Brocade Communications Systems FastIron SX 1600, Configuration Manual, Page: 231 / 593

The feature is also useful during router startup because it gives the router enough time to build up its routing table before forwarding
traffic. This can be useful where BGP is enabled on the router because it takes time for the BGP routing table to converge.
You can also configure and set a metric value for the following LSA types:
• Summary (type 3 and type 4)
• External (type 5 and type 7)
• Opaque (type 10, TE link)
OSPF 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 router 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 router 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 would 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.
The purpose of this feature is to use longer SPF scheduling values 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.
Dynamic OSPF activation and configuration
OSPF is automatically activated when you enable it. The protocol does not require a software reload.
OSPF graceful restart
FastIron Ethernet Switch Layer 3 Routing
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