OSPF VIRTUAL LINKS
OSPF Transit Area from Tunnel
When configuring OSPF with tunnel interfaces as transit areas, there are several important considerations to ensure proper routing functionality. Here's what you need to know:
Key Concepts
Transit Area: In OSPF, a transit area is an area that carries traffic that neither originates nor terminates in that area (typically Area 0, the backbone area).
Tunnel Interfaces: Tunnels (like GRE, IPsec, or others) can be used to connect OSPF areas when physical connectivity is not direct.
Important Notes
OSPF Network Type: For tunnels, it's generally recommended to use:
point-to-point
for simple connectionsbroadcast
if you need DR/BDR election (requires manual neighbor configuration)
MTU Considerations: Ensure the tunnel MTU accounts for encapsulation overhead to avoid fragmentation.
Area Design:
The tunnel should typically be in Area 0 if connecting backbone areas
Virtual links can be used for areas not physically connected to Area 0
Authentication: If using OSPF authentication, it must be configured on both ends of the tunnel.
(In this lab, we configure two virtual links and two tunnels to connect a discontiguous area to area 0. A virtual link is a logical connection built between routers; virtual links are recommended for backup or temporary connections.)
If you want to see the video, please click the link here https://youtu.be/IzitX3oBAtg?si=7ugCs2nou6MZ27q_
Topology:-

- Configure the topology as per the diagram
- Configure the IP addresses as per the topology
- Configure OSPF IGP to advertise directly connected routes (all routers)
- Configure areas as per the topology with accurate router ID
- Configure two virtual links from area 0 to router 3, and from router 3 to router 1
- Make sure router 1 installs all the routes from area 0,30, and 20
- Configure two tunnels from area 0 to router 9 and from router 9 to router 10
- Advertise the tunnel interface in OSPF area 0
- Make sure router 10 installs all the routes from area 10, 20,30,40, and 50
- Same to all routers
- Make sure router 1 can ping all the routers
- Verify with the show commands
R1(config)#interface serial 5/0
R1(config-if)#ip address 12.1.1.1 255.0.0.0
R1(config-if)#no shutdown
R1(config-if)#exit
R1(config)#interface fastethernet 0/0
R1(config-if)#ip address 10.1.1.1 255.0.0.0
R1(config-if)#no keepalive
R1(config-if)#no shutdown
R1(config-if)#exit
R1(config)#interface loopback 1
R1(config-if)#ip address 192.168.1.1 255.255.255.0
R1(config-if)#exit
R1(config)#interface loopback 2
R1(config-if)#ip address 192.168.2.1 255.255.255.0
R1(config-if)#exit
R1(config)#interface loopback 3
R1(config-if)#ip address 192.168.3.1 255.255.255.0
R1(config-if)#exit
R2(config)#interface serial 5/0
R2(config-if)#ip address 12.1.1.2 255.0.0.0
R2(config-if)#no shutdown
R2(config-if)#exit
R2(config)#interface fastethernet 0/0
R2(config-if)#ip address 20.1.1.1 255.0.0.0
R2(config-if)#no keepalive
R2(config-if)#no shutdown
R2(config-if)#exit
R2(config)#interface serial 5/1
R2(config-if)#ip address 23.1.1.1 255.0.0.0
R2(config-if)#no shutdown
R2(config-if)#exit
R3(config)#interface serial 5/0
R3(config-if)#ip address 12.1.1.2 255.0.0.0
R3(config-if)#no shutdown
R3(config-if)#exit
R3(config)#interface fastethernet 0/0
R3(config-if)#ip address 20.1.1.1 255.0.0.0
R3(config-if)#no keepalive
R3(config-if)#no shutdown
R3(config-if)#exit
R3(config)#interface serial 5/1
R3(config-if)#ip address 23.1.1.1 255.0.0.0
R3(config-if)#no shutdown
R3(config-if)#exit
R4(config)#interface serial 5/2
R4(config-if)#ip address 34.1.1.2 255.0.0.0
R4(config-if)#no shutdown
R4(config-if)#exit
R4(config)#interface FastEthernet 0/0
