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9. OSPF VIRTUAL LINK

HOW TO OSPF VIRTUAL LINK CONFIGURATION
 
Interface IP Address Configuration  
  
R1  
  
Interface  IP Address    Subnet Mask  
S0/2/0      1.1.1.1            255.0.0.0  
E 0           10.1.1.1          255.0.0.0  
  
R2  
  
Interface   IP Address    Subnet Mask  
S0             1.1.1.2            255.0.0.0  
S1             2.2.2.1            255.0.0.0  
E0             20.1.1.1          255.0.0.0  
  
R3  
  
Interface   IP Address    Subnet Mask  
S0             2.2.2.2            255.0.0.0  
E 0            30.1.1.1          255.0.0.0  
  
Task 1 :  
  
Configure OSPF in Area 0 on R1 ( S0/2/0, E0 ), R2 ( S0, E0 ).  
Configure OSPF in Area 1 on R2 ( S1 ), R3 ( S0 ).  
Configure OSPF in Area 2 on R3 ( E0 ).
 
R1                                                                 
                                                                   
Router ospf 1                                                      
Network 1.1.1.1 0.0.0.0 area 0                                   
Network 10.0.0.0 0.255.255.255 area 0                                 
 
R2
 
Router ospf 1
Network 1.1.1.2 0.0.0.0 area0  
Network 2.2.2. 1 0.0.0.0 area1
Network 20.0.0.0 0.255.255.255 area 0
                                                                    
R3  
  
Router ospf 1  
Network 2.2.2.2 0.0.0.0 area1  
Network 30.0.0.0 0.255.255.255 area 2  
  
  
Verification:  
  
Verifying the routing table on R1 in area0:  
  
R1#show ip route  
  
The output displays net 20.0.0.0 as ‘O’ and net 2.0.0.0 as ‘O IA’, but there is no net  
30.0.0.0, as it is not connected to area0.   
  
We need to configure virtual links between R2 & R3 and  this area that  connects to area0  
is called the transit area.   
  
Each router R2 & R3 point at the router-id of the other router.
 
 
Task 2:  
  
Configure Virtual Link between R2 and R3:  
  
R2                                                                    
                                                                      
Router ospf 1                                                        
Area 1 virtual-link 30.1.1.1                                          
 
R3  
 
Router ospf 1 
Area 1 virtual-link 20.1.1.1
 
  
Verifying the routing table on R1 :  
  
R1#show ip route  
  
The output displays network 30.0.0.0 as ‘O’ route because of the virtual link configured,  
the router1 assumes that net 30.0.0.0 is in the same area0.  
  
R2#show ip ospf virtual-links  
  
The output displays virtual-link to other router and as well, ‘DoNotAge’ option set.
 
 
Task 3: Configure Virtual Link when area connecting two backbone areas.  
  
(Scenario Based on Task 1)  
  
Configure OSPF in Area 0 on R1 ( E0 ).  
Configure OSPF in Area 1 on R1 ( S0/2/0 ), R2 ( S0 ).  
Configure OSPF in Area 2 on R2 ( E0, S1 ), R3 ( S0, E0 ).   
  
R1                                                                
                                                                  
Router ospf 1                                                     
Network 1.1.1.1 0.0.0.0 area 1                                    
Network 10.0.0.0 0.255.255.255 area 0                             
                                                                  
 
R2
 
Router ospf 1
Network 1.1.1.2 0.0.0.0 area 1
Network 2.2.2.1 0.0.0.0 area 0
Network 20.0.0.0 0.255.255.255 area 0
                                                                  
R3  
  
Router ospf 1  
Network 2.2.2.2 0.0.0.0 area 0  
Network 30.0.0.0 0.255.255.255 area 0 
 
Verification:  
  
Verify the routing table on R1  
  
R1#show ip route  
  
The output displays network 2.0.0.0, 20.0.0.0 and 30.0.0.0 as O IA routes.  
  
R2#show ip route  
  
R3#show ip route  
  
When we check the routing table on R3, the output does not have network 10.0.0.0 in the  
routing table.  
  
 
Task 4 :  
  
Configure Virtual Link between R1 and R2 :  
  
R1                                                                     
                                                                       
Router ospf 1                                                         
Area 1 virtual-link 20.1.1.1                                          
 
R2
 
Router ospf 1
Area 1 virtual-link 10.1.1.1
 
 
After Configuring Virtual Link:  
  
R1#show ip route  
  
R2#show ip route  
  
R3#show ip route  
  
Now, when we verify the routing table on R1, R2, R3, we see that all O IA routes are  
advertised as ‘O’ routes as the routers assume that the networks belong to the same area  
because of the virtual link.  
  
Also we can see network 10.0.0.0 in R3 routing table as O route.
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