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OSPF - Virtual Link

Oleh Hasugian Posting Komentar


Seperti yang di ketahui, proses OSPF memakan banyak resource CPU dan memori pada router, sehingga menurut rekomendasi cisco jumlah maksimum router per-OSPF area adalah 50. Sehingga, jika ada suatu area yang jumlah routernya sangat banyak (diatas 50) sangat direkomendasikan untuk memecah area tersebut menjadi beberapa area selama sesuai dengan konsep OSPF yakni seluruh area harus terhubung langsung ke area 0. Namun bagaimana jika suatu area tidak memungkinkan untuk terhubung langsung ke area 0 (misal karena jaraknya terlalu jauh)? Tentunya terdapat teknik untuk mengatasi hal ini, diantaranya adalah membangun OSPF virtual-link.


Konsepnya adalah membangun sebuah jalur virtual pada area yang menghubungkan sebuah area dengan area 0. Area dimana virtual-link dibangun dapat disebut sebagai area transit. Untuk lebih jelasnya perhatikan gambar berikut.

Pada lab ini ada area 0, area 1, area 2 dan area 3, diaman area 2 dan 3 tidak terhubung langung ke area 9 (backbone).

Router R1
 interface Loopback0
 ip address 1.1.1.1 255.255.255.255
!
interface FastEthernet0/0
 ip address 12.12.12.1 255.255.255.0
  no shu

 R1(config)#router ospf 1
R1(config-router)#network 1.1.1.1 0.0.0.0 area 0
R1(config-router)#network 12.12.12.0 0.0.0.255 area 1
R1(config-router)#router-id 1.1.1.1
R1(config-router)#area 1 virtual-link 2.2.2.2
Router R2
R2(config)#interface fa0/0
R2(config-if)#ip add
R2(config-if)#ip address 12.12.12.2 255.255.255.0
R2(config-if)#no sh
R2(config-if)#ex

 R2(config)#interface fa0/1
R2(config-if)#ip add
R2(config-if)#ip address 23.23.23.2 255.255.255.0
R2(config-if)#no sh
R2(config-if)#ex

 R2(config)#interface lo0
R2(config-if)#ip address 2.2.2.2 255.255.255.255
R2(config-if)#ex

 R2(config)#router ospf 2
R2(config-router)#network 12.12.12.0 0.0.0.255 area 1
R2(config-router)#network 2.2.2.2 0.0.0.0 area 1
R2(config-router)#network 23.23.23.2 0.0.0.0 area 2
R2(config-router)#router-id 2.2.2.2
R2(config-router)#do clear ip ospf pro
R2(config-router)#area 2 virtual-link 3.3.3.3
R2(config-router)#area 1 virtual-link 1.1.1.1


Router R3

R3(config)#interface fa0/1
R3(config-if)#ip add
R3(config-if)#ip address 23.23.23.3 255.255.255.0
R3(config-if)#no sh
R3(config-if)#ex

 R3(config)#router ospf 3
R3(config-router)#network 3.3.3.3 0.0.0.0 area 1
R3(config-router)#network 23.23.23.3 0.0.0.0 area 2
R3(config-router)#router-id 3.3.3.3
R3(config-router)#do clear ip ospf pro
R3(config-router)#area 2 virtual-link 2.2.2.2



 Verifkasi di masing-masing router

 R1#show ip route
     1.0.0.0/32 is subnetted, 1 subnets
C       1.1.1.1 is directly connected, Loopback0
     2.0.0.0/32 is subnetted, 1 subnets
O       2.2.2.2 [110/11] via 12.12.12.2, 01:36:36, FastEthernet0/0
     3.0.0.0/32 is subnetted, 1 subnets
O IA    3.3.3.3 [110/21] via 12.12.12.2, 00:02:40, FastEthernet0/0
     23.0.0.0/24 is subnetted, 1 subnets
O IA    23.23.23.0 [110/20] via 12.12.12.2, 01:34:43, FastEthernet0/0
     12.0.0.0/24 is subnetted, 1 subnets
C       12.12.12.0 is directly connected, FastEthernet0/0
R1(config-router)#


R2#show ip route
     1.0.0.0/32 is subnetted, 1 subnets
O       1.1.1.1 [110/11] via 12.12.12.1, 01:35:04, FastEthernet0/0
     2.0.0.0/32 is subnetted, 1 subnets
C       2.2.2.2 is directly connected, Loopback0
     3.0.0.0/32 is subnetted, 1 subnets
O IA    3.3.3.3 [110/11] via 23.23.23.3, 00:03:16, FastEthernet0/1
     23.0.0.0/24 is subnetted, 1 subnets
C       23.23.23.0 is directly connected, FastEthernet0/1
     12.0.0.0/24 is subnetted, 1 subnets
C       12.12.12.0 is directly connected, FastEthernet0/0
R2#



R3#show ip route
     1.0.0.0/32 is subnetted, 1 subnets
O       1.1.1.1 [110/21] via 23.23.23.2, 00:04:19, FastEthernet0/1
     2.0.0.0/32 is subnetted, 1 subnets
O IA    2.2.2.2 [110/11] via 23.23.23.2, 00:04:19, FastEthernet0/1
     3.0.0.0/32 is subnetted, 1 subnets
C       3.3.3.3 is directly connected, Loopback0
     23.0.0.0/24 is subnetted, 1 subnets
C       23.23.23.0 is directly connected, FastEthernet0/1
     12.0.0.0/24 is subnetted, 1 subnets
O IA    12.12.12.0 [110/20] via 23.23.23.2, 00:04:19, FastEthernet0/1
R3(config-router)#



Test ping dari R3 ke R1 (ip 1.1.1.1)

R3#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 = 8/41/92 ms
R3#


show ip ospf virtual-links

R2#show ip ospf  virtual-links
Virtual Link OSPF_VL1 to router 1.1.1.1 is up
  Run as demand circuit
  DoNotAge LSA allowed.
  Transit area 1, via interface FastEthernet0/0, 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 00:00:05
    Adjacency State FULL (Hello suppressed)
    Index 1/3, retransmission queue length 0, number of retransmission 3
    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
Virtual Link OSPF_VL0 to router 3.3.3.3 is up
  Run as demand circuit
  DoNotAge LSA allowed.
  Transit area 2, 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 00:00:05
    Adjacency State FULL (Hello suppressed)
    Index 2/4, 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#


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