appb_11
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Appendix BA gated Reference B.11 The Aggregate StatementsRoute aggregation is used by regional and national networks to reduce
the number of routes advertised. With careful planning, large network
providers can announce a few aggregate routes instead of hundreds of
client network routes. Enabling aggregation is the main reason that
CIDR blocks are allocated as contiguous address blocks.Most of us don't have hundreds of routes to advertise. But we may have
a classless address composed of a few class C address and we may need
to tell gated how to handle it. Older versions of gated
automatically generated an aggregate route to a natural network using
the old Class A, B, and C concept; i.e., interface address
192.168.16.1 created a route to 192.168.16.0. With the advent of
classless interdomain routing, this can be the wrong thing to
do. gated does not aggregate routes unless it is explicitly
configured with the aggregate statement:aggregate default | address [mask mask | masklen number] [preference preference] [brief] { proto proto [as as_number | tag tag | aspath aspath_regexp] [restrict] | [[preference preference] { route_filter [restrict | (preference preference)]] ;} ;Several options are available for the aggregate statement:preference preference;Defines the preference of the resulting aggregate route. The default
is 130.briefSpecifies that the AS path of the agregate route should be the longest
common AS path. The default is to build an AS path consisting of all
contributing AS paths.proto protoOnly aggregate routes learned from the specified protocol. The value
of proto may be any currently configured protocol. This includes the
"protocols" direct, static, and kernel, discussed
in the previous section; all for all possible protocols; and
aggregate for other route aggregations.as as_numberOnly aggregate routes learned from the specified autonomous system.tag tagOnly aggregate routes with the specified tag. aspath aspath_regexpOnly aggregate routes that match the specified AS path. restrictIndicates routes that are not to be aggregated.Routes that match the route filters may contribute to the aggregate
route. A route may only contribute to an aggregate route that is more
general than itself. Any given route may only contribute to one aggregate
route, but an aggregate route may contribute to a more general aggregate.A slight variation of aggregation is the generation of a route based
on the existence of certain conditions. The most common usage for this
is to create a default based on the presence of a route from a peer on
a neighboring backbone. This is done with the generate
statement.generate default | address [mask mask | masklen number] [preference preference] { proto proto [as as_number | tag tag | aspath aspath_regexp] [restrict] | [[preference preference] { route_filter [restrict | preference preference]] ; } ;} ;The generate statement uses many of the same options as the
aggregate statement. These options are described earlier in this
appendix.B.10 Control StatementsC. A named Reference[ Library Home | DNS & BIND | TCP/IP | sendmail | sendmail Reference | Firewalls | Practical Security ]