R4(config-if)#ip address 40.1.1.1 255.0.0.0
R4(config-if)#no keepalive
R4(config-if)#no shutdown
R4(config-if)#exit
R4(config)#interface serial 5/3
R4(config-if)#ip address 45.1.1.1 255.0.0.0
R4(config-if)#no shutdown
R4(config-if)#exit
R5(config)#interface serial 6/0
R5(config-if)#ip address 56.1.1.1 255.0.0.0
R5(config-if)#no shutdown
R5(config-if)#exit
R5(config)#interface FastEthernet 0/0
R5(config-if)#ip address 50.1.1.1 255.0.0.0
R5(config-if)#no keepalive
R5(config-if)#no shutdown
R5(config-if)#exit
R5(config)#interface serial 5/3
R5(config-if)#ip address 45.1.1.2 255.0.0.0
R5(config-if)#no shutdown
R5(config-if)#exit
R5(config)#interface loopback 1
R5(config-if)#ip address 192.168.51.1 255.255.255.0
R5(config-if)#exit
R5(config)#interface loopback 2
R5(config-if)#ip address 192.168.52.1 255.255.255.0
R5(config-if)#exit
R5(config)#interface loopback 3
R5(config-if)#ip address 192.168.53.1 255.255.255.0
R5(config-if)#exit
R6(config)#interface serial 6/0
R6(config-if)#ip address 56.1.1.2 255.0.0.0
R6(config-if)#no shutdown
R6(config-if)#exit
R6(config)#interface FastEthernet 0/0
R6(config-if)#ip address 60.1.1.1 255.0.0.0
R6(config-if)#no keepalive
R6(config-if)#no shutdown
R6(config-if)#exit
R6(config)#interface serial 6/1
R6(config-if)#ip address 67.1.1.1 255.0.0.0
R6(config-if)#no shutdown
R6(config-if)#exit
R6(config)#interface loopback 1
R6(config-if)#ip address 192.168.61.1 255.255.255.0
R6(config-if)#exit
R6(config)#interface loopback 2
R6(config-if)#ip address 192.168.62.1 255.255.255.0
R6(config-if)#exit
R6(config)#interface loopback 3
R6(config-if)#ip address 192.168.63.1 255.255.255.0
R6(config-if)#exit
R7(config)#interface serial 6/2
R7(config-if)#ip address 79.1.1.1 255.0.0.0
R7(config-if)#no shutdown
R7(config-if)#exit
R7(config)#interface FastEthernet 0/0
R7(config-if)#ip address 70.1.1.1 255.0.0.0
R7(config-if)#no keepalive
R7(config-if)#no shutdown
R7(config-if)#exit
R7(config)#interface serial 6/1
R7(config-if)#ip address 67.1.1.2 255.0.0.0
R7(config-if)#no shutdown
R7(config-if)#exit
R9(config)#interface serial 6/2
R9(config-if)#ip address 79.1.1.1 255.0.0.0
R9(config-if)#no shutdown
R9(config-if)#exit
R9(config)#interface FastEthernet 0/0
R9(config-if)#ip address 70.1.1.1 255.0.0.0
R9(config-if)#no keepalive
R9(config-if)#no shutdown
R9(config-if)#exit
R9(config)#interface serial 6/1
R9(config-if)#ip address 67.1.1.2 255.0.0.0
R9(config-if)#no shutdown
R9(config-if)#exit
R8(config)#interface serial 6/4
R8(config-if)#ip address 81.1.1.1 255.0.0.0
R8(config-if)#no shutdown
R8(config-if)#exit
R8(config)#interface FastEthernet 0/0
R8(config-if)#ip address 80.1.1.1 255.0.0.0
R8(config-if)#no keepalive
R8(config-if)#no shutdown
R8(config-if)#exit
R8(config)#interface serial 6/3
R8(config-if)#ip address 98.1.1.2 255.0.0.0
R8(config-if)#no shutdown
R8(config-if)#exit
R10(config)#interface serial 6/4
R10(config-if)#ip address 81.1.1.2 255.0.0.0
R10(config-if)#no shutdown
R10(config-if)#exit
R10(config)#interface FastEthernet 0/0
R10(config-if)#ip address 100.1.1.1 255.0.0.0
R10(config-if)#no keepalive
R10(config-if)#no shutdown
R10(config-if)#exit
R10(config)#interface serial 6/5
R10(config-if)#ip address 101.1.1.2 255.0.0.0
R10(config-if)#no shutdown
R10(config-if)#exit
R10(config)#interface loopback 1
R10(config-if)#ip address 192.168.101.1 255.255.255.0
R10(config-if)#exit
R10(config)#interface loopback 2
R10(config-if)#ip address 192.168.102.1 255.255.255.0
R10(config-if)#exit
R10(config)#interface loopback 3
R10(config-if)#ip address 192.168.103.1 255.255.255.0
R10(config-if)#exit
- Configure the topology as per the diagram
- Configure the IP addresses as per the topology
