Exam 350-401 topic 1 question 410 discussion

B is correct. When OSPF is running on a given loopback interface, it sees the network type of "LOOPBACK" and it knows that it can not establish an adjacency through that loopback interface with another router so it advertises that loopback as a host route or the only IP address on that logical interface. As Martin explained so nicely you can tell OSPF that it is not a loopback and it is a point-to-point network type, and OSPF says "OK I am going to advertise the interface with its correct mask". In MPLS environment where OSPF is the IGP in the core, and you have configured the loopback interface of the PE router with a /24 mask, you can have some problems, because there is a discrepancy, the loopback’s mask is 24 but OSPF is advertising a mask of /32. There are few solutions to fix this problem, one solution is to reconfigure the mask to be /32, another solution is to configure the loopback interface with “ip ospf network point-to-point”.

test(config)#int loopback 99 test(config-if)#ip add 10.99.1.1 255.255.255.0 test(config-if)#ip ospf network broadcast % OSPF: Invalid type for interface Loopback99 test(config-if)#ip ospf network non-broadcast % OSPF: Invalid type for interface Loopback99 test(config-if)# test(config-if)#ip ospf network point-to-multipoint % OSPF: Invalid type for interface Loopback99 test(config-if)#ip ospf network point-to-point ! interface Loopback99 ip address 10.99.1.1 255.255.255.0 ip ospf network point-to-point end

Loopback interfaces are normally treated as host routes (/32) in OSPF. If you want to advertise the full /24 subnet, you need to override the default behavior. The key is the command: ip ospf network broadcast This makes OSPF treat the loopback interface as a broadcast network, rather than a loopback, which allows it to advertise the subnet with the interface's configured mask (in this case, /24).

R2(config)#interface Loopback0 R2(config-if)#ip address 172.22.2.1 255.255.255.0 R2(config-if)#ip ospf network point-to-point R2(config-if)#ip ospf 100 area 0 The "point-to-point" network type will ensure that the loopback interface is advertised with its configured subnet mask, which is /24 in this case.

A. R2(config)#interface Loopback0 R2(config-if)#ip address 172.22.2.1 255.255.255.0 R2(config-if)#ip ospf network broadcast R2(config-if)#ip ospf 100 area 0 B. R2(config)#interface Loopback0 R2(config-if)#ip address 172.22.2.1 255.255.255.0 R2(config-if)#ip ospf network point-to-point R2(config-if)#ip ospf 100 area 0 C. R2(config)#interface Loopback0 R2(config-if)#ip address 172.22.2.1 255.255.255.0 R2(config-if)#ip ospf network point-to-multipoint R2(config-if)#router ospf 100 D. R2(config-router)#network 172.22.2.0 0.0.0.255 area 0 R2(config)#interface Loopback0 R2(config-if)#ip address 172.22.2.1 255.255.255.0 R2(config-if)#ip ospf 100 area 0

cisco_R5(config-if)#do s runn int loo1 Building configuration... Current configuration : 113 bytes ! interface Loopback1 ip address 55.0.0.5 255.255.255.0 ip ospf network point-to-point ip ospf 1 area 22 end cisco_R5(config-if)# far-end before " ip ospf network point-to-point" see netw mask has been changed cisco_R3(config-subif)#do s ip ro os | b Gate Gateway of last resort is not set 55.0.0.0/32 is subnetted, 1 subnets <<<<<<<<<<<<<<<<<<<<<<<<<<< O 55.0.0.5 [110/11] via 10.111.10.2, 00:00:11, Ethernet0/0.50 cisco_R3(config-subif)# after: cisco_R3(config-subif)#do s ip ro os | b Gate Gateway of last resort is not set 55.0.0.0/24 is subnetted, 1 subnets <<<<<<<<<<<<<<<<< O 55.0.0.0 [110/11] via 10.111.10.2, 00:00:06, Ethernet0/0.50 cisco_R3(config-subif)#

B is correct

answer is D

Given answer is correct