SmartSwitch Router User Reference Manual 169Chapter 11: Routing Policy Configuration GuideRoute aggregation is also used by regional and national networks to reduce the amount ofrouting 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 toknow about individual subnets which would increase the size of its routing table, the peeris 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• Route-FilterAggregate-DestinationThis component specifies the aggregate/summarized route. It also specifies the attributesassociated with the aggregate route. The preference to be associated with an aggregateroute can be specified using this component.Aggregate-SourceThis component specifies the source of the routes contributing to anaggregate/summarized route. It can also specify the preference to be associated with thecontributing routes from this source. This preference can be overridden by explicitlyspecifying a preference with the route-filter.The routes contributing to an aggregate can be identified by their associated attributes:• Protocol type (RIP, OSPF, BGP, Static, Direct, Aggregate).• Autonomous system from which the route was learned.• AS path associated with a route. When BGP is configured, all routes are assigned an ASpath when they are added to the routing table. For interior routes, this AS pathspecifies IGP as the origin and no ASs in the AS path (the current AS is added whenthe route is exported). For BGP routes, the AS path is stored as learned from BGP.• Tag associated with a route. Both OSPF and RIP version 2 currently support tags. Allother protocols have a tag of zero.In some cases, a combination of the associated attributes can be specified to identify theroutes contributing to an aggregate.
