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CCNP Route FAQ: OSPF Topology, Routes, and Convergence

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CCNP Route FAQ: OSPF Topology, Routes, and Convergence

Q1. A network design shows area 1 with three internal routers, area 0 with four internal routers, and area 2 with five internal routers. Additionally, one ABR (ABR1) connects areas 0 and 1, plus a different ABR (ABR2) connects areas 0 and 2. How many Type 1 LSAs would be listed in ABR2’s LSDB?
a. 6
b. 7
c. 15
d. 12
e. None of the other answers is correct.

Answer: D. As an ABR connected to areas 0 and 2, ABR2 will have LSDB entries for both area 0 and area 2. In Area 0, ABR2 learns Type 1 LSAs from the four routers internal to area 0, plus ABR1, and plus 1 for the area 0 Type 1 LSA ABR2 creates for itself. In area 2, ABR2 learns 1 each for the five routers internal to area 2, plus the 1 Type 1 LSA ABR2 created for itself inside area 2. The total is 12.

Q2. A network planning diagram shows a large internetwork with many routers. The configurations show that OSPF has been enabled on all interfaces, IP addresses correctly configured, and OSPF working. For which of the following cases would you expect a router to create and flood a Type 2 LSA?
a. When OSPF is enabled on a LAN interface, and the router is the only router connected to the subnet.
b. When OSPF is enabled on a point-to-point serial link, and that router has both the higher router ID and higher interface IP address on the link.
c. When OSPF is enabled on a Frame Relay point-to-point subinterface, has the lower RID and lower subinterface IP address, and otherwise uses default OSPF configuration on the interface.
d. When OSPF is enabled on a working LAN interface on a router, and the router has been elected BDR.
e. None of the other answers is correct.

Answer: E. OSPF creates a Type 2 LSA for a subnet when the router interface connected to the subnet calls for the election of a designated router (DR), and at least two routers have discovered each other and elected a DR. Then, the DR creates and floods the Type 2 LSA. IOS by default does not elect a DR on point-to-point topologies. It does on router LAN interfaces. One answer states that one router only exists in the subnet, so it does not actually find a second router and elect a DR. In the other case, a DR and BDR have been elected, but the router described in the answer is the BDR, not the DR. So, none of the other answers is correct.

Q3. A verification plan shows a network diagram with branch office Routers B1 through B100, plus two ABRs, ABR1, and ABR2, all in area 100. The branches connect to the ABRs using Frame Relay point-to-point subinterfaces. The verification plan lists the output of the show ip ospf database summary 10.100.0.0 command on a router B1, one of the branches. Which of the following is true regarding the output that could be listed for this command?
a. The output lists nothing unless 10.100.0.0 has been configured as a summary route using the area range command.
b. If 10.100.0.0 is a subnet in area 0, the output lists one Type 3 LSA, specifically the LSA with the lower metric when comparing ABR1’s and ABR2’s LSA for 10.100.0.0.
c. If 10.100.0.0 is a subnet in area 0, the output lists two Type 3 LSAs, one each created by ABR1 and ABR2.
d. None, because the Type 3 LSAs would exist only in the ABR’s LSDBs.

Answer: C. Each ABR, by definition, creates a single Type 3 LSA to represent a subnet known in one area to be advertised into another area. Assuming 10.100.0.0 is a subnet in area 0, both ABR1 and ABR2 would advertise a Type 3 LSA into area 100. The show ip ospf database summary command specifically lists type 3 network summary LSAs.

Q4. Which of the following OSPF messages contains entire complete LSAs used during the database exchange process?
a. LSR
b. LSAck
c. LSU
d. DD
e. Hello

Answer: C. The Database Description (DD) packet lists a short LSA header but not the entire LSA. The Link State Request (LSR) packet asks the neighbors for a copy of an LSA. The Link State Update (LSU) holds the LSAs. LSAck simply acknowledges received LSAs, and Hello is used for neighbor discovery and neighbor state maintenance.

Q5. Routers R1, R2, R3, and R4 connect to the same 10.10.10.0/24 LAN-based subnet. OSPF is fully working in the subnet. Later, R5, whose OSPF priority is higher than the other four routers, joins the subnet. Which of the following are true about the OSPF database exchange process over this subnet at this point? (Choose two.)
a. R5 will send its DD, LSR, and LSU packets to the 224.0.0.5 all-DR-routers multicast address.
b. R5 will send its DD, LSR, and LSU packets to the 224.0.0.6 all-DR-routers multicast address.
c. The DR will inform R5 about LSAs by sending its DD, LSR, and LSU packets to the 224.0.0.6 all-SPF-routers multicast address.
d. The DR will inform R5 about LSAs by sending its DD, LSR, and LSU packets to the 224.0.0.5 all-SPF-routers multicast address.

Answer: B and D. Because the subnet was stable before R5 arrived, the other routers will have elected a DR and BDR. OSPF does not preemptively elect a new DR nor BDR, so R5 will be neither (DROther). As a result, R5’s messages to the DR will be sent to the 224.0.0.6 all-DR-routers multicast address, and the DR’s messages directed to R5 will be sent to the 224.0.0.5 all-SPF-router address.

Q6. R1 is internal to area 1, and R2 is internal to area 2. Subnet 10.1.1.0/24 exists in area 2 as a connected subnet off R2. ABR ABR1 connects area 1 to backbone area 0, and ABR2 connects area 0 to area 2. Which of the following LSAs must R1 use when calculating R1’s best route for 10.1.1.0/24?
a. R2’s Type 1 LSA
b. Subnet 10.1.1.0/24’s Type 2 LSA
c. ABR1’s Type 1 LSA in area 0
d. Subnet 10.1.1.0/24’s Type 3 LSA in Area 0
e. Subnet 10.1.1.0/24’s Type 3 LSA in Area 1

Answer: E. R1, internal to area 1, can use LSAs only in the area 1 LSDB. R2’s Type 1 LSA exists only in area 2’s LSDB. The Type 2 LSA for subnet 10.1.1.0/24, if one exists, also only exists in area 2’s LSDB. R1 will use ABR1’s Type 1 LSA in area 1 to calculate the possible intra-area routes inside area 1, but R1 will use ABR1’s Type 1 LSA in area 1. Finally, the Type 3 LSA, created for 10.1.1.0/24, and flooded into area 1, is also needed to calculate the metric.

Figure: Comparing a Single Area LSDB to a Three Area LSDB

Q7. Which of the following LSA types describes topology information that, when changed, requires a router in the same area to perform an SPF calculation? (Choose two.)
a. 1
b. 2
c. 3
d. 4
e. 5
f. 7

Answer: A and B. OSPF builds the SPF tree based on the topology information Type 1 and Type 2 LSAs. Changes therefore require another SPF run. Changes to the other LSA types do not require an SPF calculation.

Q8. The following output was taken from Router R3. A scan of R3’s configuration shows that no bandwidth commands have been configured in this router. Which of the following answers lists configuration settings could be a part of a configuration that results in the following output? (Choose two.)

R3#show ip ospf interface brief
Interface  PID Area IP Address/Mask Cost State Nbrs F/C
Se0/0/0.2  3   34   10.10.23.3/29   647  P2P    1/1
Se0/0/0.1  3   34   10.10.13.3/29   1000 P2P    1/1
Fa0/0      3   34   10.10.34.3/24   20   BDR    1/1

a. An auto-cost reference-bandwidth 1000 command in router ospf mode
b. An auto-cost reference-bandwidth 2000 command in router ospf mode
c. An ip ospf cost 1000 interface S0/0/0.1 command in router ospf mode
d. An auto-cost reference-bandwidth 64700 command in router ospf mode

Answer: A and B. Because none of the interfaces have a bandwidth command configured, the only commands that can influence the OSPF cost are the auto-cost reference-bandwidth router subcommand and the ip ospf cost interface subcommand. To give the output shown in the question, either the interface cost could be set directly on all three interfaces listed. Alternatively, the reference-bandwidth could be set (in router configuration mode) to cause one of the interface costs to be as shown in the output, with the other two interfaces having their costs set directly.
For the wrong answers, the ip ospf cost interface s0/0/0.1 router subcommand does not exist—instead, it is an interface subcommand. An auto-cost of 64700, used as the numerator in the ref-bw/bandwidth cost calculation, does not result in any of the three listed interface costs.
For the two correct answers, with a default bandwidth of 1544 (Kbps) on the serial subinterfaces, a reference bandwidth of 1000 (Mbps) implies the math 1,000,000 / 1544, for an Interface cost of 647. With a default bandwidth of 100,000 Kbps (100 Mbps) on Fa0/0, a reference bandwidth of 2000 (MBps) implies math of 2,000 / 100 = 20.

Q9. Which of the following LSA types describe information related to topology or subnets useful for calculating routes for subnets inside the OSPF domain? (Choose three.)
a. 1
b. 2
c. 3
d. 4
e. 5
f. 7

Answer: A, B, and C. OSPF uses Types 1, 2, and 3 for calculating routes internal to the OSPF domain. OSPF uses types 4, 5, and 7 for external routes redistributed into the OSPF domain, as discussed in Chapter 9, “Basic IGP Redistribution.”