- Configure OSPF IGP to advertise directly connected routes (all routers)
- Configure areas as per the topology with accurate router ID
- Configure two virtual links from area 0 to router 3, and from router 3 to router 1
- Make sure router 1 installs all the routes from area 0,30, and 20
- Configure two tunnels from area 0 to router 9 and from router 9 to router 10
- Advertise the tunnel interface in OSPF area 0
- Make sure router 10 installs all the routes from area 10, 20,30,40, and 50
- Same to all routers
- Make sure router 1 can ping all the routers
- Verify with the show commands
R1(config-if)#ip address 12.1.1.1 255.0.0.0
R1(config-if)#no shutdown
R1(config-if)#exit
R1(config)#interface fastethernet 0/0
R1(config-if)#ip address 10.1.1.1 255.0.0.0
R1(config-if)#no keepalive
R1(config-if)#no shutdown
R1(config-if)#exit
R1(config-if)#ip address 192.168.1.1 255.255.255.0
R1(config-if)#exit
R1(config)#interface loopback 2
R1(config-if)#ip address 192.168.2.1 255.255.255.0
R1(config-if)#exit
R1(config)#interface loopback 3
R1(config-if)#ip address 192.168.3.1 255.255.255.0
R1(config-if)#exit
R2(config)#interface serial 5/0
R2(config-if)#ip address 12.1.1.2 255.0.0.0
R2(config-if)#no shutdown
R2(config-if)#exit
R2(config)#interface fastethernet 0/0
R2(config-if)#ip address 20.1.1.1 255.0.0.0
R2(config-if)#no keepalive
R2(config-if)#no shutdown
R2(config-if)#exit
R2(config)#interface serial 5/1
R2(config-if)#ip address 23.1.1.1 255.0.0.0
R2(config-if)#no shutdown
R2(config-if)#exit
R3(config)#interface serial 5/0
R3(config-if)#ip address 12.1.1.2 255.0.0.0
R3(config-if)#no shutdown
R3(config-if)#exit
R3(config)#interface fastethernet 0/0
R3(config-if)#ip address 20.1.1.1 255.0.0.0
R3(config-if)#no keepalive
R3(config-if)#no shutdown
R3(config-if)#exit
R3(config)#interface serial 5/1
R3(config-if)#ip address 23.1.1.1 255.0.0.0
R3(config-if)#no shutdown
R3(config-if)#exit
R4(config)#interface serial 5/2
R4(config-if)#ip address 34.1.1.2 255.0.0.0
R4(config-if)#no shutdown
R4(config-if)#exit
R4(config)#interface FastEthernet 0/0
R4(config-if)#ip address 40.1.1.1 255.0.0.0
R4(config-if)#no keepalive
R4(config-if)#no shutdown
R4(config-if)#exit
R4(config)#interface serial 5/3
R4(config-if)#ip address 45.1.1.1 255.0.0.0
R4(config-if)#no shutdown
R4(config-if)#exit
R5(config)#interface serial 6/0
R5(config-if)#ip address 56.1.1.1 255.0.0.0
R5(config-if)#no shutdown
R5(config-if)#exit
R5(config)#interface FastEthernet 0/0
R5(config-if)#ip address 50.1.1.1 255.0.0.0
R5(config-if)#no keepalive
R5(config-if)#no shutdown
R5(config-if)#exit
R5(config)#interface serial 5/3
R5(config-if)#ip address 45.1.1.2 255.0.0.0
R5(config-if)#no shutdown
R5(config-if)#exit
R5(config)#interface loopback 1
R5(config-if)#ip address 192.168.51.1 255.255.255.0
R5(config-if)#exit
R5(config)#interface loopback 2
R5(config-if)#ip address 192.168.52.1 255.255.255.0
R5(config-if)#exit
R5(config)#interface loopback 3
R5(config-if)#ip address 192.168.53.1 255.255.255.0
R5(config-if)#exit
R6(config)#interface serial 6/0
R6(config-if)#ip address 56.1.1.2 255.0.0.0
R6(config-if)#no shutdown
R6(config-if)#exit
R6(config)#interface FastEthernet 0/0
R6(config-if)#ip address 60.1.1.1 255.0.0.0
R6(config-if)#no keepalive
R6(config-if)#no shutdown
R6(config-if)#exit
R6(config)#interface serial 6/1
R6(config-if)#ip address 67.1.1.1 255.0.0.0
R6(config-if)#no shutdown
R6(config-if)#exit
R6(config)#interface loopback 1
R6(config-if)#ip address 192.168.61.1 255.255.255.0
R6(config-if)#exit
R6(config)#interface loopback 2
R6(config-if)#ip address 192.168.62.1 255.255.255.0
R6(config-if)#exit
R6(config)#interface loopback 3
