OSPF Convergence mechanisms

LSA Throttling (release in IOS v12.0(25)S)

Benefits of OSPF LSA Throttling:

Prior to the introduction of the OSPF LSA Throttling feature, OSPF operated with a limitation where the generation of LSAs was rate-limited to a 5-second interval. This limitation resulted in changes within LSAs being unable to propagate rapidly in milliseconds, thereby preventing the OSPF network from achieving millisecond-level convergence.
The OSPF LSA Throttling feature comes pre-enabled and offers the advantage of quicker OSPF convergence, reducing it to milliseconds. This feature is also customizable, with a single command governing LSA generation (sending) and another controlling the interval for receiving. Moreover, it offers a dynamic mechanism to decrease the frequency of LSA updates during periods of network instability.

How OSPF LSA Throttling Works:

The functioning of OSPF LSA Throttling revolves around two key commands:
- The timers throttle lsa all command regulates the generation (sending) of LSAs. It ensures that the first LSA is generated immediately following an OSPF topology change. Subsequent LSAs for the same LSA instance are generated at a rate controlled by the minimum start interval. These LSAs are then rate-limited until the maximum interval is reached. A "same LSA" is defined as an LSA instance that shares the same LSA ID number, LSA type, and advertising router ID.
- The timers lsa arrival command governs the minimum interval for accepting the same LSA. If an instance of the same LSA arrives within a time interval shorter than the one configured, the arriving LSA is discarded. It is recommended that the arrival interval is less than or equal to the hold-time interval set in the timers throttle lsa all command.

Additionally, the timers throttle lsa command in OSPF plays a crucial role in controlling the generation and propagation of LSAs during network events, such as link flaps. This command's format is timers throttle lsa initial hold max, where:

initial (default value = 0): Represents the initial delay in milliseconds before a router generates and sends out an LSA when a reconvergence event occurs., it's set to 0 milliseconds, meaning the router will send LSAs immediately upon detecting a change.
hold (default value = 5000): Specifies the hold time for duplicate LSAs. When a router generates an LSA and holds it in its queue, it will not send the same duplicate LSA within this time period., it's set to 5000 milliseconds, so if the same LSA is generated again within 5000 milliseconds, it won't be sent.
max (default value = 5000): Denotes the maximum hold time for duplicate LSAs. If there are continuous changes or flapping events causing the same LSA to be generated repeatedly, this timer ensures that after a certain maximum hold time (5000 milliseconds), the router will eventually send the LSA, even if it's a duplicate.

How to Customize OSPF LSA Throttling:

To customize the OSPF LSA Throttling settings, follow these steps:

Enable the configuration mode.
Access the terminal for configuration.
Specify the OSPF process by using router ospf process-id.
Set the parameters for LSA generation intervals using the timers throttle lsa all start-interval hold-interval max-interval command.
Adjust the minimum interval for accepting the same LSA with the timers lsa arrival milliseconds command.
Conclude the configuration.
Use the show ip ospf timers rate-limit command to display the configured rate-limiting settings.
Validate the OSPF configuration with the show ip ospf command.

For detailed information, refer to the Cisco documentation (opens in a new tab).

Benefits of SPF Throttling:

The OSPF Shortest Path First Throttling feature enables the configuration of SPF (Shortest Path First) scheduling in millisecond intervals, allowing the potential delay of SPF calculations during network instability. SPF calculations are used to determine the Shortest Path Tree (SPT) when there is a change in network topology. A single SPF run can encompass multiple topology change events.
The interval at which SPF calculations occur is dynamically determined based on the frequency of topology changes within the network. This dynamically chosen interval falls within the user-specified value ranges. In cases of network instability, SPF throttling extends the SPF scheduling intervals until the network topology stabilizes.

How SPF Throttling Works:

