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AI2524 Router Card User’s Manual
Page 8-8 August 1997
2524UM
Force the Router ID Choice with a Loopback Interface
OSPF uses the largest IP address configured on the interfaces as its
router ID. If the interface associated with this IP address is ever
brought down, or if the address is removed, the OSPF process must re-
calculate a new router ID and resend all its routing information over its
interfaces.
If a loopback interface is configured with an IP address, the Cisco IOS
software will use this IP address as its router ID, even if other inter
faces have larger IP addresses. Since loopback interfaces never go
down, greater stability in the routing table is achieved.
To configure an IP address on a loopback interface, perform thes
tasks, starting in global configuration mode:
1. Create a loopback interface, which places you in interface config-
uration mode
interface loopback 0
2. Assign an IP address to this interface.
ip address
address mask
Disable Default OSPF Metric Calculation Based on
Bandwidth
OSPF calculates the OSPF metric for an interface according to the
bandwidth of the interface. To disable this feature, type this command
in router configuration mode:
no ospf auto-cost-determination
Configure OSPF on Simplex Ethernet Interfaces
Because simplex interfaces between two devices on an Ethernet repre-
sent only one network segment, for OSPF you must configure the
transmitting interface to be a passive interface. This prevents OSPF
from sending hello packets for the transmitting interface. Both devices
are able to see each other via the hello packet generated for the receiv-
ing interface.
To configure OSPF on simplex Ethernet interfaces, type this command
in router configuration mode:
passive-interface
type number