Advanced OSPF Design Concepts

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One neighbor entry must be included in the Cisco IOS software configuration for each known nonbroadcast network neighbor. The neighbor address has to be on the primary address of the interface.

If a neighboring router has become inactive (hello packets have not been seen for the Router Dead Interval period), it might still be necessary to send hello packets to the dead neighbor.

These hello packets will be sent at a reduced rate called Poll Interval. When the router first starts up, it sends only hello packets to those routers with non-zero priority, that is, routers that are eligible to become DR and BDR. After the DR and BDR are selected, the DR and BDR will then start sending hello packets to all neighbors in order to form adjacencies.

Example: The following example declares a router at address on a nonbroadcast network, with a priority of 1 and a poll-interval of 180:

    router ospf    neighbor priority 1 poll-interval 180 

Related Commands: ip ospf priority

network area

To define the interfaces on which OSPF runs and to define the area ID for those interfaces, use the network area router configuration command. To disable OSPF routing for interfaces defined with the address wildcard-mask pair, use the no form of this command. The syntax for this command (and the no form) is as follows:

    network address wildcard-mask area area-id    no network address wildcard-mask area area-id 

Syntax Description:

address. IP address.
wildcard-mask. IP-address-type mask that includes “don’t care” bits
area-id. Area that is to be associated with the OSPF address range. It can be specified as either a decimal value or as an IP address. If you intend to associate areas with IP subnets, you can specify a subnet address as the area-id.

ospf auto-cost-determination

To control how OSPF calculates default metrics for the interface, use the ospf auto-cost-determination router configuration command. To disable this feature, use the no form of this command. The syntax for this command (and the no form) is as follows:

    ospf auto-cost-determination    no ospf auto-cost-determination 

Syntax Description: This command has no arguments or keywords.

Default: Enabled.

Command Mode: Router configuration.

Usage Guidelines: In Cisco IOS Release 10.2 and earlier, OSPF assigns default OSPF metrics to interfaces regardless of the interface bandwidth. It gives both 64K and T1 links the same metric (1562) and, thus, requires an explicit ip ospf cost command in order to take advantage of the faster link.

In Cisco IOS Release 10.3 and later, by default OSPF will calculate the OSPF metric for an interface according to the bandwidth of the interface. For example, a 64K link will get a metric of 1562, while a T1 link will have a metric of 64.

The OSPF metric is calculated as metric-scale / bandwidth, with metric-scale equal to 10 to a power of 8 by default, giving FDDI a metric of 1.

Example: The following example causes a fixed default metric assignment, regardless of interface bandwidth:

    router ospf 1    no ospf auto-cost-determination 

Related Command: ip ospf cost

ospf log-adj-changes

To configure the router to send a SYSLOG message when an OSPF neighbor state changes, perform the following task in router configuration mode:

    ospf log-adj-changes 

Configure this command if you want to know about OSPF neighbor changes without turning on the debugging command debug ip ospf adjacency. The ospf log- adj-changes command provides a higher level view of changes to the state of the peer relationship with less output.


To disable sending routing updates on an interface, use the passive-interface router configuration command. To re-enable the sending of routing updates, use the no form of this command. The syntax for this command (and the no form) is as follows:

    passive-interface type number    no passive-interface type number 

Syntax Description:

type. Interface type.
number. Interface number.

Default: Routing updates are sent on the interface.

Command Mode: Router configuration.

Usage Guidelines: If you disable the sending of routing updates on an interface, the particular subnet will continue to be advertised to other interfaces, and updates from other routers on that interface continue to be received and processed.

For OSPF, OSPF routing information is neither sent nor received through the specified router interface. The specified interface address appears as a stub network in the OSPF domain.

For IS-IS, this command instructs IS-IS to advertise the IP addresses for the specified interface without actually running IS-IS on that interface. The no form of this command for IS-IS disables advertising IP addresses for the specified address.

Enhanced IGRP is disabled on an interface that is configured as passive although it advertises the route.

Example 1: The following example sends IGRP updates to all interfaces on network except Ethernet interface 1:

    router igrp 109    network    passive-interface ethernet 1 

Example 2: The following configuration enables IS-IS on interfaces Ethernet 1 and serial 0 and advertises the IP addresses of Ethernet 0 in its link-state PDUs:

    router isis Finance    passive-interface Ethernet 0    interface Ethernet 1    ip router isis Finance    interface serial 0    ip router isis Finance 

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OSPF Network Design Solutions
OSPF Network Design Solutions
ISBN: 1578700469
EAN: 2147483647
Year: 1998
Pages: 200
Authors: Tom Thomas

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