2-13 area transit-area-id virtual-link router-id

2-13 area transit-area-id virtual-link router-id

Syntax Description:

• transit-area-id ” The OSPF area ID of the area connecting the two ABRs that the virtual link will cross. This value can be entered as a decimal number in the range of 0 to 4,294,967,295 or in IP address form in the range 0.0.0.0 to 255.255.255.255.

• router-id ” OSPF router ID of the router at the remote end of the virtual link.

Purpose: All non-zero OSPF areas must have a connection to the backbone or Area 0 and Area 0 must be contiguous. A virtual link is used to repair a segmented backbone or to connect a non-zero area that has been disconnected from Area 0. The transit area cannot be a stub area. Virtual links are used to repair a discontiguous backbone or to temporarily attach a disconnected non-zero area to the backbone. A virtual link should not be part of an initial OSPF design.

Initial Cisco IOS Software Release: 10.0

Configuration Example: Creating an OSPF Virtual Link

In Figure 2-15, Area 2 does not have a direct connection to Area 0. A virtual link is needed to repair this situation. Initially, you will configure the routers in Figure 2-15 without using a virtual link.

Figure 2-15. OSPF Virtual Link

 Router A  interface Loopback0   ip address 1.1.1.1 255.255.255.255   !   interface Serial0/1   ip address 10.1.1.1 255.255.255.252   clockrate 64000   !   router ospf 1   network 10.1.1.0 0.0.0.3 area 0   network 1.1.1.1 0.0.0.0 area 0  _______________________________________________________________________ Router B  interface Loopback0   ip address 2.2.2.2 255.255.255.255   !   interface Serial0   ip address 10.1.1.2 255.255.255.252   !   interface Serial1   ip address 10.1.1.5 255.255.255.252   clockrate 64000   !   router ospf 1   network 10.1.1.0 0.0.0.3 area 0   network 2.2.2.2 0.0.0.0 area 0   network 10.1.1.4 0.0.0.3 area 1  _______________________________________________________________________ Router C  interface Loopback0   ip address 3.3.3.3 255.255.255.255   !   interface Serial0   ip address 10.1.1.6 255.255.255.252   !   router ospf 1   network 3.3.3.3 0.0.0.0 area 2   network 10.1.1.4 0.0.0.3 area 1  

If you inspect the IP routing table on Router B, you will see that the 3.3.3.3 network from Router C is not present.

 rtrB#  show ip route  Codes: C - connected, S - static, I - IGRP, 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, E - EGP        i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default        U - per-user static route, o - ODR Gateway of last resort is not set      1.0.0.0/32 is subnetted, 1 subnets O IA    1.1.1.1 [110/65] via 10.1.1.1, 00:01:01, Serial0      2.0.0.0/32 is subnetted, 1 subnets C       2.2.2.2 is directly connected, Loopback0      10.0.0.0/30 is subnetted, 2 subnets C       10.1.1.0 is directly connected, Serial0 C       10.1.1.4 is directly connected, Serial1 

Area 2 on Router C does not have a direct connection to Area 0. A virtual link needs to be configured to correct this situation. The transit area in this case is Area 1. You also need the router IDs of the two ABRs to be able to construct the virtual link. The first method to determine the router ID is to use the show ip ospf neighbor command. This will display the router ID of the remote end of the virtual link.

 rtrB#  show ip ospf neighbor  Neighbor ID     Pri   State           Dead Time   Address         Interface 1.1.1.1           1   FULL/  -        00:00:32    10.1.1.1        Serial0  3.3.3.3           1   FULL/  -        00:00:36    10.1.1.6        Serial1  _______________________________________________________________________ rtrC#  show ip ospf neighbor  Neighbor ID     Pri   State           Dead Time   Address         Interface  2.2.2.2           1   FULL/  -        00:00:35    10.1.1.5        Serial0  

Router B has an ID of 2.2.2.2 and Router C has an ID of 3.3.3.3. The local router ID can be found by using the show ip ospf command.

 rtrB#  show ip ospf   Routing Process "ospf 1" with ID 2.2.2.2  Supports only single TOS(TOS0) routes  It is an area border router  SPF schedule delay 5 secs, Hold time between two SPFs 10 secs  Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs  Number of external LSA 0. Checksum Sum 0x0  Number of DCbitless external LSA 0  Number of DoNotAge external LSA 0  Number of areas in this router is 2. 2 normal 0 stub 0 nssa     Area BACKBONE(0)         Number of interfaces in this area is 2         Area has no authentication         SPF algorithm executed 3 times         Area ranges are         Number of LSA 3. Checksum Sum 0x1FD00         Number of DCbitless LSA 0         Number of indication LSA 0         Number of DoNotAge LSA 0     Area 1         Number of interfaces in this area is 1         Area has no authentication         SPF algorithm executed 2 times         Area ranges are         Number of LSA 5. Checksum Sum 0x2B171         Number of DCbitless LSA 0         Number of indication LSA 0         Number of DoNotAge LSA 0 _______________________________________________________________________ rtrC#  show ip ospf   Routing Process "ospf 1" with ID 3.3.3.3  Supports only single TOS(TOS0) routes  SPF schedule delay 5 secs, Hold time between two SPFs 10 secs  Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs  Number of external LSA 0. Checksum Sum 0x0  Number of DCbitless external LSA 0  Number of DoNotAge external LSA 0  Number of areas in this router is 2. 2 normal 0 stub 0 nssa     Area 1         Number of interfaces in this area is 1         Area has no authentication         SPF algorithm executed 12 times         Area ranges are         Number of LSA 5. Checksum Sum 0x2B171         Number of DCbitless LSA 0         Number of indication LSA 0         Number of DoNotAge LSA 0     Area 2         Number of interfaces in this area is 1         Area has no authentication         SPF algorithm executed 1 times         Area ranges are         Number of LSA 1. Checksum Sum 0xDDE0         Number of DCbitless LSA 0         Number of indication LSA 0         Number of DoNotAge LSA 0 

