Chapter 13: Routing Policy Configuration Guide166 SmartSwitch Router User Reference ManualA route will match the most specific filter that applies. Specifying more than one filterwith the same destination, mask, and modifiers generates an error.There are three possible formats for a route filter. Not all of these formats are available inall places. In most cases, it is possible to associate additional options with a filter. Forexample, while creating a martian, it is possible to specify the allow option, while creatingan import policy, one can specify a preference, and while creating an export policy onecan specify a metric.The three forms of a route-filter are:• Network [ exact | refines | between number,number]• Network/mask [ exact | refines | between number,number]• Network/masklen [ exact | refines | between number,number]Matching usually requires both an address and a mask, although the mask is implied inthe shorthand forms listed below. These three forms vary in how the mask is specified. Inthe first form, the mask is implied to be the natural mask of the network. In the second,the mask is explicitly specified. In the third, the mask is specified by the number ofcontiguous one bits.If no optional parameters (exact, refines, or between) are specified, any destination thatfalls in the range given by the network and mask is matched, so the mask of thedestination is ignored. If a natural network is specified, the network, any subnets, and anyhosts will be matched. Three optional parameters that cause the mask of the destination toalso be considered are:• Exact: Specifies that the mask of the destination must match the supplied mask exactly.This is used to match a network, but no subnets or hosts of that network.• Refines: Specifies that the mask of the destination must be more specified (i.e., longer)than the filter mask. This is used to match subnets and/or hosts of a network, but notthe network.• Between number, number: Specifies that the mask of the destination must be as ormore specific (i.e., as long as or longer) than the lower limit (the first numberparameter) and no more specific (i.e., as long as or shorter) than the upper limit (thesecond number). Note that exact and refines are both special cases of between.Aggregates and GeneratesRoute aggregation is a method of generating a more general route, given the presence of aspecific route. It is used, for example, at an autonomous system border to generate a routeto a network to be advertised via BGP given the presence of one or more subnets of thatnetwork learned via OSPF. The routing process does not perform any aggregation unlessexplicitly requested.
