Technical data

Gateway Routing Daemon (GATED) Configuration Reference
A.18 Control Statements
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; they are used only by the receiver, if it wishes. A router
receiving a packet that does not match one of the component routes that led
to the generation of an aggregate route is supposed to respond with an ICMP
network unreachable message. This is to prevent packets for unknown component
routes from following a default route into another network where they would be
forwarded back to the border router, and around and around again and again,
until their TTL expires. Sending an unreachable message for a missing piece of
an aggregate is only possible on systems with support for reject routes.
A slight variation of aggregation is the generation of a route based on the
existence of certain conditions. This is sometimes known as the route of last
resort. This route inherits the next hops and AS path from the contributor
specified with the lowest (most favorable) preference. The most common usage
for this is to generate a default based on the presence of a route from a peer on a
neighboring backbone.
A.18.6.1 Aggregation and Generation Syntax
The syntax of the aggregate and generation statements are as follows:
aggregate default
|(network [ ( mask mask ) | ( masklen number )])
[ preference preference ] [ brief ] {
proto [ all | direct | static | kernel | aggregate | proto ]
[(asautonomous system )|(tagtag )
| ( aspath aspath_regexp )]
restrict ;
proto [ all | direct | static | kernel | aggregate | proto ]
[(asautonomous system )|(tagtag )
| ( aspath aspath_regexp )]
[ preference preference ]{
route_filter [ restrict | ( preference preference )];
};
};
generate dffault
|(network [ ( mask mask ) | ( masklen )
[ preference preference ] [ brief ] {
proto [ all | direct | static | kernel | aggregate |
proto ]
[(asautonomous system )|(tagtag
)
| ( aspath aspath_regexp )]
restrict ;
proto [ all | direct | static | kernel | aggregate |
proto ]
[(asautonomous system )|(tagtag
)
| ( aspath aspath_regexp )]
[ preference preference ]{
route_filter [ restrict | ( preference preference )];
};
};
A–42 Gateway Routing Daemon (GATED) Configuration Reference