We now have the information we need to configure the virtual link. On Router B, the form of the command is:

  area   transit-area-id   virtual-link   router-C-ID  

And on Router C the command takes the form:

  area   transit-area-id   virtual-link   router-B-ID  

Modify the configurations on Routers B and C to construct the virtual link.

 Router B  router ospf 1    area 1 virtual-link 3.3.3.3    network 2.2.2.2 0.0.0.0 area 0   network 10.1.1.0 0.0.0.3 area 0   network 10.1.1.4 0.0.0.3 area 1  _______________________________________________________________________ Router C  router ospf 1    area 1 virtual-link 2.2.2.2    network 3.3.3.3 0.0.0.0 area 2   network 10.1.1.4 0.0.0.3 area 1  

Verification

Verify that Router C has a connection to Area 0.

 rtrC#  show ip ospf  Routing Process "ospf 1" with ID 3.3.3.3  Supports only single TOS(TOS0) routes  It is an area border router  SPF schedule delay 5 secs, Hold time between two SPFs 10 secs  Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs  Number of external LSA 0. Checksum Sum 0x0  Number of DCbitless external LSA 0  Number of DoNotAge external LSA 0  Number of areas in this router is 3. 3 normal 0 stub 0 nssa  Area BACKBONE(0)   Number of interfaces in this area is 1   Area has no authentication   SPF algorithm executed 3 times   Area ranges are   Number of LSA 6. Checksum Sum 0x45CF4   Number of DCbitless LSA 0   Number of indication LSA 0   Number of DoNotAge LSA 3  Area 1         Number of interfaces in this area is 1         Area has no authentication         SPF algorithm executed 14 times         Area ranges are         Number of LSA 10. Checksum Sum 0x4A8BD         Number of DCbitless LSA 0         Number of indication LSA 0         Number of DoNotAge LSA 0     Area 2         Number of interfaces in this area is 1         Area has no authentication         SPF algorithm executed 3 times         Area ranges are         Number of LSA 5. Checksum Sum 0x2B425         Number of DCbitless LSA 0         Number of indication LSA 0         Number of DoNotAge LSA 0 

Router C now has an interface in Area 0 and this interface is the virtual link. Verify that the virtual link is active on Routers B and C.

 rtrB#  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 1, via interface Serial1, 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 00:00:09     Adjacency State FULL (Hello suppressed) _______________________________________________________________________ rtrC#  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 1, via interface Serial0, 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) 

Finally, verify that the loopback interfaces for Routers A, B, and C are being advertised to all OSPF neighbors.

 rtrA#  show ip route  Codes: C - connected, S - static, I - IGRP, 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, E - EGP        i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default        U - per-user static route, o - ODR Gateway of last resort is not set      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 [110/65] via 10.1.1.2, 00:09:04, Serial0/1  3.0.0.0/32 is subnetted, 1 subnets  O IA    3.3.3.3 [110/129] via 10.1.1.2, 00:09:04, Serial0/1  10.0.0.0/30 is subnetted, 2 subnets C       10.1.1.0 is directly connected, Serial0/1 O IA    10.1.1.4 [110/128] via 10.1.1.2, 00:09:04, Serial0/1 _______________________________________________________________________ rtrB#  show ip route  Codes: C - connected, S - static, I - IGRP, 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, E - EGP        i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default        U - per-user static route, o - ODR Gateway of last resort is not set      1.0.0.0/32 is subnetted, 1 subnets  O       1.1.1.1 [110/65] via 10.1.1.1, 00:10:19, Serial0  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 [110/65] via 10.1.1.6, 00:10:20, Serial1  10.0.0.0/30 is subnetted, 2 subnets C       10.1.1.0 is directly connected, Serial0 C       10.1.1.4 is directly connected, Serial1 _______________________________________________________________________ rtrC#  show ip route  Codes: C - connected, S - static, I - IGRP, 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, E - EGP        i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default        U - per-user static route, o - ODR Gateway of last resort is not set      1.0.0.0/32 is subnetted, 1 subnets  O       1.1.1.1 [110/129] via 10.1.1.5, 00:11:10, Serial0  2.0.0.0/32 is subnetted, 1 subnets  O       2.2.2.2 [110/65] via 10.1.1.5, 00:11:11, Serial0  3.0.0.0/24 is subnetted, 1 subnets  C       3.3.3.0 is directly connected, Loopback0  10.0.0.0/30 is subnetted, 2 subnets O       10.1.1.0 [110/128] via 10.1.1.5, 00:11:11, Serial0 C       10.1.1.4 is directly connected, Serial0 

Troubleshooting

Step 1. Verify that there is a neighbor relationship between the OSPF routers using the show ip ospf neighbor command.

Step 2. Verify that the transit area ID used in the area virtual-link command is the proper area.

Step 3. Verify that the router IDs used in the area virtual-link are correct.