Here is the third part of the configuration of our OSPF lab. The continuation of the last article. After configuring the area 0, the area 10 with the redistribution of EIGRP and the area 23, this time we will attack the configuration of the area 30, which, unlike the OSPF logic and is not related to area 0. In other words, this article will mainly talk about setting up a OSPF Virtual Link.
 Topology for OSPF Virtual Link

OSPF Virtual Link


The situation
As shown in the topology diagram, the area 30 is not directly attached to the area 0, which runs counter to the basic operation of OSPF. Without special configuration, this area will simply be ignored by the rest of the network.

The solution
To correct this problem, it is necessary to call upon an artifice ... namely to virtually extend the area 0 to the limit of the area 30. For this there are two means:
  • Create a GRE tunnel between ABR2 and ABR3.
  • Configure a OSPF Virtual Link.
Each of these methods is useful. However, in most cases, virtual-links will be used. But why? 

The two methods rely on a principle of encapsulation of the data to convey them through the area that separates the area 0 from the "distant" area. However, virtual-links encapsulate only OSPF exchanges, while using a GRE tunnel will cause encapsulation of OSPF exchanges but also data, which causes additional data overload (since all packets that Transit from the area 30 should be encapsulated in the GRE tunnel), while with a virtual-link, the data passes normally. 

In a specific case, the GRE tunnel may prove useful. The use of a virtual-link imposes that the transit area (that which separates the distant area from the area 0) is a standard area (non-stub), if this condition cannot be met, the only A functional method will then consist in creating a GRE tunnel through the transit area so as to create a connection between the distant area and the area 0.
Now that the milestones are set, let's move on to configurations 
 
First stage
Let's start by configuring area 30 on ABR3.

ABR3#conf t
ABR3(config)#router ospf 1
ABR3(config-router)#network 192.168.30.0 0.0.0.255 area 30
ABR3(config-router)#^Z
ABR3#

Now check whether the route for the 192.168.30.0/24 network has been propagated to ABR2. 

ABR2>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

     192.168.10.0/30 is subnetted, 2 subnets
O IA    192.168.10.0 [110/782] via 10.0.0.3, 00:22:40, FastEthernet0/0
O IA    192.168.10.4 [110/1563] via 10.0.0.3, 00:22:40, FastEthernet0/0
     10.0.0.0/8 is variably subnetted, 5 subnets, 2 masks
O       10.0.1.8/30 [110/782] via 10.0.0.3, 00:23:03, FastEthernet0/0
                    [110/782] via 10.0.0.2, 00:23:03, FastEthernet0/0
C       10.0.1.12/30 is directly connected, Serial0/1
C       10.0.0.0/24 is directly connected, FastEthernet0/0
O       10.0.1.0/30 [110/782] via 10.0.0.3, 00:23:03, FastEthernet0/0
                    [110/782] via 10.0.0.1, 00:23:03, FastEthernet0/0
C       10.0.1.4/30 is directly connected, Serial0/0
C    192.168.23.0/24 is directly connected, FastEthernet0/1
O E1 192.168.100.0/24 [110/1564] via 10.0.0.3, 00:22:34, FastEthernet0/0
ABR2>

As expected, the route to network 192.168.30.0/24 (which is in area 30) has not been transmitted to ABR2 because area 30 is not directly attacked at area0.

Second step
Let's move on to the virtual-link configuration.
What you need to know is that a virtual-link is configured at both ends of the transit area. In our case, this will be on ABR2 and ABR3. On each of them, we will define a virtual-link passing through the area 23 and indicate the opposite end using the router-id configured in OSPF.

 Configuring the virtual-link on ABR2
 
ABR2#conf t
ABR2(config)#router ospf 1
ABR2(config-router)#area 23 virtual-link 4.4.4.4
ABR2(config-router)#exit
ABR2(config)#

The syntax is : area virtual-link  

Configuring the cirtual-link on ABR3

ABR3#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
ABR3(config)#router ospf 1
ABR3(config-router)#area 23 virtual-link 3.3.3.3
ABR3(config-router)#
*Mar  1 00:39:29.075: %OSPF-5-ADJCHG: Process 1, Nbr 3.3.3.3 on OSPF_VL0 from LOADING to FULL, Loading Done
ABR3(config-router)#

As you can see through the displayed message, as soon as the virtual link is configured, an adjacency relation is formed. The area 30 is now virtually connected to the area 0.  
 
