Make a habit of always going through this check list before configuring any routing protocol on your devic
There might be another instance of the same routing protocol already running on the device or there might be some policies conflicting with the configuration you’re going to make
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Configuring loopback interfaces for Router ID
R1(config)#interface loopback 1
*Nov 24 17:15:38.975: %LINEPROTO-5-UPDOWN: Line protocol on Interface Loopback1, changed state to up
R1(config-if)#ip address 10.10.10.10 255.255.255.0
R1(config-if)#exit
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R2(config)#interface loopback 1
*Nov 24 17:17:55.415: %LINEPROTO-5-UPDOWN: Line protocol on Interface Loopback1, changed state to up
R2(config-if)#ip address 20.20.20.20 255.255.255.0
R2(config-if)#exit
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R3(config)#interface loopback 1
*Nov 24 17:19:17.687: %LINEPROTO-5-UPDOWN: Line protocol on Interface Loopback1, changed state to up
R3(config-if)#ip address 30.30.30.30 255.255.255.0
R3(config-if)#exit
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Verifying Interface Status
R1#show ip interface brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 172.16.10.1 YES NVRAM up up
FastEthernet0/1 unassigned YES NVRAM administratively down down
Serial1/0 1.1.1.1 YES NVRAM up up
Serial1/1 unassigned YES manual administratively down down
Serial1/2 unassigned YES NVRAM administratively down down
Serial1/3 unassigned YES NVRAM administratively down down
Loopback1 10.10.10.10 YES manual up up
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R2#show ip interface brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 172.16.20.1 YES NVRAM up up
FastEthernet0/1 unassigned YES NVRAM administratively down down
Serial1/0 1.1.1.2 YES NVRAM up up
Serial1/1 2.1.1.1 YES NVRAM up up
Serial1/2 unassigned YES NVRAM administratively down down
Serial1/3 unassigned YES NVRAM administratively down down
Loopback1 20.20.20.20 YES manual up up
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R3#show ip interface brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 172.16.30.1 YES NVRAM up up
FastEthernet0/1 unassigned YES NVRAM administratively down down
Serial1/0 unassigned YES manual administratively down down
Serial1/1 2.1.1.2 YES NVRAM up up
Serial1/2 unassigned YES NVRAM administratively down down
Serial1/3 unassigned YES NVRAM administratively down down
Loopback1 30.30.30.30 YES manual up up
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Verifying Routing Protocols Configured
R1#show ip protocols
R1#
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R2#show ip protocols
R2#
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R3#show ip protocols
R3#
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It means that there’s no routing protocol already configured.
Configuring Routing Protocol
R1#conf terminal
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#router ospf 1
R1(config-router)#network 172.16.10.0 0.0.0.255 a 0
R1(config-router)#network 1.1.1.0 0.0.0.255 a 0
R1(config-router)#exit
*Nov 24 14:42:32.843: %SYS-5-CONFIG_I: Configured from console by console
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R2#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
R2(config)#router ospf 1
R2(config-router)#network 172.16.20.1 0.0.0.255 area 0
R2(config-router)#network 1.1.1.0 0.0.0.255 area 0
*Nov 24 14:46:59.747: %OSPF-5-ADJCHG: Process 1, Nbr 172.16.10.1 on Serial1/0 from LOADING to FULL, Loading Done
R2(config-router)#network 2.1.1.0 0.0.0.255 area 0
R2(config-router)#exit
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You can see that the adjacency between R2 and R1, where configuration has already been made is up and it is in FULL state.
R3#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
R3(config)#router ospf 1
R3(config-router)#network 172.16.30.0 0.0.0.255 area 0
R3(config-router)#network 2.1.1.0 0.0.0.255 area 0
*Nov 24 14:54:46.767: %OSPF-5-ADJCHG: Process 1, Nbr 172.16.20.1 on Serial1/1 from LOADING to FULL, Loading Done
R3(config-router)#exit
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Now another adjacency is up and running between R3 and R2
Verifying the adjacencies
R1#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
172.16.20.1 0 FULL/ - 00:00:38 1.1.1.2 Serial1/0
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On R1 Neighbor 172.16.20.1 is up and it’s in full state
R2#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
172.16.30.1 0 FULL/ - 00:00:39 2.1.1.2 Serial1/1
172.16.10.1 0 FULL/ - 00:00:30 1.1.1.1 Serial1/0
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R2 has two neighbors because it is in the middle of other two routers and they are directly connected to R2
R3#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
172.16.20.1 0 FULL/ - 00:00:39 2.1.1.1 Serial1/1
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R3 has only one directly connected neighbor and it is in full state
Write your routing table with your own hands before checking it on the router
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Checking the routing table
R1#show ip route
Codes: 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
*****OUTPUT omitted Intentionally*****
Gateway of last resort is not set
1.0.0.0/24 is subnetted, 1 subnets
C 1.1.1.0 is directly connected, Serial1/0
2.0.0.0/24 is subnetted, 1 subnets
O 2.1.1.0 [110/128] via 1.1.1.2, 00:22:51, Serial1/0
172.16.0.0/24 is subnetted, 3 subnets
O 172.16.30.0 [110/129] via 1.1.1.2, 00:22:51, Serial1/0
O 172.16.20.0 [110/65] via 1.1.1.2, 00:22:51, Serial1/0
C 172.16.10.0 is directly connected, FastEthernet0/0
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R2#show ip route
Codes: 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
*****OUTPUT omitted Intentionally*****
Gateway of last resort is not set
1.0.0.0/24 is subnetted, 1 subnets
C 1.1.1.0 is directly connected, Serial1/0
2.0.0.0/24 is subnetted, 1 subnets
C 2.1.1.0 is directly connected, Serial1/1
172.16.0.0/24 is subnetted, 3 subnets
O 172.16.30.0 [110/65] via 2.1.1.2, 01:17:29, Serial1/1
C 172.16.20.0 is directly connected, FastEthernet0/0
O 172.16.10.0 [110/65] via 1.1.1.1, 01:18:47, Serial1/0
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R3#show ip route
Codes: 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
*****OUTPUT omitted Intentionally*****
Gateway of last resort is not set
1.0.0.0/24 is subnetted, 1 subnets
O 1.1.1.0 [110/128] via 2.1.1.1, 01:18:00, Serial1/1
2.0.0.0/24 is subnetted, 1 subnets
C 2.1.1.0 is directly connected, Serial1/1
172.16.0.0/24 is subnetted, 3 subnets
C 172.16.30.0 is directly connected, FastEthernet0/0
O 172.16.20.0 [110/65] via 2.1.1.1, 01:18:00, Serial1/1
O 172.16.10.0 [110/129] via 2.1.1.1, 01:18:00, Serial1/1
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Convergence is already completed all the routers have all the networks in the routing table so we can test the connectivity with Ping test.
Verifying connectivity with ping test
R1#ping 172.16.30.1 source 172.16.10.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.30.1, timeout is 2 seconds:
Packet sent with a source address of 172.16.10.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 20/27/56 ms
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R3#ping 172.16.20.1 source 172.16.30.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.20.1, timeout is 2 seconds:
Packet sent with a source address of 172.16.30.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/24/52 ms
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If you’re using loopback interface as your router ID it’s highly recommended to advertise router ID, for management purpose Router ID should be reachable throughout the network, if a network is down Routing protocol will route you from another path.
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