BGP Selective Aggregation Overview

Table of Contents

1

Overview

When BGP aggregates routes, not only the aggregated route is advertised, but also the individual routes before the aggregation. Selective Aggregation allows control over which pre-aggregation routes are advertised along with the aggregate route.

This section describes BGP selective aggregation.

What is Selective Aggregation?

If the aggregate-address command is used to aggregate BGP routes, aggregate routes and all pre-aggregate routes will be advertised if no options are specified. If you configure the aggregate-address command with the summary-only option, pre-aggregate routes will not be advertised at all. In other words, it is All or Nothing for the pre-aggregation route, where the normal aggregation configuration is all sent and summary-only is not sent at all.

Selective aggregation means that instead of treating pre-aggregate routes in this All or Nothing manner, flexible control is given to the pre-aggregate routes that you wish to advertise along with the aggregate route. Selective aggregation allows only a selected portion of the pre-aggregation routes to be advertised with the aggregation route.

Figure: BGP Selective Aggregation Overview

Selective Aggregation Methods

There are two types of selective aggregation configurations

suppress-map
unsuppress-map

Note that these two are very confusing. First, the range of neighbors to advertise pre-aggregation routes is different. When selective aggregation is configured with suppress-map, pre-aggregation routes are advertised to all neighbors. On the other hand, selective aggregation with unsuppress-map advertises pre-aggregated routes only to a specific neighbor.

There are also different ways to specify the pre-aggregation route you want to advertise. In both cases, a route-map is specified. However, in the case of suppress-map, the pre-aggregation routes that are denied in the route-map are advertised. In the case of unsuppress-map, the opposite is true: the pre-aggregation routes which are permitted in the route-map are advertised. This point is particularly confusing and should be noted.

	suppress-map	unsuppress-map
Target Neighbor	all neighbors	a specific neighbor
Specify the pre-aggregation routes you want to advertise	deny in route-map	permit in route-map

Table: suppress-map and unsuppress-map

Summary

Points

Selective Aggregation allows control over which pre-aggregation routes are advertised along with the aggregation route.
There are two methods for configuring selective aggregation
- suppress-map
- unsuppress-map