Setting interface properties

The interface properties for OSPF define the way OSPF operates on the selected interface. If you have more than one router interface using OSPF, you need to configure each according to your network's needs and the neighboring routers. The following sections explain these interface properties. See the section titled "Setting global OSPF properties" later in this chapter to configure general OSPF properties.

General

Use the General tab shown in Figure 18-7 to specify the address on which the router interface responds, the area ID, and related properties. The following list explains the options:

♦ Enable OSPF for this address. Select this option to enable OSPF on the listed IP address for the selected interface. If you have more than one IP address bound to the selected interface, you must enable or disable, and configure options for, each IP address. The Area ID, Router priority, Cost, and Password fields apply to the IP address and can be configured differently for each address on the same interface if needed.

♦ Area ID. Select one area ID to associate with the previous address set. Each address can have a different area ID. By default, only the backbone area ID of 0.0.0.0 exists.

Create other area IDs through the Areas tab of the general OSPF properties for the router. See the section "Setting global OSPF properties" later in this chapter for more information on creating area IDs.

♦ Router priority. The router with the highest priority number becomes the designated router when multiple routers exist on the network, and takes precedence. If the priority numbers are the same (causing a tie), the router with the highest router ID value takes precedence. Specify a value to set the router's priority as needed within the context of the other routers in the network.

♦ Cost. Use this option to set the interface's metric, or relative cost. When two or more interfaces can be used for a given route, the one with the lowest metric is used by default unless it is unavailable.

♦ Password. Specify the clear text string used for identification purposes to other routers in the area. The default password is 12345678, and all routers in the same area must use the same password. This option is dimmed if plain-text passwords are disabled for the area. Open the global properties for OSPF and edit the properties for the selected area to enable the plain-text password.

♦ Broadcast. Select this option if the interface is connected to a broadcast-type network such as Ethernet, Token Ring, or FDDI.

♦ Point-to-point. Select this option if the interface is connected to a point-to-point network such as T1/E1, T3/E3, ISDN, or another dial-up network.

♦ Nonbroadcast multiple access (NBMA). Select this option if the interface is connected to an NBMA network such as Frame Relay, X.25, or ATM.

NBMA Neighbors

The NBMA Neighbors tab of the interface's OSPF properties enables you to define the neighboring routers when you select NBMA as the network type on the General page. Select an IP address for the interface (if the interface has multiple addresses bound to it), and then specify the IP address of neighboring routers.

Advanced

The Advanced tab enables you to configure various intervals, transit delay, and MTU values for each address on the selected interface. These values are explained in the following list:

♦ IP address. Select the IP address for which you want to change the settings.

♦ Transit delay. Specify the estimated number of seconds required to transmit a link state update packet over the selected interface. The default is one second.

♦ Retransmit interval. Specify the number of seconds between the retransmission of link state advertisements for adjacencies. The value should be greater than the total expected round-trip time between any two routers on the attached network.

♦ Hello interval. Specify the interval at which the router broadcasts hello packets. Hello packet transmission is a means of discovering network structure (neighboring routers). Routing metrics are configured based on the length of time it takes a packet to make the trip between the source and the destination. The hello interval value must be the same for all routers on the common network. A shorter interval enables network topology changes to be detected more quickly but has the effect of generating more OSPF traffic in a given time frame. The suggested value for X.25 is 30 seconds; for a LAN, the suggested value is 10 seconds.

♦ Dead interval. Specify the period after which adjacent routers will consider this router down. The other routers decide that the router is down when they haven't received a response to the hello packets within the specified amount of time. The value must be the same for all routers on the common network segment and should be set as a multiple of the hello interval. A common practice is to set the dead interval to four times the hello interval.

♦ Poll interval. Specify the period of time for network polls on NBMA interfaces. Network polls enable routers to determine whether a dead neighbor has come back up or the connection has been restored. The poll interval should be at least twice as long as the dead interval.

♦ Maximum transmission unit (MTU) size (bytes). Specify the maximum size that IP datagrams can be transmitted without fragmentation. The default size for Ethernet networks is 1,500.

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