WHAT ARE OSPF AREAS AND VIRTUAL LINKS 2023

WHAT ARE OSPF AREAS AND VIRTUAL LINKS 2023OSPF is an open standard link state protocol that uses Dijkstra’s algorithm.

The need for a virtual connection WHAT ARE OSPF AREAS AND VIRTUAL LINKS 2023:

It is a classless routing protocol, supports VSLM and discontinuous networks by sending subnet mask information as part of a routing advertisement. Its administrative distance value is 110.

The number of jumps is unlimited. It only supports equal cost balancing. It provides loop-free and faster convergenceThe need for a virtual connection WHAT ARE OSPF AREAS AND VIRTUAL LINKS 2023:.

It sends updates on the multicast address to 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 a very important networking training in today’s IT industry.

OSPF is a hierarchical routing protocol. Enables better management and smaller routing tables by segmenting the entire network into areas The need for a virtual connection WHAT ARE OSPF AREAS AND VIRTUAL LINKS 2023:.

Routers that are grouped together in one area are called internal routers.

All interfaces on the internal routers are directly connected to networks in the area. Within an area, routers must have an identical database.

One area, called the backbone area, must connect to all the other areas. Packages they need
the path between two non-backbone areas must go through (at least) one backbone router The need for a virtual connection WHAT ARE OSPF AREAS AND VIRTUAL LINKS 2023:.

An ABR router acts as a border router between two areas where one of the areas should be the backbone area.

ABR only advertises more concise information (prefix, subnet mask length), not full detailed information between areas. ABRs must maintain a copy of the LSDB for each area they connect to The need for a virtual connection WHAT ARE OSPF AREAS AND VIRTUAL LINKS 2023:.

Therefore, OSPF requires that all areas in must be physically connected to the backbone area (Area 0).

In large networks with many areas where direct connectivity to all backbone areas is not possible and therefore virtual links are configured to connect non-backbone areas The need for a virtual connection WHAT ARE OSPF AREAS AND VIRTUAL LINKS 2023:.

Virtual links use a transit area that contains two or more area border routers (ABRs) to forward network traffic from one neighboring area to another area, and therefore different information about the non-backbone area will eventually reach the backbone area. The transit area must have complete routing information and cannot be a stub.

Task

To create a virtual connection between two ABRSs, one ABR has a connection to Region 0 of the backbone and the other ABR is not connected to the backbone.

Steps

(1)Configure IP addresses according to the topology

(2)Provide full connectivity using OSPF

(3)Witout the use of virtual link R1 (area 0 backbone area) does not contain non-backbone area routes.

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

Observation

With the virtual link established between the R1 and R3 ABRs, all of R3’s topology data and its attached interface routes are forwarded to R1’s Area 0 backbone router and can be seen to be present in R1’s routing table.

Also, the routes of router 1 (backbone area 0) can be seen in the routing table of R3 (area 1) as seen below.

Conclusion

Using a virtual link, it is possible to connect to area routes that are not connected to the backbone area, which can help in large networks where not all areas can be connected to the backbone area, as verified by the screenshots in the observation section The need for a virtual connection WHAT ARE OSPF AREAS AND VIRTUAL LINKS 2023:.

Using a virtual link, both routers allow the use of the Do Not Age (DNA) bit, so that there is no periodic re-congestion on that virtual link. Verifying the virtual link using “show ip ospf virtual-links” can be seen in the screenshot below The need for a virtual connection WHAT ARE OSPF AREAS AND VIRTUAL LINKS 2023:.

It can also be seen from the two screenshots below that by creating a virtual link, there are two area border routers in both R1 and R3 that help connect Area 1 and Area 0 and help exchange all the routes of those areas The need for a virtual connection WHAT ARE OSPF AREAS AND VIRTUAL LINKS 2023:.

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Virtual links are used to extend Area 0 through another area. They can also be considered as tunnels for LSAs. They are deployed in cases where areas are split or the area does not border area 0. To configure a virtual link, use the following router command The need for a virtual connection WHAT ARE OSPF AREAS AND VIRTUAL LINKS 2023::

Router(config-route)#area “transit_area_id” virtual link “router_id_of_remote”

Area_id is the “transit area” that OSPF will tunnel through. The “transit area” cannot be a scrap. At the end of the tunnel will be another router terminating the virtual line; use the router ID of this router in the router_id field. Virtual links use RIDs and are another reason why we use fixed RIDs when configuring OSPF. Remember that a virtual link is actually an extension of Area 0. Think of the virtual link as a new router interface to Area 0. Configure all the interface options on the virtual link.

OSPF Virtual Link Configuration: Example Scenario
The example below will help illustrate where, when and how to use OSPF virtual link configuration in an OSPF domain The need for a virtual connection WHAT ARE OSPF AREAS AND VIRTUAL LINKS 2023: –

As shown in the diagram, we have 3 areas connected in series – Area 0 , Area 1 and Area 2 .

The IP address, subnet and interface detail in the relevant area are listed in the –ospf-virtual-link table
Once OSPF is configured according to the above table, we can see the OPSF adjacency established between all OSPF speaking routers R1, R2, R3 and R4:ospf-virtual-link

Now let’s see how many routes R1 learns from its OSPF neighbor R2:ospf-virtual-link
As we can see, R1 is not learning any inter-area (OIA) route. The reason – the OSPF rule book says that a non-spine area cannot learn OSPF routes from another non-spine area, which means that R1 (in area 2) cannot directly learn the OSPF routes of area 1 (which is again another non-spine area).

Also, let’s see if R3 is learning Area 2:ospf-virtual-link routes

And as expected, R3 did not learn any routes for networks in Area 2

Now how to fix this situation and have OSPF have R1 to learn routes from Area 1 and Area 0.

The solution is a Virtual Link – We need to extend the backbone (Area 0 ) via a virtual link up to ABR Router R2. Note – A virtual link is a logical link using the least expensive path between the ABR of the non-backbone interconnected area and the backbone ABR of the transit area.

The configuration must be done on the ABR routers of the transit area, i.e. between R2 and R3 as follows:ospf-virtual-link

Now let’s see what the ospf neighbor card looks like after configuring a virtual link between ABR R2 and R3:ospf-virtual-link
Finally, it’s time to see if R1 (Area 2) is accepting all routes after configuring a virtual link between R2 and R3 (ABR to Transit Area 1).

ospf-virtual-link
Virtual link successfully extends area 0 to area 2 (via transit area 1)ospf-virtual-link
Routers now have a complete routing table.

Another scenario where we need to implement a virtual link –

In the above topology, area 0 was divided into 2 parts and separated by area 1.

On the left side, R1 and R2 are in area 0, and on the right side, R3 and R4 are again in area 0, separated by area 1 configured on the directly connected interfaces R2 and R3.

Because region 0 should be a single backbone region unlike the above diagram. So to make it a single backbone domain, a virtual link between R2 and R3 needs to be configured. Once this configuration is done (ie one backbone area is configured), the OSPF domain will have no challenge and the database table will be consistent.

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