Topology for 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 #
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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