Cabletron Systems SmartSwitch manual Aggregate-Destination, Aggregate-Source

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.

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