Routes are filtered by specifying configuration language that will match a certain set of routes by destination, or by destination and mask. Among other places, route filters are used on martians, and in import and export statements.
The action taken when no match is found is dependent on the context. For instance import and export route filters assume an all reject ; at the end of a list.
A route will match the most specific filter that applies. Specifying more than one filter with the same destination, mask and modifiers will generate an error.
network [ exact | refines | between number and number ] network mask mask [ exact | refines | between number and number ] network masklen number [ exact | refines | between number and number ] all default host host
These are all the possible formats for a route filter. Not all of these formats are available in all places. For instance the host and default formats are not valid for martians.
In most cases it is possible to specify additional parameters relevant to the context of the filter. For example, on a martian statement it is possible to specify the allow keyword, on an import statement you can specify a preference, and on a export you can specify a metric.
If no additional parameters are specified, any destination that falls in the range given by the network and mask is matched, so the mask of the destination is ignored. If a natural network is specified, the network, any subnets, and any hosts will be matched. The three optional modifiers cause the mask of the destination to also be considered:
0.0.0.0 mask 0.0.0.0
default
0.0.0.0 mask 0.0.0.0 exact
host host
host mask 255.255.255 exact
Last updated April 26, 1997
gated@gated.merit.edu