Check
Check on ABR2 that the adjacency is well formed, the state of the virtual-link etc ...
Show ip ospf neighbors
ABR2 # sh ip ospf neighbor
Neighbor ID Primary Dead Time Address Interface
4.4.4.4 0 FULL / - - 192.168.23.254 OSPF_VL1
5.5.5.5 0 FULL / - 00:00:32 10.0.1.13 Serial0 / 1
1.1.1.1 0 FULL / - 00:00:36 10.0.1.5 Serial0 / 0
1.1.1.1 1 FULL / DROTHER 00:00:39 10.0.0.1 FastEthernet0 / 0
2.2.2.2 1 FULL / DROTHER 00:00:37 10.0.0.3 FastEthernet0 / 0
5.5.5.5 1 FULL / DR 00:00:33 10.0.0.2 FastEthernet0 / 0
4.4.4.4 1 FULL / DR 00:00:34 192.168.23.254 FastEthernet0 / 1
ABR2 #

Note that ABR2 has two adjacency relations with ABR3 (4.4.4.4), one via the Fa0 / 1 interface and the other via a special interface ... OSPF_VL1 ... a virtual interface that is used by OSPF to encapsulate The OSPF messages to pass through the area 23.

Show ip ospf virtual-links

ABR2#sh ip ospf virtual-links
Virtual Link OSPF_VL1 to router 4.4.4.4 is up
Run as demand circuit
DoNotAge LSA allowed.
Transit area 23, via interface FastEthernet0/1, Cost of using 10
Transmit Delay is 1 sec, State POINT_TO_POINT,
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:03
Adjacency State FULL (Hello suppressed)
Index 6/7, retransmission queue length 0, number of retransmission 0
First 0x0(0)/0x0(0) Next 0x0(0)/0x0(0)
Last retransmission scan length is 0, maximum is 0
Last retransmission scan time is 0 msec, maximum is 0 msec
ABR2#

This command verifies the status of the virtual-link as well as all the parameters that apply to it.
An interesting thing to analyze is the behavior of the virtual-link. If you take a look at ABR3, you can directly understand that the virtual-link extends area 0 and not area 30, simply by the fact that now ABR3 is also part of area 0

Show ip ospf (on ABR3)

ABR3#sh ip ospf
Routing Process "ospf 1" with ID 4.4.4.4
Start time: 00:00:09.392, Time elapsed: 00:52:42.496
<--- ---="" lignes="" omises="" quelques="">
Area BACKBONE(0)
Number of interfaces in this area is 1
Area has no authentication
SPF algorithm last executed 00:13:14.304 ago
SPF algorithm executed 3 times
Area ranges are
Number of LSA 11. Checksum Sum 0x071E72
Number of opaque link LSA 0. Checksum Sum 0x000000
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 8
Flood list length 0
Area 23
Number of interfaces in this area is 1
This area has transit capability: Virtual Link Endpoint
Area has no authentication
SPF algorithm last executed 00:13:14.308 ago
SPF algorithm executed 6 times
Area ranges are
Number of LSA 13. Checksum Sum 0x05A07E
Number of opaque link LSA 0. Checksum Sum 0x000000
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0
Area 30
Number of interfaces in this area is 1
Area has no authentication
SPF algorithm last executed 00:13:24.792 ago
SPF algorithm executed 3 times
Area ranges are
Number of LSA 10. Checksum Sum 0x051A75
Number of opaque link LSA 0. Checksum Sum 0x000000
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0
ABR3#

ABR3 is now part of 3 areas. The area 0, area 23 and the area 30.
The only area still missing on the board is the 40. But ... it will be for the next article

 
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