Configuring OSPF Maximum Paths

By default OSPF supports a maximum of 4 paths to be installed to a single network with an identical metric. This however can be changed to a maximum of 16. This lab will discuss and demonstrate the configuration and verification of OSPF maximum paths.

Real World Application & Core Knowledge

If you’ve completed Lab 8-6 – Configuring EIGRP Maximum Paths then you should know how maximum paths works when configured in a dynamic routing protocol.

Like EIGRP, OSPF has the same feature allowing you to administratively configure how many maximum paths OSPF will inject into the routing table with the same metric to the same destination network to load balance over.

By default, OSPF has a maximum path variable of 4, meaning that OSPF will install 4 routes into the routing table with the same metric/destination to load balance over. For example; you have 4 point-to-point T1’s from a branch location to the central office, OSPF by default will load balance across these 4 links if they are independently configured. However if they are in a PPP Multi-link group they will operate as a single 6Mbps link.

To configure the maximum paths variable you’d use the maximum-paths # command in router configuration mode.

In this lab you will configure R1 to use only 1 path to get to a destination network.

Familiarize yourself with the following new command(s);

Command Description
maximum-paths # This command is executed in router configuration mode to set how many equal metric paths that the routing can install into the routing table for load balancing.

The following logical topology shown below is used in labs found through out Section 9 – Configuring OSPF;

Lab Prerequisites

  • If you are using GNS3 than load the Stub Area Networking GNS3 topology than start devices; R1, R2, R3, R4, R5 and SW1.
  • Establish a console session with devices R1, R2, R3, R4, R5 and SW1 than load the initial configurations provided below by copying the config from the textbox and pasting it into the respected routers console.

 

Lab Objectives

  • View the routing table on R1 and check if any routes are being load balanced.
  • Configure R1 to use no more then 1 path to get to any given destination.
  • View R1’s routing table again and verify that network 10.90.45.0/30 is no longer load balanced between R4 and R5.

Lab Instruction

Objective 1. – View the routing table on R1 and check if any routes are being load balanced.

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
       * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     10.0.0.0/8 is variably subnetted, 9 subnets, 4 masks
O IA    10.90.50.1/32 [110/64767] via 10.90.245.5, 01:12:43, Serial0/0
O IA    10.90.40.1/32 [110/64767] via 10.90.245.4, 01:12:43, Serial0/0
O IA    10.90.23.0/30 [110/129532] via 10.90.245.2, 01:12:33, Serial0/0
O IA    10.90.30.1/32 [110/129533] via 10.90.245.2, 01:02:49, Serial0/0
O IA    10.90.145.0/24 [110/65766] via 10.90.245.4, 00:18:43, Serial0/0
O IA    10.90.45.0/30 [110/129532] via 10.90.245.5, 01:12:32, Serial0/0
                     [110/129532] via 10.90.245.4, 01:12:33, Serial0/0
O IA    10.90.20.1/32 [110/64767] via 10.90.245.2, 01:12:43, Serial0/0
C       10.90.10.0/24 is directly connected, Loopback0
C       10.90.245.0/29 is directly connected, Serial0/0
R1#

As you can see from R1’s routing table shown above that R1 is load balancing traffic to the 10.90.45.0/30 destination.


Objective 2. – Configure R1 to use no more then 1 path to get to any given destination.

R1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)#router ospf 1
R1(config-router)#maximum-paths 1
R1(config-router)#end
R1#
%SYS-5-CONFIG_I: Configured from console by console
R1#

Objective 3. – View R1’s routing table again and verify that network 10.90.45.0/30 is no longer load balanced between R4 and R5.

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
       * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     10.0.0.0/8 is variably subnetted, 9 subnets, 4 masks
O IA    10.90.50.1/32 [110/64767] via 10.90.245.5, 00:00:42, Serial0/0
O IA    10.90.40.1/32 [110/64767] via 10.90.245.4, 00:00:42, Serial0/0
O IA    10.90.23.0/30 [110/129532] via 10.90.245.2, 00:00:42, Serial0/0
O IA    10.90.30.1/32 [110/129533] via 10.90.245.2, 00:00:42, Serial0/0
O IA    10.90.145.0/24 [110/65766] via 10.90.245.4, 00:00:42, Serial0/0
O IA    10.90.45.0/30 [110/129532] via 10.90.245.5, 00:00:42, Serial0/0
O IA    10.90.20.1/32 [110/64767] via 10.90.245.2, 00:00:42, Serial0/0
C       10.90.10.0/24 is directly connected, Loopback0
C       10.90.245.0/29 is directly connected, Serial0/0
R1#

After configuring the maximum paths in OSPF to 1 you’ll see that R1 no longer load balances to 10.90.45.0/30 as shown above in R1’s routing table.

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