Introduction to OSPF Virtual Links

Introduction to OSPF Virtual Links

OSPF (Open Shortest Path First) is a routing protocol that establishes routing paths within a network. In an OSPF network, regions are used to organize logically related routers. Ideally, all areas should be physically connected to the backbone (Area 0), which serves as a centralized point for routing information.

⭐Related : Top OSPF Protocol Interview Questions and Answers
⭐Related : OSPF protocol : OSPF Packet Types

 What is OSPF Virtual Links ?

As we said that every area inside an OSPF autonomous system needs to be physically linked to the area 0 backbone. There may be circumstances, though, in which this physical link isn't possible. Herein comes the function of OSPF virtual links. A logical connection created between two areas via a transit area is known as an OSPF virtual link

OSPF Virtual Links
Fig 1.1- OSPF Virtual Links

Configuration on Router R1

NDNA_R1# configure terminal
NDNA_R1(config)# router ospf 1
NDNA_R1(config)#area 0
NDNA_R1(config)#area 0 virtual-link
NDNA_R1(config)#area 0 virtual-link authentication-key NDNA

Configuration on Router R2

NDNA_R2# configure terminal
NDNA_R2(config)# router ospf 1
NDNA_R2(config)# area 1
NDNA_R2(config)# area 1 virtual-link
NDNA_R2(config)# area 1 virtual-link authentication-key NDNA
NDNA_R2(config)# exit

 Purpose of OSPF virtual links

⭐1. When two areas are geographically separated and a direct physical connection isn't possible, virtual links can be used to create a logical path for routing information exchange.

⭐2. In large networks, using a single physical backbone area may become complicated. Virtual links can contribute to a more scalable network design by enabling logical connections between non-backbone locations.

⭐3. If the backbone area loses physical connectivity, virtual links can be employed to bridge the gap and maintain communication between the partitioned areas.

 How Virtual links work in OSPF ?

A router in the backbone area must function as the transit router and one of the other routers must be designated as the virtual link endpoint in order to create a virtual connection. After that, the virtual adjacency between the virtual link endpoint router and the transit router is established, enabling the flow of routing data over the backbone area.

  • ⭐Area Border Router (ABR): The router responsible for connecting a non-backbone area to the backbone or another area. One ABR acts as the virtual link endpoint.
  • ⭐Transit Area: The area through which the virtual link is established. This area must have full routing information and cannot be a stub area.
  • ⭐Transit Router: A router in the transit area that the virtual link endpoint communicates with. It acts as a bridge for routing information exchange (it may or may not be there).

The ABR in the non-backbone area is configured with the area virtual-link command, specifying the area it belongs to and the router ID of the transit router in the backbone area. The ABR initiates communication with the transit router, forming a virtual adjacency. This allows them to exchange OSPF routing messages.

The ABR sends its Link State Advertisements (LSAs), containing routing information about its connected networks, to the transit router. The transit router floods these LSAs throughout the transit area, making them accessible to the other ABR. The other ABR receives the LSAs through the virtual link and updates its routing table accordingly. This allows it to learn about networks reachable through the virtual connection.

⭐Related : OSPF NSSA Area introduction and Configuration

 Important points to consider

  • Virtual links only transmit OSPF control traffic (LSAs), not user data.
  • The cost of the virtual link is defined by the path to the transit router in the transit zone.
  • LSAs learned over a virtual link are marked as "Do Not Age," which means they will not be removed from the routing table unless explicitly withdrawn.
  • The non-backbone area's ABR becomes an ABR as a result of its connection to the virtual link.

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