Dell PowerConnect 8024, User Configuration Manual

Page: 948 / 1294

948
Configuring OSPF and OSPFv3
Static Area Range Cost
This feature allows a network operator to configure a fixed OSPF cost that is
always advertised when an area range is active. This feature applies to both
OSPFv2 and OSPFv3.
An OSPF domain can be divided into areas to limit the processing required
on each router. Area Border Routers (ABRs) advertise reachability across area
boundaries. It is common to summarize the set of prefixes that an ABR
advertises across an area boundary. RFC 2328 specifies that when an ABR
originates a type 3 LSA for an active area range, the cost in the LSA is set to
“the largest cost of any of the component networks.” Thus, when an area's
topology changes in a way that increases the largest cost, the type 3 LSA must
be re-originated. In some cases, advertising the change in cost may be less
important than preventing the topology change from propagating outside the
area (thus causing routers in other areas to process and flood a changed LSA
and rerun their routing table calculations). For this reason, it is common to
give the network administrator the option of configuring the cost for an area
range. When a static cost is configured, the cost advertised in the type 3 LSA
does not depend on the cost of the component networks. Thus, topology
changes within an area do not propagate outside the area, resulting in greater
stability within the OSPF domain.
PowerConnect switches also use area ranges to summarize type 7 LSAs when
they are translated to type 5 LSAs. The cost option may be configured on area
ranges used for type 7 to type 5 translation.
If an area range is configured for type 3 summarization and the static cost is
set to the maximum value, 16,777,215, the range is not advertised. Setting
this static cost is equivalent to configuring a range with the not-advertise
option. A summary LSA with this metric (LSInfinity) cannot be advertised,
according to RFC 2328 section 12.4.3. This behavior is consistent with the
industry standard.
If an area range is configured for type 7 to type 5 translation, a type 5 LSA is
sent if the metric is set to 16,777,215; however, other routers will not compute
a route from a type 5 LSA with this metric.
See "Configuring the Static Area Range Cost" on page 1020 for a
configuration example.