Objective

configure an OSPF virtual link so that a disconnected area can reach the backbone as required by OSPF.

Topology

Configs

SanJose1#sh run
Building configuration…

Current configuration : 1015 bytes
!
version 12.4
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname SanJose1
!
boot-start-marker
boot-end-marker
!
enable secret 5 $1$bP5E$pamhnCUi0AcSl/F9Rej651
!
no aaa new-model
!
resource policy
!
ip cef
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
interface Loopback0
ip address 192.168.1.3 255.255.255.0
!
interface Loopback1
ip address 192.168.0.3 255.255.255.0
!
interface FastEthernet0/0
ip address 192.168.224.1 255.255.255.0
duplex half
!
router ospf 1
log-adjacency-changes
network 192.168.0.0 0.0.0.255 area 0
network 192.168.1.0 0.0.0.255 area 0
network 192.168.224.0 0.0.0.255 area 51
!
no ip https server
no ip https secure-server
!
!
!
logging alarm informational
!
!
!
!
!
!
control-plane
!
!
!
!
!
!
gatekeeper
shutdown
!
!
line con 0
exec-timeout 0 0
password cisco
logging synchronous
login
stopbits 1
line aux 0
stopbits 1
line vty 0 4
exec-timeout 0 0
password cisco
logging synchronous
login
!
!
end

SanJose1#

Singapore#sh run
Building configuration…

Current configuration : 1790 bytes
!
version 12.4
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname Singapore
!
boot-start-marker
boot-end-marker
!
enable secret 5 $1$bPdv$hcFIbsP8L4rzFyuNrFoqE0
!
no aaa new-model
!
resource policy
!
ip cef
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
interface Loopback0
ip address 192.168.3.1 255.255.255.0
!
interface FastEthernet0/0
ip address 192.168.224.2 255.255.255.0
duplex half
!
interface Serial1/0
ip address 192.168.240.1 255.255.255.252
serial restart-delay 0
no dce-terminal-timing-enable
!
interface Serial1/1
no ip address
shutdown
serial restart-delay 0
no dce-terminal-timing-enable
!
interface Serial1/2
no ip address
shutdown
serial restart-delay 0
no dce-terminal-timing-enable
!
interface Serial1/3
no ip address
shutdown
serial restart-delay 0
no dce-terminal-timing-enable
!
interface Serial1/4
no ip address
shutdown
serial restart-delay 0
no dce-terminal-timing-enable
!
interface Serial1/5
no ip address
shutdown
serial restart-delay 0
no dce-terminal-timing-enable
!
interface Serial1/6
no ip address
shutdown
serial restart-delay 0
no dce-terminal-timing-enable
!
interface Serial1/7
no ip address
shutdown
serial restart-delay 0
no dce-terminal-timing-enable
!
router ospf 1
log-adjacency-changes
network 192.168.3.0 0.0.0.255 area 51
network 192.168.224.0 0.0.0.255 area 51
network 192.168.240.0 0.0.0.3 area 3
!
no ip https server
no ip https secure-server
!
!
!
logging alarm informational
!
!
!
!
!
!
control-plane
!
!
!
!
!
!
gatekeeper
shutdown
!
!
line con 0
exec-timeout 0 0
password cisco
logging synchronous
login
stopbits 1
line aux 0
stopbits 1
line vty 0 4
exec-timeout 0 0
password cisco
logging synchronous
login
!
!
end

