Here an example of 1 corporate router with connections to 3 branche routers. Each connection is with a serial interface. The routing is done with OSPF.
The example is created with Cisco Packet Tracer.
Partial running-config of CORP
CORP# ! OUTPUT OMITTED ! interface Loopback0 ip address 1.1.1.1 255.255.255.255 ! interface Tunnel0 ip address 10.0.10.1 255.255.255.252 mtu 1476 tunnel source Serial1/0 tunnel destination 194.178.28.2 ! interface Tunnel1 ip address 10.0.10.5 255.255.255.252 mtu 1476 tunnel source Serial1/1 tunnel destination 194.178.28.6 ! interface Tunnel2 ip address 10.0.10.9 255.255.255.252 mtu 1476 tunnel source Serial1/2 tunnel destination 194.178.28.10 ! interface FastEthernet0/0 ip address 192.168.10.1 255.255.255.0 duplex auto speed auto ! OUTPUT OMITTED ! interface Serial1/0 ip address 194.178.28.1 255.255.255.252 clock rate 2000000 ! interface Serial1/1 ip address 194.178.28.5 255.255.255.252 clock rate 2000000 ! interface Serial1/2 ip address 194.178.28.9 255.255.255.252 clock rate 2000000 ! OUTPUT OMITTED ! router ospf 1 network 192.168.10.0 0.0.0.255 area 0 network 10.0.0.0 0.255.255.255 area 0 network 192.168.20.0 0.0.0.255 area 0 network 192.168.30.0 0.0.0.255 area 0 ! OUTPUT OMITTED ! CORP#
Partial running-config of BR1
BR1# ! OUTPUT OMITTED ! interface Loopback0 ip address 2.1.1.1 255.255.255.0 ! interface Tunnel0 ip address 10.0.10.2 255.255.255.252 mtu 1476 tunnel source Serial1/0 tunnel destination 194.178.28.1 ! interface FastEthernet0/0 ip address 192.168.20.1 255.255.255.0 duplex auto speed auto ! OUTPUT OMITTED ! interface Serial1/0 ip address 194.178.28.2 255.255.255.252 ! OUTPUT OMITTED ! router ospf 1 network 10.0.0.0 0.255.255.255 area 0 network 192.168.20.0 0.0.0.255 area 0 ! OUTPUT OMITTED ! BR1#
Partial running-config of BR2
BR2# ! interface Loopback0 ip address 2.1.1.2 255.255.255.0 ! interface Tunnel1 ip address 10.0.10.6 255.255.255.252 mtu 1476 tunnel source Serial1/0 tunnel destination 194.178.28.5 ! interface FastEthernet0/0 ip address 192.168.30.1 255.255.255.0 duplex auto speed auto ! OUTPUT OMITTED ! interface Serial1/0 ip address 194.178.28.6 255.255.255.252 ! OUTPUT OMITTED ! router ospf 1 network 192.168.30.0 0.0.0.255 area 0 network 10.0.0.0 0.255.255.255 area 0 ! OUTPUT OMITTED ! BR2#
Partial running-config of BR3
BR3# ! OUTPUT OMITTED ! interface Loopback0 ip address 2.1.1.3 255.255.255.0 ! interface Tunnel2 ip address 10.0.10.10 255.255.255.252 mtu 1476 tunnel source Serial1/0 tunnel destination 194.178.28.9 ! interface FastEthernet0/0 ip address 192.168.40.1 255.255.255.0 duplex auto speed auto ! OUTPUT OMITTED ! interface Serial1/0 ip address 194.178.28.10 255.255.255.252 ! OUTPUT OMITTED ! router ospf 1 network 192.168.40.0 0.0.0.255 area 0 network 10.0.0.0 0.255.255.255 area 0 ! OUTPUT OMITTED ! BR3#
show ip ospf neighbor (all 4 routers)
CORP#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 2.1.1.1 0 FULL/ - 00:00:36 10.0.10.2 Tunnel0 2.1.1.2 0 FULL/ - 00:00:32 10.0.10.6 Tunnel1 2.1.1.3 0 FULL/ - 00:00:34 10.0.10.10 Tunnel2 BR1#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 1.1.1.1 0 FULL/ - 00:00:35 10.0.10.1 Tunnel0 BR2#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 1.1.1.1 0 FULL/ - 00:00:34 10.0.10.5 Tunnel1 BR3#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 1.1.1.1 0 FULL/ - 00:00:32 10.0.10.9 Tunnel2
show ip route (corp router)
CORP#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, ia - IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route
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
10.0.0.0/30 is subnetted, 3 subnets
C 10.0.10.0 is directly connected, Tunnel0
C 10.0.10.4 is directly connected, Tunnel1
C 10.0.10.8 is directly connected, Tunnel2
C 192.168.10.0/24 is directly connected, FastEthernet0/0
O 192.168.20.0/24 [110/1001] via 10.0.10.2, 00:02:23, Tunnel0
O 192.168.30.0/24 [110/1001] via 10.0.10.6, 00:02:23, Tunnel1
O 192.168.40.0/24 [110/1001] via 10.0.10.10, 00:02:23, Tunnel2
194.178.28.0/30 is subnetted, 3 subnets
C 194.178.28.0 is directly connected, Serial1/0
C 194.178.28.4 is directly connected, Serial1/1
C 194.178.28.8 is directly connected, Serial1/2