Designing Implementing an OSPF Network | OSPF Network Design Solutions

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Configuring OSPF Tunable Parameters

Cisco’s OSPF implementation enables you to alter certain interface-specific OSPF parameters, as needed. You are not required to alter any of these parameters, but some interface parameters must be consistent across all routers in an attached network. Those are the parameters set by the following commands:

    ip ospf hello-interval    ip ospf dead-interval    ip ospf authentication-key    timers spf spf-delay spf-holdtime

Therefore, be sure that if you do configure any of these parameters, the configurations for all routers on your network have compatible values.

More detailed information regarding tunable OSPF parameters was previously covered in Chapter 6. Remember that the hello packet defaults to being sent every 10 seconds on broadcast networks (Ethernet) and every 30 seconds on nonbroadcast networks (Frame Relay).

In interface configuration mode, specify any of the interface parameters as needed for your network as shown in Table 7-1.

**Table 7-1** Tunable OSPF parameters
Command	Task

`ip ospf cost cost`	Explicitly specify the cost of sending a packet on an OSPF interface
`ip ospf retransmit-interval seconds`	Specify the number of seconds between link-state advertisement retransmissions for adjacencies belonging to an OSPF interface
`ip ospf transmit-delay seconds`	Set the estimated number of seconds it takes to transmit a link-state update packet on an OSPF interface
`ip ospf priority number`	Set priority to help determine the OSPF designated router for a network
`ip ospf hello-interval seconds`	Specify the length of time, in seconds, between hello packets the Cisco IOS software sends on an OSPF interface
`ip ospf dead-interval seconds`	Set the number of seconds that a device’s hello packets must not have been seen before its neighbors declare the OSPF router down
`ip ospf authentication-key key`	Assign a specific password to be used by neighboring OSPF routers on a network segment that is using OSPF’s simple password authentication
`ip ospf message-digest-key keyid md5` key	Enable OSPF MD5 authentication

Configuring Route Calculation Timers

You can configure the delay time between when OSPF receives a topology change and when it starts a shortest path first (SPF) calculation. You can also configure the hold time between two consecutive SPF calculations. This command was added to prevent routers from computing new routing tables. This is important if you are running OSPF in a very active network that experiences a lot of interface changes or other occurrences which would cause an LSA to be sent, such as a rapidly flapping serial line.

To set the values, perform the following task in router configuration mode:

    timers spf spf-delay spf-holdtime

Creating a Loopback Interface

As previously discussed, the use of a loopback interface will force the selection by OSPF of its router ID. The default for Cisco routers is loopback interface and then the highest IP address assigned to an interface. The use of a loopback interface enables you to assign the router ID. This can be very beneficial. Because a loopback interface is not a physical interface, like Ethernet, you must create it.

You can configure a loopback interface by entering the interface loopback 0 command in the router configuration mode. The following example demonstrates the process:

    OSPF_Router# conf t    Enter configuration commands, one per line. End with CNTL/Z.    OSPF_Router(config)# interface loopback 0    OSPF_Router(config-if)# ip address 10.251.11.1 255.255.255.255    OSPF_Router(config-if)# description Configured to be OSPF Router ID

Configuring OSPF for Different Network Types

As previously discussed, OSPF classifies different media into three types of networks as a default. Each of these networks requires a slightly different configuration to optimize the performance of OSPF. This section covers the methods and procedures that are needed in order to configure OSPF over different physical networks, such as the following:

• Broadcast networks (Ethernet, Token Ring, FDDI)

• Nonbroadcast multiaccess networks (SMDS, Frame Relay, X.25)

• Point-to-point networks (HDLC, PPP)

One of the most flexible features of OSPF is that you can configure your network as either a broadcast or a nonbroadcast multiaccess network. OSPF will respond accordingly.

X.25 and Frame Relay provide an optional broadcast capability that can be configured using the map command to allow OSPF to run as a broadcast network. This command is useful if you are running a meshed network.

The specific X.25 and Frame Relay commands are outside the scope of this book. If additional information is required, see the x25 map and frame-relay map command descriptions in Cisco’s Wide-Area Networking Command Reference for more details.

Configuring your OSPF network type is one of the most functional features of OSPF. Everyone realizes that OSPF is not the perfect protocol, in fact, there will be never a protocol suitable for every situation. The strength of OSPF lies in its capability to be customized to meet certain network design requirements. The following sections will assist your understanding of customizing OSPF to your network’s design.

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