R6(config-if)#ip address 192.168.63.1 255.255.255.0
R6(config-if)#exit
R7(config)#interface serial 6/2
R7(config-if)#ip address 79.1.1.1 255.0.0.0
R7(config-if)#no shutdown
R7(config-if)#exit
R7(config)#interface FastEthernet 0/0
R7(config-if)#ip address 70.1.1.1 255.0.0.0
R7(config-if)#no keepalive
R7(config-if)#no shutdown
R7(config-if)#exit
R7(config)#interface serial 6/1
R7(config-if)#ip address 67.1.1.2 255.0.0.0
R7(config-if)#no shutdown
R7(config-if)#exit
R9(config)#interface serial 6/2
R9(config-if)#ip address 79.1.1.1 255.0.0.0
R9(config-if)#no shutdown
R9(config-if)#exit
R9(config)#interface FastEthernet 0/0
R9(config-if)#ip address 70.1.1.1 255.0.0.0
R9(config-if)#no keepalive
R9(config-if)#no shutdown
R9(config-if)#exit
R9(config)#interface serial 6/1
R9(config-if)#ip address 67.1.1.2 255.0.0.0
R9(config-if)#no shutdown
R9(config-if)#exit
R8(config)#interface serial 6/4
R8(config-if)#ip address 81.1.1.1 255.0.0.0
R8(config-if)#no shutdown
R8(config-if)#exit
R8(config)#interface FastEthernet 0/0
R8(config-if)#ip address 80.1.1.1 255.0.0.0
R8(config-if)#no keepalive
R8(config-if)#no shutdown
R8(config-if)#exit
R8(config)#interface serial 6/3
R8(config-if)#ip address 98.1.1.2 255.0.0.0
R8(config-if)#no shutdown
R8(config-if)#exit
R10(config)#interface serial 6/4
R10(config-if)#ip address 81.1.1.2 255.0.0.0
R10(config-if)#no shutdown
R10(config-if)#exit
R10(config)#interface FastEthernet 0/0
R10(config-if)#ip address 100.1.1.1 255.0.0.0
R10(config-if)#no keepalive
R10(config-if)#no shutdown
R10(config-if)#exit
R10(config)#interface serial 6/5
R10(config-if)#ip address 101.1.1.2 255.0.0.0
R10(config-if)#no shutdown
R10(config-if)#exit
R10(config)#interface loopback 1
R10(config-if)#ip address 192.168.101.1 255.255.255.0
R10(config-if)#exit
R10(config)#interface loopback 2
R10(config-if)#ip address 192.168.102.1 255.255.255.0
R10(config-if)#exit
R10(config)#interface loopback 3
R10(config-if)#ip address 192.168.103.1 255.255.255.0
R10(config-if)#exit
R1(config)#router
ospf 1
R1(config-router)#router-id 10.1.1.1
R1(config-router)#network 12.0.0.0 0.255.255.255 area 20
R1(config-router)#network 10.0.0.0 0.255.255.255 area 20
R1(config-router)#network 192.168.1.0 255.0.0.0 area 10
R1(config-router)#network 192.168.2.0 255.0.0.0 area 10
R1(config-router)#network 192.168.3.0 255.0.0.0 area 10
R1(config-router)#exit
R1(config)#exit
R2(config)#router ospf 1
R2(config-router)#router-id 20.1.1.1
R2(config-router)#network 23.0.0.0 0.255.255.255 area 20
R2(config-router)#network 12.0.0.0 0.255.255.255 area 20
R2(config-router)#network 20.0.0.0 0.255.255.255 area 20
R2(config-router)#exit
R3(config)#router ospf 1
R3(config-router)#router-id 30.1.1.1
R3(config-router)#network 34.0.0.0 0.255.255.255 area 30
R3(config-router)#network 23.0.0.0 0.255.255.255 area 20
R3(config-router)#network 30.0.0.0 0.255.255.255 area 30
R3(config-router)#exit
R4(config)#router ospf 1
R4(config-router)#router-id 40.1.1.1
R4(config-router)#network 45.0.0.0 0.255.255.255 area 30
R4(config-router)#network 45.0.0.0 0.255.255.255 area 30
R4(config-router)#network 34.0.0.0 0.255.255.255 area 30
R4(config-router)#network 40.0.0.0 0.255.255.255 area 30
R4(config-router)#exit
R5(config)#router ospf 1
R5(config-router)#router-id 50.1.1.1
R5(config-router)#network 45.1.1.0 0.255.255.255 area 30
R5(config-router)#network 50.0.0.0 0.255.255.255 area 0
R5(config-router)#network 56.0.0.0 0.255.255.255 area 0
R5(config-router)#network 192.168.51.0 255.0.0.0 area 0
R5(config-router)#network 192.168.52.0 255.0.0.0 area 0
R5(config-router)#network 192.168.53.0 255.0.0.0 area 0
R5(config-router)#exit
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