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Precedence of Commands
Configuring OSPF Areas
Copyright © 2010, Juniper Networks, Inc.
If you want to specify the cost, you can do so for both interfaces simultaneously.
host1(config)#interface fastEthernet 0/0
host1(config-if)#ip ospf cost 30
You can use address commands to create a third OSPF interface over the Ethernet
interface. When you specify a cost, you set it for only that interface.
host1(config)#interface fastEthernet 0/0
host1(config-if)#ip address 3.3.3.3 255.255.255.0 secondary
host1(config-if)#exit
host1(config)#router ospf 1
host1(config-router)#address 3.3.3.3 area 0
host1(config-router)#address 3.3.3.3 cost 25
For a single OSPF interface, when you modify the same OSPF attribute by issuing both
the ip ospf command and the address command, the value configured with the address
command takes precedence. In other words, the most specific command for a single
OSPF interface takes precedence.
Consider the following example. Suppose you have a numbered IP interface with an IP
address of 10.10.1.1/24 sitting on top of Fast Ethernet interface 0/0. Configure a single
OSPF interface on top of the IP interface.
host1(config)#router ospf 100
host1(router-config)#address 10.10.1.1 area 0
The default cost for this OSPF interface is 10. Change the cost for this OSPF interface by
using the address cost command.
host1(router-config)#address 10.10.1.1 cost 45
The cost for OSPF interface 10.10.1.1 is now 45.
Now use the ip ospf cost command to change the cost for this OSPF interface.
host1(config)#int fastEthernet 0/0
host1(config-if)#ip ospf cost 23
The cost of OSPF interface 10.10.1.1 does not change. The previously issued address cost
command is more specific for the interface and takes precedence over the ip ospf cost
command. You must use the address cost command if you want to change the cost
again.
host1(router-config)#address 10.10.1.1 cost 23
You can divide your OSPF routing domain into OSPF areas. Dividing into areas provides
the following benefits:
Reduces resource demands placed on routers and links
Reduces the router CPU usage by the OSPF routing calculation
Chapter 5: Configuring OSPF
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