What should work when we finish?
These are the three routers from the static routing lab. This time, replace static routes with OSPF so that a new LAN is learned automatically. OSPFv2 exchanges IPv4 routing information. A single-areadesign puts all participating interfaces in one area, area 0 here. Each router uses the shared area topology information to calculate its own routes. Every listed interface is addressed and up before this lab. R1 already has a static default route to the ISP (ip route 0.0.0.0 0.0.0.0 203.0.113.1); no other static routes or routing protocols remain. Save a separate copy of your earlier static-routing exercise rather than overwriting it. Use an isolated Cisco IOS or IOS XE lab image, or Packet Tracer where these commands are supported. Paper practice can predict output but cannot verify execution.
PC-1 is 192.168.1.10/24 with default gateway 192.168.1.1. PC-2 is 192.168.2.10/24 with gateway 192.168.2.1. The server is 192.168.3.10/24 with gateway 192.168.3.1. Our first goal is a ping from PC-1 to that server and back. These are IP addresses, not website URLs; no DNS lookup or web application is required for this test.
| Router | Interface | Address | Connects to |
|---|---|---|---|
| R1 | Gi0/0 | 192.168.1.1/24 | LAN 1 |
| Gi0/1 | 10.0.12.1/30 | R2 | |
| Gi0/2 | 203.0.113.2/30 | ISP (203.0.113.1) | |
| R2 | Gi0/0 | 10.0.12.2/30 | R1 |
| Gi0/1 | 10.0.23.1/30 | R3 | |
| Gi0/2 | 192.168.2.1/24 | LAN 2 | |
| R3 | Gi0/0 | 10.0.23.2/30 | R2 |
| Gi0/1 | 192.168.3.1/24 | LAN 3 (server 192.168.3.10) |
Gi abbreviates GigabitEthernet. The /24 mask is 255.255.255.0; /30 is 255.255.255.252. The router IDs 1.1.1.1, 2.2.2.2 and 3.3.3.3 name the OSPF routers. They do not replace the interface addresses used to reach a neighbour.
The ISP link remains outside OSPF. R1 advertises a default route, meaning a route for destinations without a more-specific match. Reaching the ISP router alone does not prove internet access. An actual external test also needs an external target, provider return routing and any required NAT, none of which is built by this exercise.
Task 1: plan which interfaces run OSPF
Before typing anything, decide for each interface: does it run OSPF, and should it be passive? Pick the router IDs and the area. Then decide whether the router-to-router links need a DR.
On small screens, swipe sideways or focus the diagram and use the arrow keys.
Show the plan
On small screens, swipe sideways or focus the diagram and use the arrow keys.
Router IDs 1.1.1.1, 2.2.2.2 and 3.3.3.3, everything in area 0. Each /30 link has only two routers, so set the network type to point-to-point on both ends. A designated router (DR) helps coordinate OSPF information on a shared network. It is unnecessary on these two-router links, so this network type removes the election.
Task 2: configure the three routers
Enter configure terminal from privileged EXEC mode on the named router. Each block below starts in global configuration mode. The interface commands set the link type; the router commands create process 1 and select participating interfaces. Set the router ID before the network statements enable OSPF on those interfaces.
On small screens, swipe sideways or focus the diagram and use the arrow keys.
A network statement matches local interface addresses, not remote networks to be learned. Its wildcard mask marks which address bits may vary: 0.0.0.3 matches this /30, while 0.0.0.255 matches the /24. A passive interface still contributes its selected LAN to OSPF but does not send Hellos or build neighbours there.