Singapore#

Auckland#sh run
Building configuration…

Current configuration : 1711 bytes
!
version 12.4
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname Auckland
!
boot-start-marker
boot-end-marker
!
enable secret 5 $1$W3Ux$X4cR/1jc27f9IbV5b.Qgn0
!
no aaa new-model
!
resource policy
!
ip cef
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
interface Loopback0
ip address 192.168.252.1 255.255.255.0
!
interface FastEthernet0/0
ip address 192.168.248.1 255.255.255.0
duplex half
!
interface Serial1/0
ip address 192.168.240.2 255.255.255.252
serial restart-delay 0
no dce-terminal-timing-enable
!
interface Serial1/1
no ip address
shutdown
serial restart-delay 0
no dce-terminal-timing-enable
!
interface Serial1/2
no ip address
shutdown
serial restart-delay 0
no dce-terminal-timing-enable
!
interface Serial1/3
no ip address
shutdown
serial restart-delay 0
no dce-terminal-timing-enable
!
interface Serial1/4
no ip address
shutdown
serial restart-delay 0
no dce-terminal-timing-enable
!
interface Serial1/5
no ip address
shutdown
serial restart-delay 0
no dce-terminal-timing-enable
!
interface Serial1/6
no ip address
shutdown
serial restart-delay 0
no dce-terminal-timing-enable
!
interface Serial1/7
no ip address
shutdown
serial restart-delay 0
no dce-terminal-timing-enable
!
router ospf 1
log-adjacency-changes
network 192.168.240.0 0.0.0.3 area 3
!
no ip https server
no ip https secure-server
!
!
!
logging alarm informational
!
!
!
!
!
!
control-plane
!
!
!
!
!
!
gatekeeper
shutdown
!
!
line con 0
exec-timeout 0 0
password cisco
logging synchronous
login
stopbits 1
line aux 0
stopbits 1
line vty 0 4
exec-timeout 0 0
password cisco
logging synchronous
login
!
!
end

Auckland#

Auckland#sh ip route
Codes: C – connected, S – static, R – RIP, M – mobile, B – BGP
D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area
N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2
E1 – OSPF external type 1, E2 – OSPF external type 2
i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS level-2
ia – IS-IS inter area, * – candidate default, U – per-user static route
o – ODR, P – periodic downloaded static route

Gateway of last resort is not set

192.168.240.0/30 is subnetted, 1 subnets
C 192.168.240.0 is directly connected, Serial1/0
C 192.168.252.0/24 is directly connected, Loopback0
Auckland#

See that there’s no OSPF routes distributed to the Auckland router.

Inter Area traffice must transit the backbone area. Even though area 51 and area 3 are adjacent, they do not share OSPF routing updates.

Still, Singapore and Auckland has been able to form adjacency (sh ip ospf nei).

Area 3 is not connected to the area 0.

router Singapore is the ABR for area3, router SanJose1 is the ABR for area 0. therefore we must configure virtual link on Singapore and SanJose1 via area 51 as the transit area.

On SanJose1:

!
router ospf 1
log-adjacency-changes
area 51 virtual-link 192.168.3.1 (this is the other router’s RID)
network 192.168.0.0 0.0.0.255 area 0
network 192.168.1.0 0.0.0.255 area 0
network 192.168.224.0 0.0.0.255 area 51
!

On Singapore:

!
router ospf 1
log-adjacency-changes
area 51 virtual-link 192.168.1.3 (this is the other router’s RID)
network 192.168.3.0 0.0.0.255 area 51
network 192.168.224.0 0.0.0.255 area 51
network 192.168.240.0 0.0.0.3 area 3
!

Result:

Auckland#sh ip route
Codes: C – connected, S – static, R – RIP, M – mobile, B – BGP
D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area
N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2
E1 – OSPF external type 1, E2 – OSPF external type 2
i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS level-2
ia – IS-IS inter area, * – candidate default, U – per-user static route
o – ODR, P – periodic downloaded static route

Gateway of last resort is not set

O IA 192.168.224.0/24 [110/65] via 192.168.240.1, 00:01:32, Serial1/0
192.168.240.0/30 is subnetted, 1 subnets
C 192.168.240.0 is directly connected, Serial1/0
192.168.0.0/32 is subnetted, 1 subnets
O IA 192.168.0.3 [110/66] via 192.168.240.1, 00:00:01, Serial1/0
192.168.1.0/32 is subnetted, 1 subnets
O IA 192.168.1.3 [110/66] via 192.168.240.1, 00:00:01, Serial1/0
C 192.168.252.0/24 is directly connected, Loopback0
192.168.3.0/32 is subnetted, 1 subnets
O IA 192.168.3.1 [110/65] via 192.168.240.1, 00:01:32, Serial1/0

See that finally Auckland router receives LSA from area 0 through the virtual links. You can also do the sh ip ospf virtual-links to see the virtual-links condition.

Summary

• virtual links should be configured on both edge via transit area.
• inter area traffic mus transit area backbone.