Cabletron Systems SmartSwitch manual Aggregates and Generates

Chapter 11: Routing Policy Configuration Guide

A route will match the most specific filter that applies. Specifying more than one filter with 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 in all places. In most cases, it is possible to associate additional options with a filter. For example, while creating a martian, it is possible to specify the allow option, while creating an import policy, one can specify a preference, and while creating an export policy one can 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 in the shorthand forms listed below. These three forms vary in how the mask is specified. 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 the number of contiguous one bits.

If no optional parameters (exact, refines, or between) 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. Three optional parameters that cause the mask of the destination to also 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 not the network.

•Between number, number: Specifies that the mask of the destination must be as or more specific (i.e., as long as or longer) than the lower limit (the first number parameter) and no more specific (i.e., as long as or shorter) than the upper limit (the second number). Note that exact and refines are both special cases of between.

Aggregates and Generates

Route aggregation is a method of generating a more general route, given the presence of a specific route. It is used, for example, at an autonomous system border to generate a route to a network to be advertised via BGP given the presence of one or more subnets of that network learned via OSPF. The routing process does not perform any aggregation unless explicitly requested.

168 SmartSwitch Router User Reference Manual