10-1 maximum-paths number-of-paths

10-1 maximum-paths number-of-paths

Syntax Description:

• number-of-paths ” Determines the number of parallel equal-cost paths to the same destination that OSPF will install in the IP routing table. The range of values is 1 to 6. The default is 4 paths.

Purpose: To configure the number of equal-cost parallel paths that OSPF will install in the IP routing table.

Initial Cisco IOS Software Release: 11.2

Configuration Example: Setting the Maximum Number of Equal-Cost Paths That OSPF Will Install in the IP Routing Table

In Figure 10-1, Router A has two parallel equal-cost paths to the loopback network on Router B. By default, OSPF will install both paths in the IP routing table. Configure Routers A and B as shown in the following lines of code.

Figure 10-1. OSPF Can Install Up to Six Parallel Equal-Cost Paths in the IP Routing Table

 Router A  interface Serial0/0   bandwidth 64   ip address 10.1.1.6 255.255.255.252   !   interface Serial0/1   bandwidth 64   ip address 10.1.1.1 255.255.255.252   clockrate 64000   !   router ospf 1   network 10.1.1.0 0.0.0.3 area 0   network 10.1.1.4 0.0.0.3 area 0  ______________________________________________________ Router B  interface Loopback0   ip address 2.2.2.2 255.255.255.255   !   interface Serial0   bandwidth 64   ip address 10.1.1.2 255.255.255.252   !   interface Serial1   bandwidth 64   ip address 10.1.1.5 255.255.255.252   !   router ospf 1   network 10.1.1.0 0.0.0.3 area 0   network 10.1.1.4 0.0.0.3 area 0   network 2.2.2.2 0.0.0.0 area 2  

Verify that OSPF on Router A has installed the two paths to network 2.2.2.2 on Router B.

 rtrA#  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      2.0.0.0/32 is subnetted, 1 subnets  O IA    2.2.2.2 [110/1563] via 10.1.1.5, 00:00:36, Serial0/0   [110/1563] via 10.1.1.2, 00:00:36, Serial0/1  10.0.0.0/30 is subnetted, 2 subnets C       10.1.1.0 is directly connected, Serial0/1 C       10.1.1.4 is directly connected, Serial0/0 

To prove that the paths must have equal cost, change the bandwidth on Serial 0/1 on Router A to 63.

 Router A  interface Serial0/0    bandwidth 63    ip address 10.1.1.6 255.255.255.252  

The OSPF cost for Serial 0/0 is now 100,000,000/63,000 = 1587. The OSPF cost of Serial 0/1 is 100,000,000/64,000 = 1562. Display the routing table on Router A to determine the effect of changing the cost of interface Serial 0/0.

 rtrA#  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      2.0.0.0/32 is subnetted, 1 subnets  O IA    2.2.2.2 [110/1563] via 10.1.1.2, 00:04:27, Serial0/1  10.0.0.0/30 is subnetted, 2 subnets C       10.1.1.0 is directly connected, Serial0/1 C       10.1.1.4 is directly connected, Serial0/0 

The cost of the two paths to reach 2.2.2.2 on Router B is no longer equal, so OSPF will install the route with the lowest cost.

What happens if OSPF has more equal-cost paths to a destination than are allowed by the maximum-paths command? In Figure 10-1 there are two equal-cost paths to network 2.2.2.2 (if we reset the bandwidth on Serial 0/0 to 64). If we set the maximum paths variable to 1, which path will be installed in the IP routing table? Modify the configuration on Router A so Serial 0/0 has a bandwidth of 64 and the maximum-paths variable is set to 1.

 Router A  interface Serial0/0    bandwidth 64    ip address 10.1.1.6 255.255.255.252   !   router ospf 1   network 10.1.1.0 0.0.0.3 area 0   network 10.1.1.4 0.0.0.3 area 0    maximum-paths 1   

Verification

Verify that OSPF has installed only one route to 2.2.2.2 on Router A.

 rtrA#  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      2.0.0.0/32 is subnetted, 1 subnets  O IA    2.2.2.2 [110/1563] via 10.1.1.2, 00:02:00, Serial0/1  10.0.0.0/30 is subnetted, 2 subnets C       10.1.1.0 is directly connected, Serial0/1 C       10.1.1.4 is directly connected, Serial0/0 

Troubleshooting

Nothing should go wrong with this command if configured correctly (famous last words?).