OSPF: Virtual Links – Part II

OSPF: Virtual Links – Part II

·         In the previous post OSPF: Virtual Links - Part I, we have seen how to create Virtual Links in case of an Area not connected to Area 0.

·         In this post, let me take the case of partitioned Backbone area to create Virtual Links.

·         Topology:

·         R1-R2 link is part of Area 0

·         R2-R3 link is part of Area 23

·         R2 is an ABR for Area0 & Area 23

·         R3-R4 link is part of Area 0

·         R1’s loopback 1.1.1.1/32 is advertised into Area 0

·         R2’s loopback 2.2.2.2/32 is advertised into Area 0

·         R3’s loopback 3.3.3.3/32 is advertised into Area 23

·         R4’s loopback 4.4.4.4/32 is advertised into Area 0

·         Let’s see what happens before creating virtual link

R1#show run | begin router ospf

router ospf 1

 router-id 1.1.1.1

 log-adjacency-changes

 network 1.1.1.1 0.0.0.0 area 0

 network 100.1.12.1 0.0.0.0 area 0

!

R2#show run | begin router ospf

router ospf 1

 router-id 2.2.2.2

 log-adjacency-changes

 network 2.2.2.2 0.0.0.0 area 0

 network 100.1.12.2 0.0.0.0 area 0

 network 100.1.23.2 0.0.0.0 area 23

!

R3#show run | begin router ospf

router ospf 1

 router-id 3.3.3.3

 log-adjacency-changes

 network 3.3.3.3 0.0.0.0 area 23

 network 100.1.23.3 0.0.0.0 area 23

 network 100.1.34.3 0.0.0.0 area 0

!

R4#show run | begin router ospf

router ospf 1

 router-id 4.4.4.4

 log-adjacency-changes

 network 4.4.4.4 0.0.0.0 area 0

 network 100.1.34.4 0.0.0.0 area 0

!

R1#show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface

2.2.2.2           0   FULL/  -        00:00:30    100.1.12.2      Serial1/0

R1#

R2#show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface

1.1.1.1           0   FULL/  -        00:00:32    100.1.12.1      Serial1/0

3.3.3.3           0   FULL/  -        00:00:33    100.1.23.3      Serial1/1

R2#

R3#show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface

4.4.4.4           0   FULL/  -        00:00:35    100.1.34.4      Serial1/0

2.2.2.2           0   FULL/  -        00:00:39    100.1.23.2      Serial1/1

R3#

R4#show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface

3.3.3.3           0   FULL/  -        00:00:37    100.1.34.3      Serial1/0

R4#

Observations:

·         OSPF has been configured as per the Topology.

·         All neighbors are up.

Let’s see the OSPF routing tables:

R1#show ip route ospf

     2.0.0.0/32 is subnetted, 1 subnets

O       2.2.2.2 [110/65] via 100.1.12.2, 00:09:16, Serial1/0

     100.0.0.0/24 is subnetted, 2 subnets

O IA    100.1.23.0 [110/128] via 100.1.12.2, 00:09:16, Serial1/0

     3.0.0.0/32 is subnetted, 1 subnets

O IA    3.3.3.3 [110/129] via 100.1.12.2, 00:09:16, Serial1/0

R1#

R1#

R2#show ip route ospf

     1.0.0.0/32 is subnetted, 1 subnets

O       1.1.1.1 [110/65] via 100.1.12.1, 00:09:22, Serial1/0

     3.0.0.0/32 is subnetted, 1 subnets

O       3.3.3.3 [110/65] via 100.1.23.3, 00:08:53, Serial1/1

R2#

R3#show ip route ospf

     4.0.0.0/32 is subnetted, 1 subnets

O       4.4.4.4 [110/65] via 100.1.34.4, 00:08:38, Serial1/0

R3#

R4#show ip route ospf

     100.0.0.0/24 is subnetted, 2 subnets

O IA    100.1.23.0 [110/128] via 100.1.34.3, 00:08:37, Serial1/0

     3.0.0.0/32 is subnetted, 1 subnets

O IA    3.3.3.3 [110/65] via 100.1.34.3, 00:08:37, Serial1/0

R4#

R4#

R2#show ip ospf database

            OSPF Router with ID (2.2.2.2) (Process ID 1)

                Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count

1.1.1.1         1.1.1.1         1297        0x80000004 0x000494 3

2.2.2.2         2.2.2.2         548         0x80000002 0x00C8EB 4

3.3.3.3         3.3.3.3         1     (DNA) 0x80000003 0x002569 3

4.4.4.4         4.4.4.4         704   (DNA) 0x80000002 0x00E55D 3

                Summary Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum

3.3.3.3         2.2.2.2         1292        0x80000001 0x004F98

3.3.3.3         3.3.3.3         720   (DNA) 0x80000001 0x00AE75

100.1.23.0      2.2.2.2         1292        0x80000001 0x00ACCB

100.1.23.0      3.3.3.3         720   (DNA) 0x80000001 0x008EE5

                Router Link States (Area 23)

Link ID         ADV Router      Age         Seq#       Checksum Link count

2.2.2.2         2.2.2.2         548         0x80000002 0x003255 2

3.3.3.3         3.3.3.3         549         0x80000005 0x001151 3

                Summary Net Link States (Area 23)

Link ID         ADV Router      Age         Seq#       Checksum

1.1.1.1         2.2.2.2         1294        0x80000001 0x00AB44

2.2.2.2         2.2.2.2         1294        0x80000001 0x00FA31

4.4.4.4         3.3.3.3         1250        0x80000001 0x0003DC

100.1.12.0      2.2.2.2         1294        0x80000001 0x00265D

100.1.34.0      3.3.3.3         1267        0x80000001 0x001554

R2#

R3#show ip ospf database

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

                Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count

1.1.1.1         1.1.1.1         749   (DNA) 0x80000004 0x000494 3

2.2.2.2         2.2.2.2         1     (DNA) 0x80000002 0x00C8EB 4

3.3.3.3         3.3.3.3         1474        0x80000003 0x002569 3

4.4.4.4         4.4.4.4         326         0x80000003 0x00E35E 3

                Summary Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum

3.3.3.3         2.2.2.2         744   (DNA) 0x80000001 0x004F98

3.3.3.3         3.3.3.3         325         0x80000002 0x00AC76

100.1.23.0      2.2.2.2         744   (DNA) 0x80000001 0x00ACCB

100.1.23.0      3.3.3.3         325         0x80000002 0x008CE6

                Router Link States (Area 23)

Link ID         ADV Router      Age         Seq#       Checksum Link count

2.2.2.2         2.2.2.2         1475        0x80000002 0x003255 2

3.3.3.3         3.3.3.3         1474        0x80000005 0x001151 3

                Summary Net Link States (Area 23)

Link ID         ADV Router      Age         Seq#       Checksum

1.1.1.1         2.2.2.2         315         0x80000002 0x00A945

2.2.2.2         2.2.2.2         315         0x80000002 0x00F832

4.4.4.4         3.3.3.3         326         0x80000002 0x0001DD

100.1.12.0      2.2.2.2         315         0x80000002 0x00245E

100.1.34.0      3.3.3.3         326         0x80000002 0x001355

R3#

Observations:

·         R1 and R2 don’t have R3-R4 link(100.1.34.0/24, which is part of another Area 0) in OSPF routing table

·         R1 and R2 don’t have R4’s loopback(4.4.4.4/32, which is part of another Area 0) in OPSF routing table.

o    This is because, R2(which is an ABR for Area 0 and 23), has not installed the routes received by R3, in its OSPF table). This is because; it has full-neighbor relationship in Area 0 (with R1).

o    We can find in these prefixes in R2’s OSPF database, but it will not install them in its OSPF routing table.

o    This is inbuilt inter-area loop prevention mechanism in OSPF.

·         R3 and R4 don’t have R1-R2 link (100.1.12.0/24, which is part of another Area 0) in OSPF routing table.

·         R3 and R4 don’t have R1’s loopback 1.1.1.1/32, which is part of another Area 0) in OSPF routing table.

·         R3 and R4 don’t have r2’s loopback 2.2.2.2/32, which is part of another Area 0) in OSPF routing table.

o    This is because, R3(which is an ABR for Area 0 and 23), has not installed the routes received by R2, in its OSPF table). This is because; it has full-neighbor relationship in Area 0 (with R4).

o    We can find in these prefixes in R3’s OSPF database, but it will not install them in its OSPF routing table.

o    This is inbuilt inter-area loop prevention mechanism in OSPF.

