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244 CHAPTER 16: IP ROUTING PROTOCOL OPERATION
The value of interval should be bigger than the interval in which a packet can be
transmitted and returned between two routers.
An LSA retransmission interval that is too small will cause unnecessary retransmission.
Setting a Shortest Path First (SPF) Calculation Interval for OSPF
Whenever the OSPF LSDB changes, the shortest path requires recalculation.
Calculating the shortest path after the change consumes enormous resources and
affects the operating efficiency of the router. Adjusting the SPF calculation interval,
however, can restrain the resource consumption due to frequent network changes.
Perform the following configuration in OSPF View.
By default, the interval for SPF recalculation is 5 seconds.
Configuring STUB Area of OSPF
STUB areas are special LSA areas in which the ABRs do not propagate the learned
external routes of the AS. In these areas, the routing table sizes of routers and the
routing traffic are significantly reduced.
The STUB area is an optional configuration attribute, but not every area conforms to
the configuration condition. Generally, STUB areas, located at the AS boundaries, are
those non-backbone areas with only one ABR. Even if this area has multiple ABRs, no
virtual links are established between these ABRs.
To ensure that the routes to destinations outside the AS are still reachable, the ABR in
this area will generate a default route (0.0.0.0) and advertise it to the non-ABR
routers in the area.
Note the following items when you configure a STUB area:
■ The backbone area cannot be configured as a STUB area, and virtual links cannot
pass through the STUB area.
■ If you want to configure an area as a STUB area, all the routers in this area should
be configured with the
stub command.
■ No ASBR can exist in a STUB area, and the external routes of the AS cannot be
propagated in the STUB area.
Perform the following configuration in OSPF Area View.
Table 233 Setting the SPF calculation interval
Operation Command
Set the SPF calculation interval spf-schedule-interval seconds
Restore the SPF calculation interval undo spf-schedule-interval seconds
Table 234 Configuring and OSPF STUB area
Operation Command
Configure an area to be the STUB area stub [ no-summary ]
Remove the configured STUB area undo stub
Configure the cost of the default route transmitted by
OSPF to the STUB area
default-cost value
Remove the cost of the default route to the STUB area undo default-cost