Show the configuration
interface GigabitEthernet0/1
ip ospf network point-to-point
!
router ospf 1
router-id 1.1.1.1
auto-cost reference-bandwidth 10000
passive-interface GigabitEthernet0/0
network 192.168.1.0 0.0.0.255 area 0
network 10.0.12.0 0.0.0.3 area 0
default-information originateR1. No network statement matches 203.0.113.2, so OSPF stays off the ISP link. default-information originate advertises the existing static default route.
interface GigabitEthernet0/0
ip ospf network point-to-point
interface GigabitEthernet0/1
ip ospf network point-to-point
!
router ospf 1
router-id 2.2.2.2
auto-cost reference-bandwidth 10000
passive-interface GigabitEthernet0/2
network 10.0.12.0 0.0.0.3 area 0
network 10.0.23.0 0.0.0.3 area 0
network 192.168.2.0 0.0.0.255 area 0R2: two router links and one passive LAN.
interface GigabitEthernet0/0
ip ospf network point-to-point
!
router ospf 1
router-id 3.3.3.3
auto-cost reference-bandwidth 10000
passive-interface default
no passive-interface GigabitEthernet0/0
network 10.0.23.0 0.0.0.3 area 0
network 192.168.3.0 0.0.0.255 area 0R3, written the other way round: everything passive, then hellos switched back on towards R2.
The same reference bandwidth on all three routers keeps their costs comparable: 10 for each gigabit interface.
The transit interfaces must exchange Hellos; the host LANs must be advertised without building host-side neighbours. The reference bandwidth is 10 000 Mbps on every router. A gigabit interface with configured bandwidth 1000 Mbps therefore has cost 10, unless an explicit interface cost overrides it.
default-information originate requests an external default advertisement from R1 while R1 has an installed default route. It does not enable OSPF towards the ISP, and the existence of that static route is not an application-health check.
Task 3: verify, bottom-up
Check neighbours first, then routes. On R2 you expect two neighbours, both Full:
On small screens, swipe sideways or focus the diagram and use the arrow keys.
R2#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 3.3.3.3 0 FULL/ - 00:00:35 10.0.23.2 GigabitEthernet0/1 1.1.1.1 0 FULL/ - 00:00:38 10.0.12.1 GigabitEthernet0/0
FULL/ -: fully adjacent, and the dash means no DR or BDR role on a point-to-point link. Neighbor ID names the remote OSPF process; Address is the remote link IP. Interface is R2's local interface. The falling Dead Time is refreshed by valid Hellos. R1 and R3 each have only one transit neighbour.Then, on R3, inspect the OSPF-learned remote networks and default. R3's own connected LAN and transit network are not O entries and will not appear in this filtered command.
R3#show ip route ospf Gateway of last resort is 10.0.23.1 to network 0.0.0.0 O*E2 0.0.0.0/0 [110/1] via 10.0.23.1, 00:02:41, GigabitEthernet0/0 O 10.0.12.0/30 [110/20] via 10.0.23.1, 00:02:51, GigabitEthernet0/0 O 192.168.1.0/24 [110/30] via 10.0.23.1, 00:02:51, GigabitEthernet0/0 O 192.168.2.0/24 [110/20] via 10.0.23.1, 00:02:51, GigabitEthernet0/0
Puzzle: why is the default route's cost 1, not 21?
It is an E2 (external type 2) route. Its metric is the value R1 set when it advertised it (1 by default) and doesn't grow as it crosses the network. An E1 route would add the internal cost along the way.
Task 4: test from end to end
On PC-1, ping its gateway first, then 192.168.3.10. Check the return direction from the server as well. If the routers have routes but the host test fails, check the host gateway, LAN connection and ICMP permissions before changing OSPF. The following trace is a Windows-style command; other lab tools may use traceroute.
On small screens, swipe sideways or focus the diagram and use the arrow keys.
C:\> tracert -d 192.168.3.10 Tracing route to 192.168.3.10 over a maximum of 30 hops 1 1 ms 1 ms 1 ms 192.168.1.1 2 1 ms 1 ms 1 ms 10.0.12.2 3 2 ms 1 ms 1 ms 10.0.23.2 4 2 ms 2 ms 2 ms 192.168.3.10 Trace complete.
Task 5: three faults that stop a neighbourship
These are three useful lab faults. Pick each one and watch what happens on the R2–R3 link, then say which command shows it and how you would fix it.
On small screens, swipe sideways or focus the diagram and use the arrow keys.
