Brocade Communications Systems ICX 7250 series, Руководство по настройке

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OSPFv2 Shortest Path First throttling
Rapid triggering of SPF calculations with exponential back-off to offer the advantages of rapid convergence without sacrificing stability.
As the delay increases, multiple topology changes can occur within a single SPF. This dampens network activity due to frequent topology
changes.
This scheduling method starts with an initial value after which a configured delay time is followed. If a topology change event occurs the
SPF is schedule after the time specified by the initial value, the device starts a timer for the time period specified by a configured hold
time value. If no topology events occur during this hold time, the router returns to using the initial delay time.
If a topology event occurs during the hold time period, the next hold time period is recalculated to a value that is double the initial value. If
no topology events occur during this extended hold time, the device resets to its initial value. If an event occurs during this extended hold
time, the next hold time is doubled again. The doubling occurs as long as topology events occur during the calculated hold times until a
configured maximum delay time value is reached or no event occurs (which resets the router to the initial hold time). The maximum value
is then held until the hold time expires without a topology change event occurring. At any time that a hold time expires without a topology
change event occurring, the router reverts to the initial hold value and begins the process all over again.
For example, if you set the initial delay timer to 100 milliseconds, the hold timer to 300 and the maximum hold timer to 2000
milliseconds, the following will occur:
If a topology change occurs the initial delay of 100 milliseconds will be observed. If a topology change occurs during the hold time of
300 milliseconds the hold time is doubled to 600 milliseconds. If a topology change event occurs during the 600 millisecond period,
the hold time is doubled again to 1200 milliseconds. If a topology change event occurs during the 1200 millisecond period, the hold
time is doubled to 2400 milliseconds. Because the maximum hold time is specified as 2000, the value will be held at 2000. This 2000
millisecond period will then repeat as long as topology events occur within the maximum 2000 millisecond hold time. When a maximum
hold time expires without a topology event occurring, the router reverts to the initial delay time and the cycle repeats as described.
Therefore, longer SPF scheduling values can be used during network topology instability.
IETF RFC and internet draft support
The implementation of OSPF Graceful Restart supports the following IETF RFC:
• RFC 3623: Graceful OSPF Restart
NOTE
A secondary management module must be installed for the device to function as a graceful restart device. If the device
functions as a graceful restart helper device only, there is no requirement for a secondary management module.
OSPFv2 non-stop routing
OSPFv2 can continue operation without interruption during hitless failover when the OSPFv2 non-stop routing (NSR) feature is enabled.
During graceful restart (GR), the restarting neighbors must help build routing information during a failover. However, GR may not be
supported by all devices in a network. NSR eliminates this dependency.
NSR does not require support from neighboring devices to perform hitless failover, and OSPF can continue operation without
interruption.
NOTE
NSR does not support IPv6-over-IPv4 tunneling and virtual links, so traffic loss is expected while performing hitless
failover.
OSPFv2 Shortest Path First throttling
Brocade FastIron Layer 3 Routing Configuration Guide
224 53-1003903-04