What Are OSPF Areas and Virtual Links?

Introduction
OSPF is an open standard link state protocol which uses Dijkstra algorithm. It is a classless routing protocol, supports VSLM and discontinuous networks by sending subnet mask information as part of a route advertisement. Its administrative distance value is 110. Its hop count is unlimited. It supports only equal cost load balancing. It provides loop free and faster convergence. It sends updates on multicast address on 224.0.0.5 for all OSPF routers and 224.0.0.6 for DR and BDR routers. This is a very important topic of CCNA  which is very important networking training in current IT industry.

Need of virtual-link

OSPF is a hierarchical routing protocol. It enables better administration and smaller routing tables due to segmentation of entire network in to areas. Routers that are grouped in one area are called internal routers. All interfaces on internal routers are directly connected to networks within an area. Within an area, routers must have an identical database.

 

One area, called the backbone area, must connect to all other areas. Packets that need to
pass between two non-backbone areas must pass through (at least) one backbone router. ABR router acts as the boundary router between the two areas where one of the areas should be a backbone area. ABR just advertises the briefer information (prefix, subnet mask length), not the full detailed information between areas. The ABRs must keep a copy of the LSDB for each area to which they attach.

 

Hence OSPF requires that all areas in must be physically connected to the backbone area (area 0). In large networks with many areas, in which direct connectivity to all areas with the backbone area is not possible and hence configuration of virtual links is done to connect non-backbone areas. Virtual links use a transit area that contains two or more area border routers (ABRs) to pass network traffic from one adjacent area to another area and hence the different non-backbone area information then eventually reaches the backbone area. The transit area must have full routing information and it cannot be a stub area.

 

Task

To create a virtual link between two ABRS, one ABR having connection to area 0 of backbone area and other ABR not connected to the backbone area.

Steps

(1)Configure IP addresses as shown in the topology

(2)Provide full connectivity using OSPF

(3)Without using virtual link R1 (area 0 backbone area) does not contain the routes of non-backbone area.

(4)Create a virtual link between R1 (area 0 backbone area) and R3 of same connected area

 

 

Observation

Due to the virtual link created between R1 and R3 ABR’s, all the R3 topology data and its connected interface routes are passed on to R1’s area 0 backbone router and can be seen to be present in R1 routing table.

Also Router 1 (Area 0 Backbone area) routes can be seen in R3 (Area1) routing table as can be seen below.

Conclusion

Using virtual link connecting to the routes of an area which are not connected to backbone area  is possible which is able to help in large networks where all the areas cannot be connected to backbone areas which the screenshots in observation section verifies. Using virtual link the routers both allow the use of the Do Not Age (DNA) bit, so periodic reflooding does not occur over this virtual link .Verification of virtual link using “show ip ospf virtual-links” can be seen from the below screenshot.

From the below two screenshot it as well can be seen that due to the creation of virtual link two area border routers  exists in both R1 and R3 which help in linking  Area 1 and Area 0 and helps in exchanging all the routes of these areas.

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