, an ASBR connected to the Internet and to backbone area 0, has been configured with a default-information originate command. Which of the following is true about the effects of this configuration command?
a. R1 will always create and flood a default route into the OSPF domain.
b. R1 will create and flood an LSA for prefix/length 0.0.0.0/0 into the OSPF domain if R1’s IP routing table has a route to 0.0.0.0/0.
c. R1 will set a flag on the LSA for the subnet between itself and one of the ISPs, noting this subnet as a default network, regardless of whether R1 has a default route.
d. R1 will set a flag on the LSA for the subnet between itself and one of the ISPs, noting this subnet as a default network, but only if R1 has a route to 0.0.0.0/0.

Answer: B. Without the always parameter, the default-information originate command generates an LSA for a default route, with prefix 0.0.0.0/0, but only if its own IP routing table has a route for 0.0.0.0/0. It does not flag another LSA as being used as a candidate default route.

Q7. Which of the following is true about routers internal to a totally NSSA area? (Choose two.)
a. Routers cannot redistribute external routes into the area.
b. Routers should have zero Type 3 LSAs in their LSDBs.
c. Routers should have zero Type 5 LSAs in their LSDBs.
d. Routers should learn default routes from the ABRs attached to the area.

Answer: C and D. Both types of NSSA stubby areas allow the redistribution of external routes into the area, but these routes are advertised as Type 7 LSAs. As a totally NSSA area, the ABR should flood no Type 5 LSAs into the area and flood no Type 3 LSAs into the area, except for the Type 3 LSAs used to advertise the default route into the area. As such, a router internal to a totally stubby are should see zero Type 5 LSAs, and a small number of Type 3 LSAs for the default route(s) advertised by the ABR(s).

Q8. ABR R1 has been configured with an area 1 stub no-summary command. Which stubby area type is area 1?
a. Stub
b. Totally stubby
c. NSSA
d. Totally NSSA

Answer: B. The stub keyword means either a stub area or totally stubby. The no-summary command means the area is totally stubby.

CCNP Route FAQ: OSPF Route Summarization, Filtering, and Default Routing

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