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BSCI – OSPF virtual-link Lab

I’m almost done with OSPF and it is not as bad as I thought. Actually I like it, the labs are real fun and the theory is complicated but in a reasonable way while the commands are simple and straight forward.

I want to show my Virtual Link lab as I find this topic very interesting and relatively neglected. I built a lab as shown in the diagram.

OSPF Virtual-Link Lab

The lab has three routers: R1 & R2 share area 0, R2 & R3 share area 1 and a Loopback Interface on R3 that represent area 2 – the bad guy who mess up the traditional OSPF setup.
You can check the show run output of R1, R2 and R3.

Since it is an advanced lab I will not repeat steps I and II which include Interface configuration and ping test. You can check both my EIGRP and OSPF labs for this part and we’ll start from Step III.

Step III:
I’ve configured the Router ID value for each router using the router number. This is an example for R1:
router ospf 1
router-id 1.1.1.1

At this point I had working OSPF areas where routers can see their neighbors. The problem is area 2 on R3 since this router has no direct link to area 0.

REMINDER: OSPF domain is built from area 0 – the backbone of the domain and as many other areas that MUST have one ABR (Area Border Router) with direct connection to the backbone.
This is an output from R3 before I added area 2:

R3#sh ip ospf database

OSPF Router with ID (3.3.3.3) (Process ID 1)

Router Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum Link count
2.2.2.2         2.2.2.2         36          0x80000003 0x0061DE 2
3.3.3.3         3.3.3.3         35          0x80000002 0x00E35B 2

Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
10.1.14.0       2.2.2.2         46          0x80000001 0x00944A
R3#sh ip ospf interface
Serial0/0 is up, line protocol is up
Internet Address 10.1.24.1/30, Area 1
Process ID 1, Router ID 3.3.3.3, Network Type POINT_TO_POINT, Cost: 64
Transmit Delay is 1 sec, State POINT_TO_POINT,
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:03
Supports Link-local Signaling (LLS)
Index 1/1, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 1
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 2.2.2.2
Suppress hello for 0 neighbor(s)

R3#sh ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2           0   FULL/  –        00:00:36    10.1.24.2       Serial0/0

Part IV:
Adding area 2 is simple, using the following command:
router ospf 1
network 10.1.34.0 0.0.0.255 area 2

this is the changed output on R3:

R3#sh ip ospf interface
Serial0/0 is up, line protocol is up
Internet Address 10.1.24.1/30, Area 1
Process ID 1, Router ID 3.3.3.3, Network Type POINT_TO_POINT, Cost: 64
Transmit Delay is 1 sec, State POINT_TO_POINT,
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:02
Supports Link-local Signaling (LLS)
Index 1/1, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 1
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 2.2.2.2
Suppress hello for 0 neighbor(s)
Loopback1 is up, line protocol is up
Internet Address 10.1.34.3/24, Area 2
Process ID 1, Router ID 3.3.3.3, Network Type LOOPBACK, Cost: 1
Loopback interface is treated as a stub Host

R3#sh ip ospf database

OSPF Router with ID (3.3.3.3) (Process ID 1)

Router Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum Link count
2.2.2.2         2.2.2.2         372         0x80000003 0x0061DE 2
3.3.3.3         3.3.3.3         372         0x80000002 0x00E35B 2

Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
10.1.14.0       2.2.2.2         383         0x80000001 0x00944A

Router Link States (Area 2)

Link ID         ADV Router      Age         Seq#       Checksum Link count
3.3.3.3         3.3.3.3         141         0x80000001 0x00B534 1

Step V:
As you can see in the diagram, there is no ABR to connect area 2 with the backbone and this is where the virtual-link command come into the picture.
The way to fix this problem (some call it band-aid as it is a wrong design) is by using the virtual-link command.
When I configured the following on R3
router ospf 1
area 1 virtual-link 2.2.2.2

R2 showed this error:

*Mar  1 00:07:51.007: %OSPF-4-ERRRCV: Received invalid packet: mismatch area ID,
from backbone area must be virtual-link but not found from 10.1.24.1, Serial0/1

Then I completed the command on the neighbor, R2
router ospf 1
area 1 virtual-link 3.3.3.3
This is what popped on R2

*Mar  1 00:09:21.383: %OSPF-5-ADJCHG: Process 1, Nbr 3.3.3.3 on OSPF_VL0 from LOADING to FULL, Loading Done

and R3

*Mar  1 00:09:21.123: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on OSPF_VL0 from LOADING to FULL, Loading Done

At this point the Virtual Link is up, area 2 his connected to area 0, virtual but as good as a direct one. This is how the final output look on R3:

R3#show ip ospf virtual-link
Virtual Link OSPF_VL0 to router 2.2.2.2 is up
Run as demand circuit
DoNotAge LSA allowed.
Transit area 1, via interface Serial0/0, Cost of using 64
Transmit Delay is 1 sec, State POINT_TO_POINT,
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:01
Adjacency State FULL (Hello suppressed)
Index 1/2, retransmission queue length 0, number of retransmission 1
First 0x0(0)/0x0(0) Next 0x0(0)/0x0(0)
Last retransmission scan length is 1, maximum is 1
Last retransmission scan time is 0 msec, maximum is 0 msec

R3#show 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

10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
O       10.1.14.0/30 [110/128] via 10.1.24.2, 00:01:38, Serial0/0
C       10.1.24.0/30 is directly connected, Serial0/0
C       10.1.34.0/24 is directly connected, Loopback1

and on R2:

R2#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

10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
C       10.1.14.0/30 is directly connected, Serial0/0
C       10.1.24.0/30 is directly connected, Serial0/1
O IA    10.1.34.3/32 [110/65] via 10.1.24.1, 00:02:16, Serial0/1

This is a nice way to fix a problem caused by wrong design or forced real life changes. The goal is always to use Virtual Links as a temporary solution until you can fix the design and get a direct connection between each area and the backbone.

I’m about to complete my OSPF studies. It took me almost a week but since I combined theory and lab I feel that I know this topic very good and can move forward knowing that both EIGRP and OSPF are done and does not need much more time.

Find more labs here

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