Riverstone Networks WICT1-12 - Manual (Page 435)

Riverstone Networks RS Switch Router User Guide Release 8.0 18-5
Routing Policy Configuration Preference
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.
18.1.4 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.
Route aggregation is also used by regional and national networks to reduce the amount of routing information passed
around. With careful allocation of network addresses to clients, regional networks can just announce one route to
regional networks instead of hundreds.
Aggregate routes are not actually used for packet forwarding by the originator of the aggregate route, but only by the
receiver (if it wishes). Instead of requiring a route-peer to know about individual subnets which would increase the
size of its routing table, the peer is only informed about an aggregate-route which contains all the subnets.
Like export policies, aggregate-routes can have up to three components:
• Aggregate-Destination
• Aggregate-Source