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 was filtered by both ABRs. - C. The output lists all of B1's routes for
subnets within the range 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 or deny
whether the subnet was filtered. B1's route for 10.1.2.0/24 is an intra-area route by virtue of not listing
an inter-area (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.
The following command output was gathered from Router R1, an ABR between area 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 configured. 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: 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 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. - 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 through 10.1.2.255, with prefix lengths in the range 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.
Use the same scenario as the previous question, with one change. Instead of the distribute list
configured on R1, R1's OSPF process has an area 1 filter list 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: 3 entries
seq 5 deny 10.1.2.0/24 ge 25 le 27
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