B. Architectural Constants

Several OSPF protocol parameters have fixed architectural values. These parameters have been referred to in the text by names such as LSRefreshTime. The same naming convention is used for the configurable protocol parameters. They are defined in Appendix C.

The name of each architectural constant follows, together with its value and a short description of its function.

LSRefreshTime

The maximum time between distinct originations of any particular link state advertisement. When the LS age field of one of the router's self-originated advertisements reaches the value LSRefreshTime, a new instance of the link state advertisement is originated, even though the contents of the advertisement (apart from the link state header) will be the same. The value of LSRefreshTime is set to 30 minutes.

MinLSInterval

The minimum time between distinct originations of any particular link state advertisement. The value of MinLSInterval is set to 5 seconds.

MaxAge

The maximum age that a link state advertisement can attain. When an advertisement's LS age field reaches MaxAge, it is reflooded in an attempt to flush the advertisement from the routing domain (See Section 14). Advertisements of age MaxAge are not used in the routing table calculation. The value of MaxAge must be greater than LSRefreshTime. The value of MaxAge is set to 1 hour.

CheckAge

When the age of a link state advertisement (that is contained in the link state database) hits a multiple of CheckAge, the advertisement's checksum is verified. An incorrect checksum at this time indicates a serious error. The value of CheckAge is set to 5 minutes.

MaxAgeDiff

The maximum time dispersion that can occur, as a link state advertisement is flooded throughout the AS. Most of this time is accounted for by the link state advertisements sitting on router output queues (and therefore not aging) during the flooding process. The value of MaxAgeDiff is set to 15 minutes.

LSInfinity

The metric value indicating that the destination described by a link state advertisement is unreachable. Used in summary link advertisements and AS external link advertisements as an alternative to premature aging (see Section 14.1). It is defined to be the 24-bit binary value of all ones: 0xffffff.

DefaultDestination

The Destination ID that indicates the default route. This route is used when no other matching routing table entry can be found. The default destination can only be advertised in AS external link advertisements and in stub areas' type 3 summary link advertisements. Its value is the IP address 0.0.0.0.