Solution: Create a Virtual-link through Area 23 (between R2 and R3) to connect the partitioned Area 0:

R2(config)#router ospf 1

R2(config-router)#

R2(config-router)#area 23 virtual-link 3.3.3.3

R2(config-router)#

*Jul  8 15:44:20.611: %OSPF-5-ADJCHG: Process 1, Nbr 3.3.3.3 on OSPF_VL0 from LOADING to FULL, Loading Done

R2(config-router)#

R3(config)#router ospf 1

R3(config-router)#area 23 virtual-link 2.2.2.2

R3(config-router)#

*Jul  8 15:44:20.767: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on OSPF_VL0 from LOADING to FULL, Loading Done

R3(config-router)#

R2#show ip ospf virtual-links

Virtual Link OSPF_VL0 to router 3.3.3.3 is up

  Run as demand circuit

  DoNotAge LSA allowed.

  Transit area 23, via interface Serial1/1, 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 2/3, 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

R2#

R3#show ip ospf virtual-links

Virtual Link OSPF_VL0 to router 2.2.2.2 is up

  Run as demand circuit

  DoNotAge LSA allowed.

  Transit area 23, via interface Serial1/1, 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 00:00:00

    Adjacency State FULL (Hello suppressed)

    Index 2/3, 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

R3#

R1#show ip route ospf

     2.0.0.0/32 is subnetted, 1 subnets

O       2.2.2.2 [110/65] via 100.1.12.2, 00:01:22, Serial1/0

     100.0.0.0/24 is subnetted, 3 subnets

O       100.1.34.0 [110/192] via 100.1.12.2, 00:01:22, Serial1/0

O IA    100.1.23.0 [110/128] via 100.1.12.2, 00:01:22, Serial1/0

     3.0.0.0/32 is subnetted, 1 subnets

O IA    3.3.3.3 [110/129] via 100.1.12.2, 00:01:22, Serial1/0

     4.0.0.0/32 is subnetted, 1 subnets

O       4.4.4.4 [110/193] via 100.1.12.2, 00:01:22, Serial1/0

R1#

R1#ping 4.4.4.4

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 4.4.4.4, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 60/85/164 ms

R1#

R2#show ip route ospf

     1.0.0.0/32 is subnetted, 1 subnets

O       1.1.1.1 [110/65] via 100.1.12.1, 00:02:42, Serial1/0

     100.0.0.0/24 is subnetted, 3 subnets

O       100.1.34.0 [110/128] via 100.1.23.3, 00:02:42, Serial1/1

     3.0.0.0/32 is subnetted, 1 subnets

O       3.3.3.3 [110/65] via 100.1.23.3, 00:02:42, Serial1/1

     4.0.0.0/32 is subnetted, 1 subnets

O       4.4.4.4 [110/129] via 100.1.23.3, 00:02:42, Serial1/1

R2#

R3#show ip route ospf

     1.0.0.0/32 is subnetted, 1 subnets

O       1.1.1.1 [110/129] via 100.1.23.2, 00:02:44, Serial1/1

     2.0.0.0/32 is subnetted, 1 subnets

O       2.2.2.2 [110/65] via 100.1.23.2, 00:02:44, Serial1/1

     100.0.0.0/24 is subnetted, 3 subnets

O       100.1.12.0 [110/128] via 100.1.23.2, 00:02:44, Serial1/1

     4.0.0.0/32 is subnetted, 1 subnets

O       4.4.4.4 [110/65] via 100.1.34.4, 00:02:44, Serial1/0

R3#

R4#show ip route ospf

     1.0.0.0/32 is subnetted, 1 subnets

O       1.1.1.1 [110/193] via 100.1.34.3, 00:02:45, Serial1/0

     2.0.0.0/32 is subnetted, 1 subnets

O       2.2.2.2 [110/129] via 100.1.34.3, 00:02:45, Serial1/0

     100.0.0.0/24 is subnetted, 3 subnets

O       100.1.12.0 [110/192] via 100.1.34.3, 00:02:45, Serial1/0

O IA    100.1.23.0 [110/128] via 100.1.34.3, 00:02:45, Serial1/0

     3.0.0.0/32 is subnetted, 1 subnets

O IA    3.3.3.3 [110/65] via 100.1.34.3, 00:02:45, Serial1/0

R4#

R4#ping 1.1.1.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 52/95/152 ms

R4#

Observation:

·         I have successfully repaired the problem in OSPF partitioned Area 0