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6.3 | ![]() |
OSPF Configuration and Verification | |
6.3.2 | ![]() |
Optional configuration commands |
Configuring a Loopback Address When the OSPF process starts, the Cisco IOS uses the highest local IP address as its OSPF router ID. If a loopback interface is configured, that address is used, regardless of its value. An IP can be assigned to a loopback interface with the following commands:
A loopback derived router ID ensures stability because that interface is immune to link failure. The loopback interface must be configured before the OSPF process starts, to override the highest interface IP address. It is recommended that the loopback address be used on all key routers in the OSPF based network. To avoid routing problems, it is good practice to use a 32-bit subnet mask when configuring a loopback IP address, shown as follows:
A 32-bit mask is sometimes called a
host mask, because it specifies a single host and not a network or
subnetwork.
Note: To prevent propagation of bogus or fake
routes, OSPF always advertises loopback addresses as host routes, with
a 32-bit mask. Modifying OSPF Router Priority
To set the E0 FastEthernet interface on a router with a priority of zero (0) use the commands that follow:
For the priority value to figure into
the election, it must be set before the election takes place. The priority value and other key information
on the interface can be displayed
with the show ip ospf interface command.
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