An Open Shortest Path First (OSPF) autonomous system all areas
must be physically connected to the backbone area (Area 0). In some cases, if
this is not possible, we can use a virtual link to connect to the backbone (Area
0) through a non-backbone area. We can also use the virtual links to connect
two parts of a partitioned backbone (Area 0) through a non-backbone area. The
area through which we configure the virtual link, known as a transit area,
must have full routing information. Remember the transit area cannot be a stub
area.
Virtual link must be configuring both sides with the same
area ID and the corresponding virtual link OSPF neighbor router ID. Show IP
OSPF neighbors command give you the router ID information.
area-id
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Is identifier for the OSPF area assigned to the transit
area for the virtual link. OSPF area ID can be from 0 to 4294967295 or an IP
address.
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router-id
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Router ID is associated with the virtual link neighbor.
Specify as an IP address. The router ID appears in the show IP OSPF
neighbors command.
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In short: virtual link are used to connect a discontiguous area to area 0, virtual link is a logical connection built between routers, virtual links are recommended for backup or temporary connections
Before we start BGP configuration check out some important of BGP CCIE exam topics.
Here are the lists of all OSPF labs and theories CCNA to CCIE.
configuration virtual link:
Topology:
GOAL:
- Configure interfaces and assigned IP addresses as per the Topology
- advertise the interfaces using multiple areas defined as per the Topology
- configure the manual router id (RID) R1-11.1.1.1, R2-22.1.1.1, and R3-33.1.1.1
- make sure that the loop-back of R1- 11.0.0.0 routes should be seen on all the other routers
R1#show ip interface brief
Interface IP-Address OK? Method Status Protocol
FastEthernet 0/0 10.1.1.1 YES manual up up
Serial3/0 1.1.1.1 YES manual up up
Loopback0 11.0.0.1 YES manual up up
Loopback1 11.0.1.1 YES manual up up
Loopback2 11.0.2.1 YES manual up up
Loopback3 11.0.3.1 YES manual up up
R2#show ip interface brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 20.1.1.1 YES manual up up
Serial3/0 1.1.1.2 YES manual up up
Serial3/1 2.2.2.1 YES manual up up
R3#show ip interface brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 30.1.1.1 YES manual up up
Serial3/1 2.2.2.2 YES manual up up
Serial3/2 3.3.3.1 YES manual up up
R4#show ip interface brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 40.1.1.1 YES manual up up
Serial3/2 3.3.3.2 YES manual up up
Loopback0 14.0.0.1 YES manual up up
Loopback1 14.0.1.1 YES manual up up
Loopback2 14.0.2.1 YES manual up up
Loopback3 14.0.3.1 YES manual up up
R1(config)#router ospf 1
R1(config-router)#router-id 11.1.1.1
R1(config-router)#NETwork 1.0.0.0 0.255.255.255 Area 20
R1(config-router)#NETwork 10.0.0.0 0.255.255.255 Area 20
R1(config-router)#NETwork 11.0.0.0 0.0.0.255 area 10
R2(config)#router ospf 1
R2(config-router)#router-id 22.1.1.1
R2(config-router)#network 1.0.0.0 0.255.255.255 area 20
R2(config-router)#network 2.0.0.0 0.255.255.255 area 20
R2(config-router)#network 20.0.0.0 0.255.255.255 area 20
R3(config)#router ospf 1
R3(config-router)#router-id 33.1.1.1
R3(config-router)#network 2.0.0.0 0.255.255.255 a 20
R3(config-router)#network 3.0.0.0 0.255.255.255 area 0
R3(config-router)#network 30.0.0.0 0.255.255.255 area 20
R4(config)#router ospf 1
R4(config-router)#network 3.0.0.0 0.255.255.255 area 0
R4(config-router)#network 40.0.0.0 0.255.255.255 area 0
R4(config-router)#network 14.0.0.0 0.0.0.255 area 0
R2#show ip route ospf
Codes: L - local, 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, H - NHRP, l - LISP
+ - replicated route, % - next hop override
Gateway of last resort is not set
O IA 3.0.0.0/8 [110/128] via 2.2.2.2, 00:28:48, Serial3/1
O 10.0.0.0/8 [110/65] via 1.1.1.1, 00:31:39, Serial3/0
14.0.0.0/32 is subnetted, 1 subnets
O IA 14.0.0.1 [110/129] via 2.2.2.2, 00:26:24, Serial3/1
O 30.0.0.0/8 [110/65] via 2.2.2.2, 00:29:55, Serial3/1
O IA 40.0.0.0/8 [110/129] via 2.2.2.2, 00:26:39, Serial3/1
R3#show ip route ospf
Codes: L - local, 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, H - NHRP, l - LISP
+ - replicated route, % - next hop override
Gateway of last resort is not set
O 1.0.0.0/8 [110/128] via 2.2.2.1, 00:29:41, Serial3/1
O 10.0.0.0/8 [110/129] via 2.2.2.1, 00:29:41, Serial3/1
14.0.0.0/32 is subnetted, 1 subnets
O 14.0.0.1 [110/65] via 3.3.3.2, 00:27:06, Serial3/2
O 20.0.0.0/8 [110/65] via 2.2.2.1, 00:29:41, Serial3/1
O 40.0.0.0/8 [110/65] via 3.3.3.2, 00:27:21, Serial3/2
R4#show ip route ospf
Codes: L - local, 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, H - NHRP, l - LISP
+ - replicated route, % - next hop override
Gateway of last resort is not set
O IA 1.0.0.0/8 [110/192] via 3.3.3.1, 00:28:06, Serial3/2
O IA 2.0.0.0/8 [110/128] via 3.3.3.1, 00:28:06, Serial3/2
O IA 10.0.0.0/8 [110/193] via 3.3.3.1, 00:28:06, Serial3/2
O IA 20.0.0.0/8 [110/129] via 3.3.3.1, 00:28:06, Serial3/2
O IA 30.0.0.0/8 [110/65] via 3.3.3.1, 00:28:06, Serial3/2
No routes from 11.0.0.0 coming in to routing table of R2, R3, R3, and R4 as it belong to area 10 which in not directly connected to area 0
in order to make sure that the loopback of R1 (11.0.0.0) routes should be seen on all the other routes we need to configure Virtual-link which allows area 10 to area 0 virtually over area 20 virtual area.
