Configuring OSPF Interface Cost

In this lab you will learn how to configure manipulate Open Shortest Path First route metrics by statically configuring the OSPF interface cost.

Real World Application & Core Knowledge

Unlike the metric in RIP which is determined by hop count and EIGRP’s crazy mathematical formulated metric, OSPF is a little more simple. The default formula to calculate the cost for the OSPF metric is (10^8/BW).

By default the metric’s reference cost is 100Mbps, so any link that is 100Mbps will have a metric of 1. a T1 interface will have a metric of 64 so in this case if a router is trying to get to a FastEthernet network on a router that is through a T1 the metric would be 65 (64 +1).

You do however have the ability to statically specify a metric on a per interface basis by using the ip ospf cost # where the cost is an integer between 1-65535.

So the big question is why would you want to statically configure a metric?

The biggest advantage of statically configuring an OSPF metric on an interface is to manipulate which route will be chosen dynamically via OSPF. In a nut shell its like statically configuring a dynamic protocol to use a specific route. Pretty cool huh?

In this lab you’ll increase the interface cost on R5′s FastEthernet0/0 interface to make R4 the preferred route on R1 to get to the 10.45.1.0/24 network.

Familiarize yourself with the following new command(s);

ip ospf cost # – This command is executed in interface configuration mode to statically configure the OSPF interface cost of the specified interface.

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

Lab Prerequisites

• If you completed lab 9-9 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 9-10 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  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 9-10 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  log-adjacency-changes  area 3 stub no-summary ! line con 0  logging sync  no exec-timeout ! end

!################################################## !#  Free CCNA Workbook Lab 9-10 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  area 3 stub ! line con 0  logging sync  no exec-timeout ! end

!################################################## !#  Free CCNA Workbook Lab 9-10 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  log-adjacency-changes ! line con 0  logging sync  no exec-timeout ! end

!################################################## !#  Free CCNA Workbook Lab 9-10 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 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 ! 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

• Observe the routing table on R1, The route to 10.45.1.0/24 should be load balanced via R4 and R5.
• Configure R5′s 10.45.1.0/24 interface with the OSPF cost 100; afterwards verify R1′s routing table to see if R1 is using the R4 to get to the 10.45.1.0/24 route.

Lab Instruction

Objective 1. – Observe the routing table on R1, The route to 10.45.1.0/24 should be load balanced via 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 [110/65] via 10.1.245.5, 02:09:40, Serial1/0 O IA 10.55.40.1/32 [110/65] via 10.1.245.4, 02:09:40, Serial1/0 O IA 10.1.23.0/30 [110/128] via 10.1.245.2, 02:01:57, Serial1/0 O IA 10.55.30.1/32 [110/129] via 10.1.245.2, 00:47:17, Serial1/0 O IA 10.45.1.0/24 [110/65] via 10.1.245.5, 02:09:40, Serial1/0 [110/65] via 10.1.245.4, 00:11:37, Serial1/0 O IA 10.1.45.0/30 [110/128] via 10.1.245.5, 02:09:40, Serial1/0 [110/128] via 10.1.245.4, 02:09:40, Serial1/0 O IA 10.55.20.1/32 [110/65] via 10.1.245.2, 02:01:57, Serial1/0 C 10.55.10.0/24 is directly connected, Loopback0 C 10.1.245.0/29 is directly connected, Serial1/0 R1#

Objective 2. – Configure R5′s 10.45.1.0/24 interface with the OSPF cost 100; afterwards verify R1′s routing table to see if R1 is using the R4 to get to the 10.45.1.0/24 route.

In order to make R4 the preferred route on R1, you must increase R5′s cost to the 10.45.1.0/24 network as both routers cost to the 10.45.1.0/24 network is 1 since 100Mbps is the default reference bandwidth of OSPF.

R5#configure terminal Enter configuration commands, one per line. End with CNTL/Z. R5(config)#interface FastEthernet0/0 R5(config-if)#ip ospf cost 100 R5(config-if)#end R5# %SYS-5-CONFIG_I: Configured from console by console R5#

After the cost has been changed on R5 verify that R1 is now using R4 as the next hop to get to the 10.45.1.0/24 network as shown below;

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 [110/65] via 10.1.245.5, 02:14:01, Serial1/0
O IA    10.55.40.1/32 [110/65] via 10.1.245.4, 02:14:01, Serial1/0
O IA    10.1.23.0/30 [110/128] via 10.1.245.2, 02:06:18, Serial1/0
O IA    10.55.30.1/32 [110/129] via 10.1.245.2, 00:51:38, Serial1/0
O IA    10.45.1.0/24 [110/65] via 10.1.245.4, 00:15:59, Serial1/0
O IA    10.1.45.0/30 [110/128] via 10.1.245.5, 02:14:01, Serial1/0
                    [110/128] via 10.1.245.4, 02:14:01, Serial1/0
O IA    10.55.20.1/32 [110/65] via 10.1.245.2, 02:06:18, Serial1/0
C       10.55.10.0/24 is directly connected, Loopback0
C       10.1.245.0/29 is directly connected, Serial1/0
R1#