Cabletron Systems SmartSwitch Router, Руководство пользователя

Страница: 169 / 338

SmartSwitch Router User Reference Manual 169
Chapter 11: Routing Policy Configuration Guide
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
• Route-Filter
Aggregate-Destination
This component specifies the aggregate/summarized route. It also specifies the attributes
associated with the aggregate route. The preference to be associated with an aggregate
route can be specified using this component.
Aggregate-Source
This component specifies the source of the routes contributing to an
aggregate/summarized route. It can also specify the preference to be associated with the
contributing routes from this source. This preference can be overridden by explicitly
specifying 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 AS
path when they are added to the routing table. For interior routes, this AS path
specifies IGP as the origin and no ASs in the AS path (the current AS is added when
the 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. All
other protocols have a tag of zero.
In some cases, a combination of the associated attributes can be specified to identify the
routes contributing to an aggregate.