Introduction to OSPF Digital Hyperlink
Digital hyperlinks are used to increase Space 0 throughout one other space. Additionally they may be regarded as tunnels for LSAs. They’re deployed in instances when areas grow to be partitioned or an space doesn’t border Space 0. To configure a digital hyperlink, use the next router command:
Router(config-route)#space “transit_area_id” virtual-link “router_id_of_remote”
The area_id is the “transit space” that OSPF will tunnel by. The “transit space” can’t be a stub space of any type. On the finish of the tunnel will likely be one other router terminating the digital hyperlink; use the router ID of that router within the router_id subject. Digital hyperlinks use RIDs, and they’re one more reason why we use fastened RIDs when configuring OSPF. Keep in mind that a digital hyperlink is definitely an extension of Space 0. Consider the digital hyperlink because the router’s new interface into Space 0. Configure all interface choices on the digital hyperlink.
OSPF digital hyperlink configuration : Instance Situation
The Beneath instance situation will assist illustrate the place , when and tips on how to use OSPF digital hyperlink configuration within the OSPF area –
As depicted within the diagram , now we have 3 areas linked in sequence – Space 0 , Space 1 and Space 2.
The IP deal with , subnets and interface element in respective space is proven within the desk –
As soon as the OSPF is configured as per above desk , we are able to see the OPSF neighborship fashioned amongst all of the OSPF talking routers R1 , R2, R3 and R4: 
Now Lets see , what number of routes R1 learns from its OSPF neighbor R2:
As we see, no Inter space (OIA) route is being learnt by R1 . The explanation – OSPF rule ebook says , a non-backbone space can’t be taught OSPF routes from one other non spine space , which suggests R1 (in Space 2) can’t be taught OSPF routes of Space 1 (which once more is one other non-backbone space) instantly.
Additionally , lets see whether or not R3 learns Space 2 routes: 
And as anticipated , no routes discovered by R3 for networks in Space 2
Now , how can we right this case and have OSPF to have R1 to be taught routes from Space 1 and Space 0.
The answer is Digital Hyperlink – We have to lengthen the Spine (Space 0 ) by way of digital hyperlink as much as ABR Router R2.Word – Digital hyperlink is a logical hyperlink utilizing the least value path between the ABR of the non-backbone linked space and the spine ABR of the transit space.
The configuration must be carried out within the transit space ABR routers ie between R2 and R3 as follows: 
Now , lets see how the ospf neighbor tabe appears like after Digital hyperlink is configured between ABRs R2 and R3: 
Lastly , its time to see whether or not R1 (Space 2) receives all of the routes publish configuration of digital hyperlink between R2 and R3 (ABRs on transit Space 1).
The digital hyperlink efficiently extends Space 0 as much as Space 2 (by way of transit space 1)
The Routers now have the entire routing desk.
One other situation the place we have to implement digital hyperlink –
Within the above topology, Space 0 has been break up into 2 components and separated by Space 1.
On the left aspect, R1 & R2 are in Space 0 and on the precise aspect, R3 & R4 are once more in Space 0, separated by Space 1 configured on instantly linked interfaces of R2 & R3.
Since, Space 0 ought to be single spine space in contrast to the above diagram. So, so as to render it a single spine area, a digital hyperlink must be configured between R2 & R3. As soon as this configuration is completed (i.e. single spine space is configured) OSPF area is not going to have any problem and the database desk can be constant.
Proceed Studying:
OSPF LSA Varieties: The Final Information
