Fine-Tuning RAR Configurations
You can fine-tune RAR configurations within a MANET by converting the link metrics to OSPFv3 link costs and configuring a hysteresis threshold. Configuring a hysteresis threshold on the resultant link costs helps minimize the propagation of LSAs responding to link-metric changes.
Metrics can be weighted during the configuration process to emphasize or de-emphasize particular characteristics. For example, if throughput is highly important, the metric for Current Data Rate (CDR) could be weighted more heavily into the composite metric. Similarly, a metric that is of no concern can be omitted.
Link metrics can change rapidly, often by very small degrees, which can result in a flood of meaningless routing updates. In a worst case scenario, the network will churn almost continuously as it struggles to react to minor variations in link quality. To alleviate this concern, Cisco provides a tunable dampening mechanism that allows the user to configure threshold values. Any metric change that falls below the threshold is ignored.
A tunable hysteresis mechanism allows users to adjust the threshold to the routing changes that occur when the router receives a signal that a new peer has been discovered, or that an existing peer is unreachable. The tunable metric is weighted and is adjusted dynamically to account for the following characteristics:
- Current and Maximum Bandwidth
- Latency
- Resources
- Hysteresis
Individual weights can be deconfigured and all weights cleared so that the cost is set back to the default value for the interface type. Based on the routing changes that occur, cost can be determined by the application of these metrics.
The dynamic cost metric used for interfaces is computed based on the Layer 2 (L2) feedback to Layer 3 (L3), where the metric calculations are as follows:
OC = maximum-data-rate
S1 = ospfv3 6 dynamic weight throughput (Bandwidth component)
S2 = ospfv3 6 dynamic weight resources (Resources component)
S3 = ospfv3 6 dynamic weight latency (Latency component)
S4 = ospfv3 6 dynamic weight L2 factor (L2 factor component)
Note While the commands and output in this section reflect IPv6 configurations, all examples and commands work for IPv4 as well.
Throughput = (current-data-rate)/(maximum-data-rate)
Router-dynamic cost = OC + (S1) + (S2) + (S3) + (S4)
For a dynamic cost to have the same cost as a default cost, all parameters must equal zero.
Each L2 feedback can contribute a cost in the range of 0 to 65535. To tune down this cost range, use the optional weight keyword in conjunction with the throughput, resources, latency, or L2-factor keyword. Each of these weights has a default value of 100 percent and can be configured in a range from 0 to 100. When 0 is configured for a specific weight, that weight does not contribute to the OSPFv3 cost.
Because cost components can change rapidly, you may need to dampen the amount of changes in order to reduce network-wide churn. Use the optional hysteresis keyword with the threshold threshold-value keyword and argument to set a cost change threshold. Any cost change below this threshold is ignored.
You can use the hysteresis keyword to specify a hysteresis value based on the percentage of change of the currently stored value in the routing table for the peer.
Each time the router receives a new PADQ packet from the radio for a peer, a new cost will be calculated for it. The hysteresis keyword specifies the amount of change required before saving the new value.
The hysteresis percent calculated is performed as follows:
If the absolute value of (new_cost - saved_cost) is greater than (hysteresis_percent*saved_cost), then the new_cost will be saved.
Because cost components can change rapidly, it might be necessary to dampen the volume of changes to reduce network-wide churn. The recommended values for S2, S3, and S4 are based on network simulations that may reduce the rate of network changes. The recommended value for S1 is zero to eliminate this variable from the route cost calculation.
While each network might have unique characteristics that require different settings to optimize actual network performance, these are recommended values intended as a starting point for optimizing a OSPFv3 network. Table 10-1 lists the recommended value settings for OSPFv3 cost metrics.
Table 10-1 Recommended Value Settings for OSPFv3 Cost Metrics
|
|
|
|
S1 |
ospfv3 6 dynamic weight throughout |
100 |
0 |
S2 |
ospfv3 6 dynamic weight resources |
100 |
29 |
S3 |
ospfv3 6 dynamic weight latency |
100 |
29 |
S4 |
ospfv3 6 dynamic weight L2-factor |
100 |
29 |
The overall link cost is computed using the following formula: