Configure OSPF Network Optimization

Configuration Condition

Before configuring OSPF network optimization, ensure that:

• Interface IP addresses have been configured so that neighbor nodes are reachable at the network layer.
• Enable OSPF.

Configure the Keepalive Time of an OSPF Neighbor

OSPF Hello packets are used to set up neighbor relations and keep the relations alive. The default transmission interval of Hello packets is determined by the network type. For broadcast networks and P2P networks, the default transmission interval of Hello packets is 10s. For P2MP networks and NBMA networks, the default transmission interval of Hello packets is 30s.

Neighbor dead time is used to determine the validity of a neighbor. By default, the neighbor dead time is four times the Hello interval. If an OSPF device fails to receive Hello packets from a neighbor after the neighbor dead time times out, the OSPF device regards the neighbor as invalid, and then it deletes the neighbor in an active manner.

Table 7-25 Configuring the Keepalive Time of an OSPF Neighbor

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  The Hello interval and neighbor dead time of OSPF neighbors must be the same; otherwise, they cannot set up neighbor relations.
• When you modify the Hello interval, if the current neighbor dead time is four times the Hello interval, the neighbor dead time is automatically modified to be still four times the new Hello interval. If the current neighbor dead time is not four times the Hello interval, the neighbor dead time keeps unchanged.
• If you modify the neighbor dead time, the Hello interval is not affected.

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Configure an OSPF Passive Interface

The dynamic routing protocol adopts a passive interface to effectively decrease the network bandwidth consumed by the routing protocol. After an OSPF passive interface is configured, you can use the network command to advertise the routes of the directly connected network segment in which the interface is located, but the receiving and transmitting of OSPF packets are damped on the interface.

Table 7-26 Configuring an OSPF Passive Interface

Configure an OSPF Demand Circuit

On P2P and P2MP links, to decrease the line cost, you can configure an OSPF demand circuit to suppress periodical transmitting of Hello packets and periodical update of LSA packets. This function is mainly applied on charged links such as ISDN, SVC, and X.25.

Table 7-27 Configuring an OSPF Demand Circuit

Configure the Priority of an OSPF Interface

Interface priorities are mainly used in election of Designated Router (DR), and Backup Designated Router (BDR) in broadcast networks and NBMA networks. The value range is 0-255. The larger the value is, the higher the priority is. The default value is 1.

The DR and BDR are selected from all devices in a network segment based on interface priorities and Router IDs through Hello packets. The rules are as follows:

• First, the device whose interface has the highest priority is elected as the DR, and the device whose interface has the second highest priority is elected as the BDR. The device whose interface has the priority 0 does not participate in the election.
• If the interface priorities of two devices are the same, the device with the largest Router ID is elected as the DR, and the device with the second largest Router ID is elected as the BDR.
• If the DR fails, the BDR becomes the DR immediately, and a new BDR is elected.

Table 7-28 Configuring the Priority of an OSPF Interface

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  Interface priorities affect only an election process. If the DR and BDR have already been elected, modification of interface priorities does not affect the election result; instead, it affects the next election of DR or BDR. Therefore, the DR may not have the interface with the highest priority, and the BDR may not have the interface with the second highest priority.

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Configure the MTU of an OSPF Interface

In encapsulating OSPF packets, to prevent fragmentation, you need to limit the packet size to equal to or smaller than Maximum Transmission Unit (MTU) of the interface.

When adjacent OSPF devices exchange DD packets, MTUs are checked by default. If the MTUs are different, the devices cannot form a neighbor relation. If you have configured OSPF to ignore interface MTU check, even if MTUs are different, they can set up a neighbor relation.

Table 7-29 Configuring the MTU of an OSPF Interface

Configure the LSA Transmit Delay of an OSPF Interface

LSA transmit delay refers to the time it takes for an LSA to flood to other devices. The device that sends the LSA adds the interface transmit delay to the LSA aging time. By default, once the flooding LSA passes a device, the aging time is increased by 1. You can configure the LSA transmit delay according to the network conditions. The value range is 1-840. LSA transmit delay is usually configured on low-speed links.

Table 7-30 Configuring the LSA Transmit Delay of an OSPF Interface

Configure OSPF LSA Retransmission

To ensure the reliability of data exchange, OSPF adopts the acknowledgement mechanism. If an LSA floods on a device interface, the LSA is added into the retransmission list of the neighbor. If no acknowledgement message is received from the neighbor after the retransmission time times out, the LSA is retransmitted until an acknowledgement message is received.

Table 7-31 Configuring OSPF LSA Retransmission

Configure OSPF to Prevent LSA Flooding

In actual network applications, redundant links may be used between OSPF neighbors under some circumstances. This configuration helps to decrease flooding of OSPF update packets on redundant links.

Table 7-32 Configuring OSPF to Prevent LSA Flooding

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  Configuring OSPF to prevent LSA spreading may result in loss of some routing information.

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Configure OSPF SPF Calculation Time

If the OSPF network topology changes, routes need to be re-calculated. When the network continues to change, frequent route calculation occupies a lot of system resources. You can adjust the SPF calculation time parameters to prevent frequent network changes from consuming too many system resources. Table 7-33 Configuring OSPF SPF Calculation Time

• The parameter delay-time indicates the initial calculation delay, hold-time indicates the suppression time, and max-time indicates the maximum waiting time between two SPF calculations. If network changes are not frequent, you can shorten the continuous route calculation interval to delay-time. If network changes are frequent, you can adjust the parameters, increase the suppression time to hold-time×2n-2 (n is the number of route calculation trigger times), extend the waiting time based on the configured hold-time increment and the maximum value must not exceed max-time.

Configure OSPF Database Overflow

OSPF database overflow is used to limit the number of Type-5 LSAs in the database.

Table 7-34 Configuring OSPF Databases Overflow

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  After the database overflow function is enabled, the databases in the OSPF area may become inconsistent, and some routes get lost.

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