In Cisco's OSPF implementation, the standard initial SPF schedule delay is 5000 milliseconds, equivalent to a 5-second interval you can confirm this using the show ip ospf command. This brief pause serves as a protective measure, mitigating network turbulence and avoiding superfluous recalculations triggered by rapid, consecutive changes. Its purpose is to guarantee that OSPF doesn't hastily respond to minor or fleeting network alterations, which could potentially introduce routing instability.
Minimum Hold Time between Two Consecutive SPFs (Minimum SPF Schedule Delay): The "Minimum hold time between two consecutive SPFs" indicates the minimum waiting period that OSPF will observe between running successive SPF calculations in response to network topology changes. In the provided output, it's set at 10,000 milliseconds (10 seconds), ensuring a minimum waiting time of 10 seconds before initiating the next SPF calculation, even if further changes occur within that timeframe. This delay is implemented to prevent excessive, rapid SPF calculations triggered by minor or temporary network alterations, promoting network stability.
Maximum Wait Time between Two Consecutive SPFs (Maximum SPF Schedule Delay): The "Maximum wait time between two consecutive SPFs" defines an upper limit on the time OSPF will wait before launching the next SPF calculation when no changes in network topology have occurred. In the output, this value is also set to 10,000 milliseconds (10 seconds), ensuring that even in the absence of topology changes, OSPF will not postpone SPF calculations for more than 10 seconds. This parameter safeguards against extended SPF calculation delays that might result in routing inefficiencies during periods of network stability.
To see the default in the running config issue the command show run all | se router ospf
SPF calculations are governed by the timers throttle spf command. This command sets the wait interval, which indicates the time to wait until the next SPF calculation occurs. Subsequent wait intervals are twice as long as the previous one until the wait interval reaches the maximum wait time specified.
Here's an example to illustrate SPF timing settings: The start interval is set to 5 milliseconds (ms), the wait interval is 1000 milliseconds, and the maximum wait time is configured as 90,000 milliseconds.
The options for this command, in order, are spf-start, spf-hold, and spf-max-weight. This timer throttles the SPF algorithm, especially in situations where factors like link flapping or a high volume of LSAs being sent require SPF calculations. When the first LSA arrives, the timer initiates SPF calculations after spf-start milliseconds. If another event occurs within the spf-hold interval, the timer is doubled. This doubling process is repeated if subsequent events occur within this hold-time; it essentially acts as an exponential timer. The spf-max-wait value serves as a ceiling to prevent the timer from becoming excessively high. If there are no events for two times the max-wait, the timer reverts back to spf-start.

How to Customize SPF Throttling:

To customize SPF throttling settings, follow these steps:

under router config mode timers throttle spf 10 100 1000
Access the OSPF interface by using the interface command ip ospf transmit-delay seconds command. This command estimates the time needed to send a link-state update over a link. It becomes significant primarily on low-bandwidth links. LSAs that are sent will have their age incremented by the specified amount before being transmitted.
Use the interface command ip ospf retransmit-interval seconds command to dictate how long to wait before retransmitting unacknowledged LSAs.
Conclude the configuration.
Refer to the Cisco documentation (opens in a new tab) for additional details on SPF Throttling.

This customization allows you to fine-tune SPF calculations and LSA transmission settings to optimize network performance, especially in scenarios with specific bandwidth requirements or the need to control retransmissions.

OSPF Support for Fast Hello Packets

OSPF Hello Interval and Dead Interval

OSPF hello packets are integral components of OSPF network connectivity. These packets are exchanged between OSPF processes and their neighboring routers to maintain connectivity. Hello packets are sent at user-configurable intervals, typically measured in seconds. The default values are 10 seconds for Ethernet links and 30 seconds for non-broadcast links. Hello packets include a list of all neighboring routers for which a hello packet has been received within the "dead interval," another configurable time frame (in seconds). The default dead interval is set at four times the value of the hello interval. It is crucial to note that all routers within a network must have the same hello interval, and the same holds true for the dead interval.

These two intervals, the hello interval and the dead interval, collaborate to affirm the operational status of network links. If a router fails to receive a hello packet from a neighbor within the dead interval, it considers that neighbor as being down.

OSPF Fast Hello Packets

OSPF fast hello packets refer to hello packets transmitted at intervals less than 1 second. To comprehend the significance of fast hello packets, a prior understanding of the relationship between OSPF hello packets and the dead interval is essential. Please refer to the section titled "OSPF Hello Interval and Dead Interval" for a comprehensive explanation.

Fast hello packets in OSPF are enabled through the ip ospf dead-interval command. This command reduces the dead interval to 1 second, and the hello-multiplier value specifies the number of hello packets to be sent within that 1-second time frame, effectively providing subsecond or "fast" hello packets.

When fast hello packets are configured on an interface, the hello interval advertised in the hello packets sent from that interface is set to 0. The hello interval in the hello packets received on that interface is disregarded.

Consistency in the dead interval is essential on a network segment, whether it is configured as 1 second for fast hello packets or set to another value. The hello-multiplier does not need to be uniform across the entire segment as long as at least one hello packet is sent within the dead interval.

Benefits of OSPF Fast Hello Packets

The OSPF Support for Fast Hello Packets feature provides a notable benefit to OSPF networks by significantly improving convergence times compared to networks without fast hello packets. With this feature, lost neighbors can be detected within a mere 1 second. It is particularly valuable in LAN segments where neighbor loss might go unnoticed by lower OSI layers, such as the physical layer and data-link layer.

How to Configure OSPF Fast Hello Packets:

Specify the interface using interface type number.
Utilize the ip ospf dead-interval minimal hello-multiplier multiplier command to configure fast hello packets.
we can change the retransmit-time for LSAcks from the default of 5 seconds to something quicker ip ospf retransmit-interval [1-65535 second]
Verify the OSPF interface settings using the show ip ospf interface [interface-type interface-number] command.

For more details and comprehensive configuration steps, please refer to the Cisco documentation (opens in a new tab).

Incremental Spf Ospf Filtering