Virtual Links


Virtual Links

To define the virtual link, each router configures the other Router’s RID and a reference to the area through which the virtual link passes. The ABRs connected over a virtual link act mostly like any other ABR, with a couple of differences.
The first difference is that ABRs send all OSPF messages as unicasts to the IP address of the router on the other end of the link.
Second, the routers also mark the Do Not Age (DNA) bit in the LSAs, meaning that all routers on the other side of the virtual link will not expect the LSAs to be reflooded over the virtual link on the usual 30-minute refresh interval. This helps reduce overhead over the virtual link, which often runs over slower links and less-powerful routers. The router also assigns an OSPF cost to the virtual link, just as it would for an interface.

Most of the optional configuration settings relate to features that would normally be configured on the interface connecting two neighboring routers, but with a virtual link, there is no such interface, so the parameters must be added to the area virtual-link command.

 


The following list summarizes the key configuration options on the area virtual-link router subcommand:

The remote-RID in the area area-num virtual-link remote-RID command refers to the other router’s RID.

The area-num in the area area-num virtual-link remote-RID command refers to the transit area over which the packets flow between the two routers.

The transit area over which the two routers communicate must not be a stubby area.

The optional configuration of OSPF neighbor authentication parameters, normally configured as interface subcommands, must be configured as additional parameters on the area virtual-link command.

The optional configuration of Hello and Dead intervals, normally configured as inter-face subcommands, must be configured as additional parameters on the area virtual-link command.

The router assigns the virtual link an OSPF cost as if it were a point-to-point link. The router calculates the cost as the cost to reach the router on the other end of the link, as calculated using the transit area’s LSDB.

Note: OSPF does not require that the RID IP address range be advertised as a route in OSPF. As a result, the RID listed in the area virtual-link command may not be pingable, but the virtual link still work.

( I have configured virtual-link by the name of Router Ids that RouterIds are not configured on any interface\loopbacks).


Configuration for Virtual-Links

Router 4

router ospf 1
 router-id 4.4.4.4
 log-adjacency-changes
 area 511 virtual-link 2.2.2.2

Router 2

router ospf 1
 router-id 4.4.4.4
 log-adjacency-changes
 area 511 virtual-link 2.2.2.2

Where area 511 is the transit area for virtual link between Router2 and 4.

 
To configure the authentication on Virtual-Links

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