Configuring OSPF Maximum Paths

Pre-requisite: Open Shortest Path First (OSPF) Protocol fundamentals, Open shortest path first (OSPF) router roles and configuration

OSPF Maximum Paths :

• Open Shortest Path First (OSPF) does equal cost load balancing.
• It is used to perform dynamic routing by exchanging routing table information using LSAs within a single Autonomous System (AS) in a network.
• And if you have multiple routes with equal cost paths to the same destination, then OSPF will do load sharing.
• OSPF allows entry of up to 4 multiple paths of a single network with identical metrics by default, and can be increased to a maximum of 16 paths and a minimum of 1 path. 
• Default settings and range of Maximum-paths vary depending on router platform/model.

OSPF Maximum Paths command :

R1(config-router)#maximum-paths <1-16>

Configuring OSPF Maximum Paths :

Step 1: Create the physical topology in GNS3 as shown in the image below :

Step 2: Configure the IPv4 address on the physical interface on the routers :

On R1 :

R1(config)#interface FastEthernet0/0
R1(config-if)#ip address 12.1.1.1 255.255.255.0
R1(config-if)#no shutdown
R1(config-if)#interface FastEthernet1/0
R1(config-if)#ip address 13.1.1.1 255.255.255.0
R1(config-if)#no shutdown
R1(config-if)#interface FastEthernet1/1
R1(config-if)#ip address 14.1.1.1 255.255.255.0
R1(config-if)#no shutdown

On R2 :

R2(config)#interface FastEthernet0/0
R2(config-if)#ip address 12.1.1.2 255.255.255.0
R2(config-if)#no shutdown
R2(config-if)#interface FastEthernet1/0
R2(config-if)#ip address 10.1.1.2 255.255.255.0
R2(config-if)#no shutdown

On R3 :

R3(config)#interface FastEthernet0/0
R3(config-if)#ip address 13.1.1.2 255.255.255.0
R3(config-if)#no shutdown
R3(config-if)#interface FastEthernet1/0
R3(config-if)#ip address 10.1.1.3 255.255.255.0
R3(config-if)#no shutdown

On R4 :

R4(config)#interface FastEthernet0/0
R4(config-if)#ip address 14.1.1.2 255.255.255.0
R4(config-if)#no shutdown
R4(config-if)#interface FastEthernet1/0
R4(config-if)#ip address 10.1.1.4 255.255.255.0
R4(config-if)#no shutdown 

On SERVER :

SERVER(config)#interface FastEthernet0/0
SERVER(config-if)ip address 10.1.1.1 255.255.255.0
SERVER(config-if)#no shutdown

Step 3: Configuring the OSPF process on all the routers with the command router ospf <process-id> :

R1(config)#router ospf 1
R1(config-router)#exit

R2(config)#router ospf 1
R2(config-router)#exit

R3(config)#router ospf 1
R3(config-router)#exit

R4(config)#router ospf 1
R4(config-router)#exit

SERVER(config)#router ospf 1
SERVER(config-router)#exit

Step 4: Enabling OSPF on the interfaces with the interface level sub-command ip ospf <process-id> area <area-id> :

R1(config)#interface FastEthernet0/0
R1(config-if)#ip ospf 1 area 0
R1(config-if)#interface FastEthernet1/0
R1(config-if)#ip ospf 1 area 0
R1(config-if)#interface FastEthernet1/1
R1(config-if)#ip ospf 1 area 0

R2(config)#interface FastEthernet0/0
R2(config-if)#ip ospf 1 area 0
R2(config-if)#interface FastEthernet1/0
R2(config-if)#ip ospf 1 area 0

R3(config)#interface FastEthernet0/0
R3(config-if)#ip ospf 1 area 0
R3(config-if)#interface FastEthernet1/0
R3(config-if)#ip ospf 1 area 0

R4(config)#interface FastEthernet0/0
R4(config-if)#ip ospf 1 area 0
R4(config-if)#interface FastEthernet1/0
R4(config-if)#ip ospf 1 area 0

SERVER(config)#interface FastEthernet0/0
SERVER(config-if)ip ospf 1 area 0

Step 5: Checking the IPv4 routing table on the routers and seeing OSPF route entry :

R1#show ip route

R2#show ip route

R3#show ip route

R4#show ip route

SERVER#show ip route

Step 6: Configure R1 to use no more than 1 path to get to any given destination, with router subcommand maximum-paths <1-16> :

R1(config)#router ospf 1
R1(config-router)#maximum-paths 1

Step 7: Checking the IPv4 routing table of R1 again to verify the changes in the maximum-paths :

R1#show ip route

Step 8: Verifying the connectivity throughout the topology :