Chapter 6: Routing Policy Configuration Guide6 - 6SSR User Reference ManualThe 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 impliedin the shorthand forms listed below. These three forms vary in how the mask isspecified. In the 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 thenumber of contiguous 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, andany hosts will be matched. Three optional parameters that cause the mask of thedestination to also be considered are:• Exact: Specifies that the mask of the destination must match the supplied maskexactly. 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 not the 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 presenceof a specific route. It is used, for example, at an autonomous system border to generatea route to a network to be advertised via BGP given the presence of one or moresubnets of that network learned via OSPF. The routing process does not perform anyaggregation unless explicitly requested.Route aggregation is also used by regional and national networks to reduce the amountof routing information passed around. With careful allocation of network addresses toclients, regional networks can just announce one route to regional networks instead ofhundreds.Aggregate routes are not actually used for packet forwarding by the originator of theaggregate route, but only by the receiver (if it wishes). Instead of requiring a route-peer
