Virtual link

1.10 Troubleshoot OSPF (v2/v3)

1.10.c Network types, area types, and router types

📘CCNP Enterprise – ENARSI (300-410)

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OSPF Virtual Link (Exam Focus)

1. What is an OSPF Virtual Link?

• A Virtual Link is like a logical connection between two OSPF areas when a normal physical link does not exist.
• Specifically, it connects an OSPF area to the backbone area (Area 0) through another area.
• Remember: All OSPF areas must connect to Area 0. If an area is isolated from Area 0, OSPF cannot properly route between areas. A virtual link fixes this.

Key point for the exam:

Virtual links are only configured between ABRs (Area Border Routers).

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2. Why do we need a Virtual Link?

• OSPF requires all areas to communicate via Area 0 (backbone).
• Sometimes, due to network design or topology changes, an area might lose direct connectivity to Area 0.
• Instead of redesigning the network physically, a virtual link creates a tunnel through another area to reach Area 0.

IT environment example:

• You have a branch office in Area 2. Area 2 cannot connect directly to Area 0. But Area 1 is connected to both Area 2 and Area 0. A virtual link allows Area 2 to “tunnel” through Area 1 to reach Area 0.

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3. Key Rules of OSPF Virtual Links

1. Only between two ABRs.
2. Must traverse a non-backbone area (transit area).
3. Both ABRs must share a common area that is not stub or totally stubby.
4. The virtual link uses OSPF authentication and hello/dead timers—like normal OSPF adjacency.
5. Virtual links do not create a physical interface—they are logical.

Exam tip: Remember that you cannot create a virtual link through a stub or totally stubby area.

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4. How it Works

Think of a virtual link as a tunnel between two ABRs:

1. ABR1 in Area 0 + Area X
2. ABR2 in Area 0 + Area X
3. The tunnel (virtual link) passes through Area X.
4. ABR1 and ABR2 treat the virtual link as if Area 0 is directly connected, even though the traffic goes through Area X.

Exam diagrams often show this:

[Area 0]---ABR1       ABR2---[Area 2]
              \     /
              [Area 1]

• Area 2 connects to Area 0 logically via ABR1-ABR2 virtual link through Area 1.

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5. Virtual Link Configuration (Cisco IOS/IOS-XE)

Step 1: Identify the transit area.

• Must be a non-stubby area.

Step 2: Configure the virtual link on both ABRs.

On ABR1:

router ospf 1
area 1 virtual-link <ABR2-Router-ID>

On ABR2:

router ospf 1
area 1 virtual-link <ABR1-Router-ID>

• <ABR2-Router-ID> is the router ID of the remote ABR.

Exam tip: The virtual link uses the router IDs of ABRs, not IP addresses.

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6. Verification Commands

1. Check virtual link status:

show ip ospf virtual-links

• Look for up in state.

2. Check OSPF neighbor status through virtual link:

show ip ospf neighbor

• The virtual link creates an adjacency.

3. Check OSPF routes through virtual link:

show ip route ospf

• Ensure routes from disconnected area are visible.

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7. Important Exam Facts

• Virtual links must not pass through stub or totally stub areas.
• They use the same OSPF authentication and hello/dead timers as the OSPF process.
• They are logical connections, so no new physical interface is created.
• Often used during OSPF backbone failures or migration scenarios.

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✅ Summary Table for Exam

Feature	Details
Purpose	Connect non-backbone area to Area 0 logically
Type	Logical (tunnel-like)
Configured on	ABRs only
Can traverse	Only normal/non-stub areas
Config method	area <transit-area> virtual-link <remote-ABR-RID>
Verification	show ip ospf virtual-links, show ip ospf neighbor, show ip route ospf