OSPF Virtual Link to Disconnected Area Not Working
Presenting Symptom
A router in area 1 cannot reach a router in area 0, even though OSPF is configured and neighbors are up.
Network Context
The network consists of three routers: R1 (area 0), R2 (area 0 and area 1), and R3 (area 1). R2 is an ABR connecting area 0 and area 1. Area 1 is a stub area. R3 is in area 1 and cannot communicate with R1 in area 0. All routers run IOS 15.x and are in a small branch office topology.
Diagnostic Steps
Check OSPF neighbors on R3
show ip ospf neighborNeighbor ID Pri State Dead Time Address Interface 10.0.0.2 1 FULL/DR 00:00:34 10.0.1.2 GigabitEthernet0/0
If neighbor state is FULL, the adjacency is established. If not, there is a Layer 2 or OSPF configuration issue. Here, neighbor is FULL, so the problem is not adjacency.
Check OSPF routes on R3
show ip route ospfO IA 10.0.0.0/24 [110/2] via 10.0.1.2, 00:10:00, GigabitEthernet0/0 O E2 0.0.0.0/0 [110/1] via 10.0.1.2, 00:10:00, GigabitEthernet0/0
R3 should have inter-area (IA) routes for area 0 networks. If only a default route (E2) appears, the ABR is not injecting inter-area routes. This suggests a virtual link issue if area 1 is not directly connected to area 0.
Check OSPF database on R3
show ip ospf database OSPF Router with ID (10.0.3.3) (Process ID 1)
Router Link States (Area 1)
Link ID ADV Router Age Seq# Checksum Link count
10.0.3.3 10.0.3.3 100 0x80000003 0x00A1B2 1
10.0.2.2 10.0.2.2 200 0x80000004 0x00C3D4 1
Summary Net Link States (Area 1)
Link ID ADV Router Age Seq# Checksum
10.0.0.0 10.0.2.2 300 0x80000001 0x00E5F6If there are no summary LSAs for area 0 networks, the ABR is not generating them. This could be due to a missing virtual link if area 1 is not directly connected to area 0.
Check OSPF virtual-link configuration on R2 (ABR)
show running-config | section router ospfrouter ospf 1 network 10.0.0.0 0.0.0.255 area 0 network 10.0.1.0 0.0.0.255 area 1 area 1 stub area 1 virtual-link 10.0.3.3
If the virtual-link command is missing or misconfigured, the ABR cannot communicate with the remote area. The virtual-link must be configured on both the ABR and the remote router (R3) using the router-id of the other end.
Check OSPF virtual-link status on R2
show ip ospf virtual-linksVirtual Link OSPF_VL0 to router 10.0.3.3 is down Run as demand circuit DoNotAge LSA allowed.
If the virtual link is down, check the neighbor reachability and authentication. The virtual link requires the transit area (area 1) to have full connectivity between the endpoints.
Root Cause
The OSPF virtual link between the ABR (R2) and the remote router (R3) is not configured or is misconfigured. Area 1 is a stub area, and the virtual link is required to allow R3 to receive inter-area routes from area 0. The virtual link must be configured on both routers with the correct router-id of the neighbor.
Resolution
Verification
On R2: show ip ospf virtual-links Expected output: Virtual Link OSPF_VL0 to router 10.0.3.3 is up On R3: show ip route ospf Expected output: O IA 10.0.0.0/24 [110/2] via 10.0.1.2, 00:00:10, GigabitEthernet0/0 Also verify that R3 can ping R1's loopback (e.g., 10.0.0.1).
Prevention
1. Always configure virtual links on both endpoints using the correct router-id. 2. Ensure the transit area (area 1) has full IP connectivity between the virtual link endpoints. 3. Avoid using stub areas as transit areas for virtual links unless necessary; consider using a regular area instead.
CCNA Exam Relevance
On the CCNA 200-301 exam, OSPF virtual link troubleshooting may appear in multiple-choice questions or drag-and-drop scenarios. The exam tests understanding of when a virtual link is needed (disconnected area) and the configuration requirements. Key fact: Virtual links require the router-id of the neighbor and must be configured on both routers.
Exam Tips
Remember that virtual links are configured in the transit area, not area 0.
The virtual link endpoint is identified by the router-id, not the IP address.
If a virtual link is down, check that both ends have the correct router-id and that the transit area has full connectivity.
Commands Used in This Scenario
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