Brocade FastIron Ethernet Switch Layer 3 Routing Configuration Guide - Manual (Page 253)

The feature is useful for avoiding a loss of traffic during short periods when adjacency failures are
detected and traffic is rerouted. Using this feature, traffic can be rerouted before an adjacency failure
occurs due to common services interruptions such as a router being shutdown for maintenance.
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.
OSPF Shortest Path First throttling
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