Juniper IGP - CONFIGURATION GUIDE V11.1.X, Configuration Manual

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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.
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
Configuring OSPF Areas
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
■
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
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:
■
Areas hide information, which can result in less-than-optimal data paths.
266 ■
Configuring OSPF Areas
JUNOSe 11.0.x IP, IPv6, and IGP Configuration Guide