R1#show ip protocols
*** IP Routing is NSF aware ***
Routing Protocol is "ospf 1"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Router ID 11.1.1.1
Number of areas in this router is 2. 2 normal 0 stub 0 nssa
Maximum path: 4
Routing for Networks:
1.0.0.0 0.255.255.255 area 20
10.0.0.0 0.255.255.255 area 20
11.0.0.0 0.0.0.255 area 10
Routing Information Sources:
Gateway Distance Last Update
22.1.1.1 110 00:37:30
33.1.1.1 110 00:32:35
Distance: (default is 110)
R3#show ip protocols
*** IP Routing is NSF aware ***
Routing Protocol is "ospf 1"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Router ID 33.1.1.1
It is an area border router
Number of areas in this router is 2. 2 normal 0 stub 0 nssa
Maximum path: 4
Routing for Networks:
2.0.0.0 0.255.255.255 area 20
3.0.0.0 0.255.255.255 area 0
30.0.0.0 0.255.255.255 area 20
Routing Information Sources:
Gateway Distance Last Update
11.1.1.1 110 00:35:57
14.0.3.1 110 00:33:22
22.1.1.1 110 00:35:57
Distance: (default is 110)
R1(config)#router ospf 1
R1(config-router)#area 20 virtual-link 33.1.1.1
R3(config)#router ospf 1
R3(config-router)#area 20 virtual-link 11.1.1.1
R2#show ip route ospf
Codes: L - local, 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, H - NHRP, l - LISP
+ - replicated route, % - next hop override
Gateway of last resort is not set
O IA 3.0.0.0/8 [110/128] via 2.2.2.2, 00:40:25, Serial3/1
O 10.0.0.0/8 [110/65] via 1.1.1.1, 00:43:16, Serial3/0
11.0.0.0/32 is subnetted, 1 subnets
O IA 11.0.0.1 [110/65] via 1.1.1.1, 00:02:47, Serial3/0
14.0.0.0/32 is subnetted, 1 subnets
O IA 14.0.0.1 [110/129] via 2.2.2.2, 00:38:01, Serial3/1
O 30.0.0.0/8 [110/65] via 2.2.2.2, 00:41:32, Serial3/1
O IA 40.0.0.0/8 [110/129] via 2.2.2.2, 00:38:16, Serial3/1
R3#show ip route ospf
Codes: L - local, 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, H - NHRP, l - LISP
+ - replicated route, % - next hop override
Gateway of last resort is not set
O 1.0.0.0/8 [110/128] via 2.2.2.1, 00:41:43, Serial3/1
O 10.0.0.0/8 [110/129] via 2.2.2.1, 00:41:43, Serial3/1
11.0.0.0/32 is subnetted, 1 subnets
O IA 11.0.0.1 [110/129] via 2.2.2.1, 00:03:18, Serial3/1
14.0.0.0/32 is subnetted, 1 subnets
O 14.0.0.1 [110/65] via 3.3.3.2, 00:39:08, Serial3/2
O 20.0.0.0/8 [110/65] via 2.2.2.1, 00:41:43, Serial3/1
O 40.0.0.0/8 [110/65] via 3.3.3.2, 00:39:23, Serial3/2
R4#show ip route ospf
Codes: L - local, 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, H - NHRP, l - LISP
+ - replicated route, % - next hop override
Gateway of last resort is not set
O IA 1.0.0.0/8 [110/192] via 3.3.3.1, 00:40:10, Serial3/2
O IA 2.0.0.0/8 [110/128] via 3.3.3.1, 00:40:10, Serial3/2
O IA 10.0.0.0/8 [110/193] via 3.3.3.1, 00:40:10, Serial3/2
11.0.0.0/32 is subnetted, 1 subnets
O IA 11.0.0.1 [110/193] via 3.3.3.1, 00:03:35, Serial3/2
O IA 20.0.0.0/8 [110/129] via 3.3.3.1, 00:40:10, Serial3/2
O IA 30.0.0.0/8 [110/65] via 3.3.3.1, 00:40:10, Serial3/2
R1#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
33.1.1.1 0 FULL/ - - 2.2.2.2 OSPF_VL0
22.1.1.1 0 FULL/ - 00:00:32 1.1.1.2 Serial3/0
R1#show ip ospf virtual-links
Virtual Link OSPF_VL0 to router 33.1.1.1 is up
Run as demand circuit
DoNotAge LSA allowed.
Transit area 20, via interface Serial3/0
Topology-MTID Cost Disabled Shutdown Topology Name
0 128 no no Base
Transmit Delay is 1 sec, State POINT_TO_POINT,
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:04
Adjacency State FULL (Hello suppressed)
Index 1/2, 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
R4#ping 11.0.0.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 11.0.0.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 80/131/172 ms
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