DELL N3000 User Manual

1116

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.
Dell Networking 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

configuration example.