The diagram choices simulate the symptoms; they do not alter a router. If you reproduce a fault on your own lab, start from the healthy saved state, change only the named interface and restore it before testing another fault. Otherwise several simultaneous faults can hide one another.
Device key: ISP (network / service) · R1, R2, R3 (router) · LAN 1, LAN 2, LAN 3 (network / site)
Lines show the example’s network connections. Thick links: uplinks/WAN, as labelled. The coloured path and envelope follow the current step; its text identifies the sender, receiver and traffic type.
Reading the addresses
IP labels identify interfaces or destinations. Subnet labels identify networks; /n is the prefix length. A router ID (RID) identifies a routing process and need not be a reachable interface.
An IP address alone is not a URL. A web URL includes a scheme such as https://; an address on a router or printer does not imply a website.
- ISP: IP 203.0.113.1
- R1: RID 1.1.1.1
- R2: RID 2.2.2.2
- R3: RID 3.3.3.3
- LAN 1: Subnet 192.168.1.0/24
- LAN 2: Subnet 192.168.2.0/24
- LAN 3: Subnet 192.168.3.0/24
- 1. Hellos both ways: each hello lists the neighbours the sender has heard, so both reach 2-Way.
- 2. Databases synchronised: they exchange database descriptions and any missing LSAs, then reach Full.
- 3. Routes installed: R3 learns 192.168.1.0/24 at cost 30 and can reach LAN 1.
Which command would you run first for each fault?
- Area mismatch:
show ip ospf interface briefon both routers: compare the Area column. - Passive link:
show ip protocols: the interface is under Passive Interface(s). - MTU mismatch: EXSTART or EXCHANGE directs you to database negotiation. Compare the IP MTU using
show ip interfaceon both ends and inspect the interface configuration. MTU is one possible cause, not a diagnosis from the state alone.
The MTU fault is the one that tends to eat time in real networks: the neighbour does appear, the hellos look healthy, and the problem only shows in the state column.
Optional: reproduce one fault on your isolated lab
These are three separate exercises. Do not paste all three fault blocks into one configuration. After each experiment, restore the healthy state and confirm the same two Full neighbours on R2 before continuing.
interface GigabitEthernet0/0
ip ospf 1 area 1Area fault on R3, starting in global configuration mode. Compare the interface's area with R2 Gi0/1. Restore the effective area using ip ospf 1 area 0 on R3 Gi0/0.
router ospf 1
passive-interface GigabitEthernet0/1Passive fault on R2. The transit link remains physically up, but Hellos stop. Restore with no passive-interface GigabitEthernet0/1 under this OSPF process.
interface GigabitEthernet0/0
ip mtu 1400MTU fault on R3. First confirm the healthy IP MTU is 1500 on both endpoints. After comparing negotiation states and IP MTUs, restore this lab's original default using no ip mtu and verify the actual value.
Bonus: a cost change that only works one way
On R2, you set ip ospf cost 100 on Gi0/1 (towards R3). Which router's routes change: R1's route to LAN 3, R3's route to LAN 1, or both?
Show answer
Of the two routes asked about, only R1's route to LAN 3 changes. R2's own route to LAN 3 changes too. Cost counts on the way out of an interface, and R2 Gi0/1 is outgoing only for traffic towards R3. R1's cost to 192.168.3.0/24 becomes 10 + 100 + 10 = 120. R3's cost to LAN 1 stays 30, because that traffic leaves R2 through Gi0/0.
Try the cost change only after the baseline works. Compare show ip route 192.168.3.10 on R1 and show ip route 192.168.1.10 on R3. Restore the calculated baseline with no ip ospf cost on R2 Gi0/1. This line topology has no alternate path, so changing a metric cannot create failover around a broken link.
Finish with evidence, not just saved commands
Record the neighbour counts, the cost-30 route from R3 to LAN 1 and the successful two-way host test. Only then save your lab with copy running-config startup-config. Keep deliberate fault snapshots separate from the healthy configuration.
Technical references: OSPFv2, RFC 2328, Cisco database-exchange troubleshooting and Cisco OSPF configuration questions. Output here is written for teaching; no running-device execution is claimed.