Precedence Of Commands; Configuring Ospf Areas - Juniper IGP - CONFIGURATION GUIDE V11.1.X Configuration Manual

JUNOSe 11.0.x IP, IPv6, and IGP Configuration Guide

Precedence of Commands

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.
The default cost for this OSPF interface is 10. Change the cost for this OSPF interface
by using the address cost command.
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.
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.

Configuring OSPF Areas

You can divide your OSPF routing domain into OSPF areas. Dividing into areas
provides the following benefits:
You must attach each area in your routing domain to an area called the backbone
area (0.0.0.0).
Disadvantages of using OSPF areas include the following:
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Configuring OSPF Areas
host1(config)#router ospf 100
host1(router-config)#address 10.10.1.1 area 0
host1(router-config)#address 10.10.1.1 cost 45
host1(config)#int fastEthernet 0/0
host1(config-if)#ip ospf cost 23
host1(router-config)#address 10.10.1.1 cost 23
Reduces resource demands placed on routers and links
Reduces the router CPU usage by the OSPF routing calculation
Reduces the amount of memory used for link-state databases
Hides subnets within areas from the rest of the routing domain
Increases routing security within the area
Areas hide information, which can result in less-than-optimal data paths.