Defining The Spf Route Calculation Level - Juniper JUNOSE SOFTWARE FOR E SERIES 11.3.X - IP-IPV6-IGP CONFIGURATION GUIDE 2010-10-31 Configuration Manual

JunosE 11.3.x IP, IPv6, and IGP Configuration Guide
spf-interval

Defining the SPF Route Calculation Level

366
time. When many changes are taking place, a slower response to each change makes
more sense.
IS-IS enables the router to respond quickly to an isolated network event, but to slow the
response exponentially when many triggering events are taking place in rapid succession.
SPF calculations are performed at exponentially increasing intervals until the maximum
interval set by the spf-interval command is reached.
The first SPF calculation is performed immediately when the LDSB changes. If another
calculation-triggering event occurs, the router waits 1 second before performing the SPF
calculation. If another event occurs, the router waits 2 seconds before performing the
SPF calculation. The interval between a triggering event and the corresponding SPF
calculation continues to increase exponentially: 4 seconds, 8 seconds, 16 seconds, and
so on. When the maximum configured interval is reached, the interval reverts back to
immediate response mode for the next triggering event.
If no calculation-triggering network events have occurred by the end of any given back-off
interval, the router reverts back to immediate response mode.
Use to set the maximum interval between SPF calculations.
You can select an interval value in the range 0–120 seconds.
The default value is 5 seconds.
If you do not specify level-1 or level-2, the interval applies to both level 1 and level 2.
SPF calculations are performed only when the topology of the area changes. They are
not performed when external routes change.
Example
host1(config-router)#spf-interval level-2 30
Use the no version to restore the default value, 5 seconds.
See spf-interval
The IS-IS protocol uses the Dijkstra algorithm to compute IP node metrics when a change
occurs within the IS-IS network. This calculation results in the IS-IS router containing a
shortest-path tree (SPT) that maps the shortest path to each node in the IS-IS network.
By default, the router uses a partial route calculation (PRC) SPF to determine the next
hop (when required). This partial computation occurs when the router receives link-state
PDUs (LSPs) with only changes relating to IP prefixes (for example, the addition of a
new IP prefix, change in attributes of an existing IP prefix, or the removal of an existing
IP prefix).
Because changes in IP prefixes happen more frequently than other events, using the PRC
SPF results in faster IS-IS convergence and saves router resources. However, you can
Copyright © 2010, Juniper Networks, Inc.