CCNA 3- ENSA Module 2 - OSPFv2

What will an OSPF router prefer to use first as a router ID?

• any IP address that is configured using the router-id command

• 
  a loopback interface that is configured with the highest IP address on the router

• 
  the highest active interface IP that is configured on the router

• 
  the highest active interface that participates in the routing process because of a specifically configured network statement

Answer explanation

• The first preference for an OSPF router ID is an explicitly configured 32-bit address. This address is not included in the routing table and is not defined by the network command. If a router ID that is configured through the router-id command is not available, OSPF routers next use the highest IP address available on a loopback interface, as loopbacks used as router IDs are also not routable addresses. Lacking either of these alternatives, an OSPF router will use the highest IP address from its active physical interfaces.

What are the two purposes of an OSPF router ID? (Choose two.)

• • to facilitate the establishment of network convergence

  
  

• • to uniquely identify the router within the OSPF domain

  
  

• • to facilitate the transition of the OSPF neighbor state to Full

  
  

• • to facilitate router participation in the election of the designated router

  
  

• • to enable the SPF algorithm to determine the lowest cost path to remote networks

  
  

Refer to the exhibit. If no router ID was manually configured, what would router Branch1 use as its OSPF router ID?

• 10.0.0.1

• • 10.1.0.1

  
  

• • 192.168.1.100

  
  

• • 209.165.201.1

  
  

Answer explanation

• In OSPFv2, a Cisco router uses a three-tier method to derive its router ID. The first choice is the manually configured router ID with the router-id command. If the router ID is not manually configured, the router will choose the highest IPv4 address of the configured loopback interfaces. Finally if no loopback interfaces are configured, the router chooses the highest active IPv4 address of its physical interfaces.

A network technician issues the following commands when configuring a router:
R1(config)# router ospf 11
R1(config-router)# network 10.10.10.0 0.0.0.255 area 0

What does the number 11 represent?

• the autonomous system number to which R1 belongs

• the area number where R1 is located

• 
  the cost of the link to R1

• the OSPF process ID on R1

• 
  the administrative distance that is manually assigned to R1

Answer explanation

• There is no autonomous system number to configure on OSPF. The area number is located at the end of the network statement. The cost of a link can be modified in the interface configuration mode. The process ID is local to the router.

An OSPF router has three directly connected networks; 172.16.0.0/16, 172.16.1.0/16, and 172.16.2.0/16. Which OSPF network command would advertise only the 172.16.1.0 network to neighbors?

• • router(config-router)# network 172.16.1.0 0.0.255.255 area 0

  
  

• • router(config-router)# network 172.16.0.0 0.0.15.255 area 0

  
  

• • router(config-router)# network 172.16.1.0 255.255.255.0 area 0

  
  

• • router(config-router)# network 172.16.1.0 0.0.0.0 area 0
    

  
  

Answer explanation

• To advertise only the 172.16.1.0/24 network the wildcard mask used in the network command must match the first 24-bits exactly. To match bits exactly, a wildcard mask uses a binary zero. This means that the first 24-bits of the wildcard mask must be zero. The low order 8-bits can all be set to 1.

Which command will a network engineer issue to verify the configured hello and dead timer intervals on a point-to-point WAN link between two routers that are running OSPFv2?

• • show ip ospf neighbor

  
  

• • show ip ospf interface serial 0/0/0

  
  

• • show ipv6 ospf interface serial 0/0/0

  
  

• • show ip ospf interface fastethernet 0/1

  
  

Answer explanation

• The show ip ospf interface serial 0/0/0 command will display the configured hello and dead timer intervals on a point-to-point serial WAN link between two OSPFv2 routers. The show ipv6 ospf interface serial 0/0/0 command will display the configured hello and dead timer intervals on a point-to-point serial link between two OSPFv3 routers. The show ip ospf interface fastethernet 0/1 command will display the configured hello and dead timer intervals on a multiaccess link between two (or more) OSPFv2 routers. The show ip ospf neighbor command will display the dead interval elapsed time since the last hello message was received, but does not show the configured value of the timer.

Refer to the exhibit. Which three statements describe the results of the OSPF election process of the topology that is shown in the exhibit? (Choose three.)

• R2 will be elected DR.

• • R1 will be elected BDR.

  
  

• 
  R3 will be elected BDR.

• 
  The R4 router ID is 172.16.1.1.

• • The router ID on R2 is the loopback interface.

  
  

Answer explanation

• R2 will be elected DR because it has the highest priority of 255, all of the others have a priority of 1. R3 will be elected BDR because it has the numerically highest router-ID of 192.168.1.4. The R4 router-ID is 172.16.1.1 because it is the IPv4 address attached to the loopback 0 interface.

Refer to the exhibit. Suppose that routers B, C, and D have a default priority, and router A has a priority 0. Which conclusion can be drawn from the DR/BDR election process?​

• • Router A will become the DR and router D will become the BDR.​

  
  

• • If the DR fails, the new DR will be router B.

  
  

• • If a new router with a higher priority is added to this network, it will become the DR.

  
  

• • If the priority of router C is changed to 255, then it will become the DR.

  
  

Refer to the exhibit. If the switch reboots and all routers have to re-establish OSPF adjacencies, which routers will become the new DR and BDR?

• • Router R4 will become the DR and router R1 will become the BDR.

  
  

• • Router R2 will become the DR and router R3 will become the BDR.

  
  

• • Router R1 will become the DR and router R2 will become the BDR.

  
  

• • Router R4 will become the DR and router R3 will become the BDR.

  
  

Answer explanation

• OSPF elections of a DR are based on the following in order of precedence:

  • highest pritority from 1 -255 (0 = never a DR)

  • highest router ID

  • highest IP address of a loopback or active interface in the absence of a manually configured router ID. Loopback IP addresses take higher precedence than other interfaces.

  In this case routers R4 and R1 have the highest router priority. Between the two, R3 has the higher router ID. Therefore, R4 will become the DR and R1 will become the BDR.

By default, what is the OSPF cost for any link with a bandwidth of 100 Mb/s or greater?

• 1

• 100

• 10000

• 100000000

Answer explanation

• OSPF uses the formula: Cost = 100,000,000 / bandwidth. Because OSPF will only use integers as cost, any bandwidth of 100 Mb/s or greater will all equal a cost of 1.