OSPF virtual-link

• Virtual-link are seem as point-to-point link
• Belonging  to area 0
• Base on the router-ids and intra-area shortest-path tree
• Virtual-link are only used to flood specific LASs: router, network, summary LSA => Stub Area.
• LSA type 5 flood to all area expect stub area. So no need to duplicate information across the virtual-link


• Chu y:  Rat quan trong, thuong xuyen gap trong CCIE Lab






R2#show run | se router ospf router ospf 1  log-adjacency-changes  area 1 virtual-link 3.3.3.3

R3#show run | se router ospf router ospf 1  log-adjacency-changes  area 1 virtual-link 2.2.2.2



R2 va R3 thiet lap neighbor qua virtual-link. Nhung neighbor se down khi truong hop tang cost duoi interface f0/1 (R2) va f0/0 (R3) qua max trong cmd " ip ospf cost 65535" thi neighbor se down.'


The cost of the virtual-link is calculated dynamically, based on the cost to transit the underlying intra-area topology. If this topology changes, so does the virtual-link cost. Sine virtual link is a normal OSPF link advertised in type-1 LSA, it could not have the cost above 65535. However, it is possible to the intra-area path to exceed this cost, e.g. as a result of cost manipulation or changing the reference bandwidth. In this situation, both virtual-link endpoints will declare it down and stop using. That is, if you have a virtual-link across the area, the maximum path cost in this area should not exceed 65535.