IP Connectivity Fix · Medium ~15 min · 3 devices · 4 tasks · CCNA v1.1 and v2.0

OSPF triangle: two neighbors missing and a loopback not advertised

Three routers are wired in a triangle and run OSPF process 1. Right now only R1 and R2 are Full neighbors, and R3 has no route to R1’s loopback 1.1.1.1. All IP addresses and cables are correct. Three OSPF configuration mistakes are hiding on three different routers. Find and fix them without changing any IP address.

R1 Lo0 1.1.1.1/32 Gi0/0 10.0.12.1/30 to R2 Gi0/0 10.0.12.2/30 Lo0 2.2.2.2/32 R2
R1 Gi0/1 10.0.13.1/30 to R3 Gi0/0 10.0.13.2/30 Lo0 3.3.3.3/32 R3
R2 Gi0/1 10.0.23.1/30 to R3 Gi0/1 10.0.23.2/30

Topology

Tasks

○R1 and R3 are Full OSPF neighbors
○R2 and R3 are Full OSPF neighbors
○R3 learns 1.1.1.1/32 through OSPF
○ping 1.1.1.1 from R3 succeeds

Hints (open one at a time)

Hint 1: where to start

Run show ip ospf neighbor on every router. Note which pairs are missing, then compare the two ends of each broken link.

Hint 2: R1 to R3

On R3 run show ip ospf interface brief. Both ends of a link must be in the same OSPF area, and R1 has 10.0.13.0/30 in area 0.

Hint 3: R2 to R3

On R2 run show running-config (press Space at --More--, or run terminal length 0 first) and read the router ospf 1 section. A passive interface sends no hellos, so it never forms a neighbor.

Hint 4: the loopback

Run show ip route ospf on R3. If 1.1.1.1/32 is missing, check whether R1 has a network statement that covers Lo0.

Full solution

! R3: move the R1 link into area 0
R3# configure terminal
R3(config)# router ospf 1
R3(config-router)# no network 10.0.13.0 0.0.0.3 area 1
R3(config-router)# network 10.0.13.0 0.0.0.3 area 0
R3(config-router)# end

! R2: stop suppressing hellos toward R3
R2# configure terminal
R2(config)# router ospf 1
R2(config-router)# no passive-interface GigabitEthernet0/1
R2(config-router)# end

! R1: advertise the loopback
R1# configure terminal
R1(config)# router ospf 1
R1(config-router)# network 1.1.1.1 0.0.0.0 area 0
R1(config-router)# end

R3# ping 1.1.1.1

Neighbors are recalculated after every command. Both ends of a link need matching areas, and neither end can be passive.

Stuck on a lab, or not sure what to practice next?

Talk it through with a mentor on a free 20-minute call. We look at your lab, your exam date and your weak spots, and you leave with a plan.

Book a free session

Cisco Mentorship

CompTIA Foundation

Firewall Security

Short Courses

More

CCNA

Network+

NSE 4

CCNP

OSPF triangle: two neighbors missing and a loopback not advertised

Stuck on a lab, or not sure what to practice next?