Configuring OSPF Maximum Paths

In this lab you will learn how to configure an Open Shortest Path First (OSPF) passive interface to prevent OSPF from forming neighbor relationships on an OSPF enabled interface.

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);

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 completed lab 9-12 you can start where you’ve left off or you can load the Free CCNA Workbook GNS3 topology; start and establish a console session with R1, R2, R3, R4, R5 and SW1 then load their initial configurations included below by copying the config from the textbox and pasting it into the routers console.

Initial Configurations

!################################################## !#  Free CCNA Workbook Lab 9-13 R1 Initial Config # !################################################## ! enable configure terminal ! hostname R1 no ip domain-lookup ! interface Loopback0  description ### SIMULATED NETWORK ###  ip address 10.55.10.1 255.255.255.0  ip ospf 1 area 1 ! interface Serial1/0  description ### PHYSICAL FRAME RELAY INTERFACE ###  ip address 10.1.245.1 255.255.255.248  ip ospf 1 area 0  encapsulation frame-relay  serial restart-delay 0  frame-relay map ip 10.1.245.5 125 broadcast  frame-relay map ip 10.1.245.4 124 broadcast  frame-relay map ip 10.1.245.2 122 broadcast  no frame-relay inverse-arp  no shut  exit ! router ospf 1  router-id 1.1.1.1  log-adjacency-changes  auto-cost reference-bandwidth 100000  neighbor 10.1.245.2  neighbor 10.1.245.4  neighbor 10.1.245.5 ! line con 0  logging sync  no exec-timeout ! end

!################################################## !#  Free CCNA Workbook Lab 9-13 R2 Initial Config # !################################################## ! enable configure terminal ! hostname R2 no ip domain-lookup ! interface Loopback0  description ### SIMULATED NETWORK ###  ip address 10.55.20.1 255.255.255.0  ip ospf 1 area 2 ! interface Serial1/0  description ### PHYSICAL FRAME RELAY INTERFACE ###  ip address 10.1.245.2 255.255.255.248  encapsulation frame-relay  ip ospf priority 0 ip ospf 1 area 0  serial restart-delay 0  no frame-relay inverse-arp  frame map ip 10.1.245.1 221 broadcast  frame map ip 10.1.245.4 221  frame map ip 10.1.245.5 221  no shut ! interface Serial1/2  ### POINT-TO-POINT LINK TO R3 ###  ip address 10.1.23.1 255.255.255.252  ip ospf 1 area 3  ip ospf hello-interval 1  encapsulation ppp  serial restart-delay 0  no shut  exit ! router ospf 1  router-id 2.2.2.2  auto-cost reference-bandwidth 100000  log-adjacency-changes  area 3 stub no-summary ! line con 0  logging sync  no exec-timeout ! end

!################################################## !#  Free CCNA Workbook Lab 9-13 R3 Initial Config # !################################################## ! enable configure terminal ! hostname R3 no ip domain-lookup ! interface Loopback0  description ### SIMULATED NETWORK ###  ip address 10.55.30.1 255.255.255.0  ip ospf 1 area 3 ! interface Serial1/1  description ### POINT-TO-POINT LINK TO R2 ###  ip address 10.1.23.2 255.255.255.252  ip ospf 1 area 3  ip ospf hello-interval 1  encapsulation ppp  no shut  exit ! router ospf 1  router-id 3.3.3.3  log-adjacency-changes  auto-cost reference-bandwidth 100000  area 3 stub ! line con 0  logging sync  no exec-timeout ! end

!################################################## !#  Free CCNA Workbook Lab 9-13 R4 Initial Config # !################################################## ! enable configure terminal ! hostname R4 no ip domain-lookup ! interface Loopback0  description ### SIMULATED NETWORK ###  ip address 10.55.40.1 255.255.255.0  ip ospf 1 area 4 ! interface FastEthernet0/0  description ### REAL NETWORK ###  ip address 10.45.1.1 255.255.255.0  ip ospf 1 area 45  no shut ! interface Serial1/0  description ### PHYSICAL FRAME RELAY INTERFACE ###  ip address 10.1.245.4 255.255.255.248  encapsulation frame-relay  ip ospf priority 0  ip ospf 1 area 0  serial restart-delay 0  no frame-relay inverse-arp  frame map ip 10.1.245.1 421 broadcast  frame map ip 10.1.245.2 421  frame map ip 10.1.245.5 421  no shut ! interface Serial1/1  description ### POINT-TO-POINT LINK TO R5 ###  ip address 10.1.45.1 255.255.255.252  ip ospf 1 area 45  encapsulation ppp  serial restart-delay 0  no shut  exit ! router ospf 1  router-id 4.4.4.4  auto-cost reference-bandwidth 100000  log-adjacency-changes  passive-interface fastethernet0/0 ! line con 0  logging sync  no exec-timeout ! end

