Virtual Link And Transit Area For Ospf - Cisco NCS 6000 Series Configuration Manual

Virtual Link and Transit Area for OSPF

Virtual Link and Transit Area for OSPF
In OSPF, routing information from all areas is first summarized to the backbone area by ABRs. The same
ABRs, in turn, propagate such received information to their attached areas. Such hierarchical distribution of
routing information requires that all areas be connected to the backbone area (Area 0). Occasions might exist
for which an area must be defined, but it cannot be physically connected to Area 0. Examples of such an
occasion might be if your company makes a new acquisition that includes an OSPF area, or if Area 0 itself
is partitioned.
In the case in which an area cannot be connected to Area 0, you must configure a virtual link between that
area and Area 0. The two endpoints of a virtual link are ABRs, and the virtual link must be configured in both
routers. The common nonbackbone area to which the two routers belong is called a transit area. A virtual link
specifies the transit area and the router ID of the other virtual endpoint (the other ABR).
A virtual link cannot be configured through a stub area or NSSA.
Figure 16: Virtual Link to Area 0
This figure illustrates a virtual link from Area 3 to Area 0.
Passive Interface
Setting an interface as passive disables the sending of routing updates for the neighbors, hence adjacencies
will not be formed in OSPF. However, the particular subnet will continue to be advertised to OSPF neighbors.
Use the passive command in appropriate mode to suppress the sending of OSPF protocol operation on an
interface.
It is recommended to use passive configuration on interfaces that are connecting LAN segments with hosts
to the rest of the network, but are not meant to be transit links between routers.
Routing Configuration Guide for Cisco NCS 6000 Series Routers, IOS XR Release 6.4.x
268
Implementing OSPF