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2-13 area transit-area-id virtual-link router-idSyntax Description:
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 LinkIn 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 VerificationVerify 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
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