!################################################## !#  Free CCNA Workbook Lab 9-13 R5 Initial Config # !################################################## ! enable configure terminal ! hostname R5 no ip domain-lookup ! interface Loopback0  description ### SIMULATED NETWORK ###  ip address 10.55.50.1 255.255.255.0  ip ospf 1 area 5 ! interface FastEthernet0/0  description ### REAL NETWORK ###  ip address 10.45.1.2 255.255.255.0  ip ospf cost 65535  ip ospf 1 area 45  no shut ! interface Serial1/0  description ### PHYSICAL FRAME RELAY INTERFACE ###  ip address 10.1.245.5 255.255.255.248  encapsulation frame-relay  ip ospf priority 0  ip ospf 1 area 0  serial restart-delay 0  no frame-relay inverse-arp  frame map ip 10.1.245.1 521 broadcast  frame map ip 10.1.245.2 521  frame map ip 10.1.245.4 521  no shut ! interface Serial1/1  description ### POINT-TO-POINT LINK TO R4 ###  ip address 10.1.45.2 255.255.255.252  ip ospf 1 area 45  encapsulation ppp  serial restart-delay 0  no shut  exit ! router ospf 1  router-id 5.5.5.5  log-adjacency-changes  auto-cost reference-bandwidth 100000  passive-interface fastethernet0/0 ! line con 0  logging sync  no exec-timeout ! end

!################################################## !#  Free CCNA Workbook Lab 9-X SW1 Initial Config # !################################################## ! enable configure terminal ! hostname SW1 no ip domain-lookup ! line con 0  logging sync  no exec-timeout ! end

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 10.1.45.0/30 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 i - IS-IS, su - IS-IS summary, 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 10.0.0.0/8 is variably subnetted, 9 subnets, 4 masks O IA 10.55.50.1/32 [110/64767] via 10.1.245.5, 01:12:43, Serial1/0 O IA 10.55.40.1/32 [110/64767] via 10.1.245.4, 01:12:43, Serial1/0 O IA 10.1.23.0/30 [110/129532] via 10.1.245.2, 01:12:33, Serial1/0 O IA 10.55.30.1/32 [110/129533] via 10.1.245.2, 01:02:49, Serial1/0 O IA 10.45.1.0/24 [110/65766] via 10.1.245.4, 00:18:43, Serial1/0 O IA 10.1.45.0/30 [110/129532] via 10.1.245.5, 01:12:32, Serial1/0 [110/129532] via 10.1.245.4, 01:12:33, Serial1/0 O IA 10.55.20.1/32 [110/64767] via 10.1.245.2, 01:12:43, Serial1/0 C 10.55.10.0/24 is directly connected, Loopback0 C 10.1.245.0/29 is directly connected, Serial1/0 R1#

As you can see from R1′s routing table shown above that R1 is load balancing traffic to the 10.1.45.0/30 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 10.1.45.0/30 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 i - IS-IS, su - IS-IS summary, 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 10.0.0.0/8 is variably subnetted, 9 subnets, 4 masks O IA 10.55.50.1/32 [110/64767] via 10.1.245.5, 00:00:42, Serial1/0 O IA 10.55.40.1/32 [110/64767] via 10.1.245.4, 00:00:42, Serial1/0 O IA 10.1.23.0/30 [110/129532] via 10.1.245.2, 00:00:42, Serial1/0 O IA 10.55.30.1/32 [110/129533] via 10.1.245.2, 00:00:42, Serial1/0 O IA 10.45.1.0/24 [110/65766] via 10.1.245.4, 00:00:42, Serial1/0 O IA 10.1.45.0/30 [110/129532] via 10.1.245.5, 00:00:42, Serial1/0 O IA 10.55.20.1/32 [110/64767] via 10.1.245.2, 00:00:42, Serial1/0 C 10.55.10.0/24 is directly connected, Loopback0 C 10.1.245.0/29 is directly connected, Serial1/0 R1#

After configuring the maximum paths in OSPF to 1 you’ll see that R1 no longer load balances to 10.1.45.0/30 as shown above in R1′s routing table.