Overview of BGP Aggregation

BGP route aggregation
is a powerful feature that helps reduce the size of routing tables by
summarizing multiple specific routes into a single, broader route. This is
especially useful in large-scale networks where route optimization and
scalability are critical.

In Cisco IOS,
the aggregate-address command provides flexible options to control
how and when summary routes are advertised. Whether you’re looking to advertise
only the summary, retain specific routes, or apply custom attributes, this
command gives you granular control over BGP route announcements.

By default, when you use the aggregate-address command,
the router advertises both the aggregate route and the more specific routes
that fall under it

Let’s dive in and understand how each option can be used to fine-tune your BGP
advertisements.

  1. as-set

• Purpose: Includes the AS numbers of the contributing routes in the AS path
  of the aggregate.
• Use Case: When you want to preserve AS path information for loop prevention
  or policy decisions.
• Effect: The aggregate route will have an AS_SET attribute, which is a
  list of AS numbers from the contributing routes.

2. summary-only

• Purpose: Suppresses the advertisement of the more specific routes.
• Use Case: When you want to advertise only the summarized route and hide the
  specifics.
• Effect: Only the aggregate route is advertised; specific routes are not
  sent to BGP peers.

3. suppress-map
<map-name>

• Purpose: Selectively suppress specific routes from being advertised.
• Use Case: When you want to suppress some specific prefixes but still
  advertise others along with the aggregate.
• Effect: Routes matching the route-map are suppressed; others are
  advertised.

4. advertise-map
<map-name>

• Purpose: Controls which specific routes are used to generate the
  aggregate.
• Use Case: When you want the aggregate to be created only if certain routes
  exist.
• Effect: Aggregate is advertised only if routes matching the map are
  present in the BGP table.

5. attribute-map
<map-name>

• Purpose: Applies specific BGP attributes to the aggregate route.
• Use Case: When you want to set attributes like MED, community, or local
  preference on the aggregate.
• Effect: The aggregate route inherits attributes defined in the route-map.