OSPF Area 0

control

In an OSPF network, do ALL areas need to have a router interface attached to Area 0? Or is it possible to have say Area 1, which has an Interface in Area 0, but then say an area 5 which is connected off of Area 1 (but has not interface in Area 0)?

Roguetadhg

Areas Not Physically Connected to Area 0

As mentioned earlier, area 0 has to be at the center of all other areas. In some rare case where it is impossible to have an area physically connected to the backbone, a virtual link is used. The virtual link will provide the disconnected area a logical path to the backbone. The virtual link has to be established between two ABRs that have a common area, with one ABR connected to the backbone. This is illustrated in the following example:

In this example, area 1 does not have a direct physical connection into area 0. A virtual link has to be configured between RTA and RTB. Area 2 is to be used as a transit area and RTB is the entry point into area 0. This way RTA and area 1 will have a logical connection to the backbone. In order to configure a virtual link, use the area <area-id> virtual-link <RID> router OSPF sub-command on both RTA and RTB, where area-id is the transit area. In the above diagram, this is area 2. The RID is the router-id. The OSPF router-id is usually the highest IP address on the box, or the highest loopback address if one exists. The router-id is only calculated at boot time or anytime the OSPF process is restarted. To find the router-id, use theshow ip ospf interface command. Assuming that 1.1.1.1 and 2.2.2.2 are the respective RIDs of RTA and RTB, the OSPF configuration for both routers would be:

RTA#router ospf 10area 2 virtual-link 2.2.2.2RTB#router ospf 10area 2 virtual-link 1.1.1.1



Source: http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094e9e.shtml

control

That is a great explanation!

Thanks a lot for that

iamme4eva

Virtual links won't be covered at CCNA level though. And, they aren't recommended. You should try and design your network to avoid them.