Route Summary using aggregate-address command

The aggregate-address command can be used to generate BGP aggregate routes. The syntax of the command is as follows

BGP aggregate-address

(config)#router bgp <AS>
(config-router)#aggregate-address <network-address> <subnet-mask>

<AS> : AS number
<network-address> : Network address of aggregate route
<subnet-mask> : Subnet mask of aggregate route

The route specified in the aggregate-address command is placed in the BGP table, becomes the best path, and is advertised to other BGP neighbors. However, the condition for using the aggregate-address command is that at least one route within the scope of the aggregate route must exist in the BGP table. If no routes within the scope of the aggregation exist in the BGP table, the aggregate route cannot be generated and advertised.

Figure aggregate-address command
Figure aggregate-address command

Path Attributes for Aggregate Route

Recall that BGP route has several path attributes attached to it. Naturally, aggregate route generated by the aggregate-address command will also have path attributes added to it. The following attributes are added by default to the aggregate route generated by aggregate-address command.

  • AS_PATH: empty
  • WEIGHT:32768
  • MED: none
  • AGGREGATOR: Local AS number and Router ID

The Well Known Mandatory attribute (ORIGIN/NEXT_HOP/AS_PATH) of the aggregate route generated by the aggregate-address command is the same as the route generated by the network command. ATOMIC-AGGREGATE is the flag indicating that it is an aggregated route. The AGGREGATOR is marked with the AS number and router ID of the local router that generated the aggregated route.

aggregate-address command options

And the aggregate-address command has various options. If you type “?” in the middle of a command and get help, you will see the following.

R1(config-router)#aggregate-address ?
advertise-map  Set condition to advertise attribute
as-set         Generate AS set path information
attribute-map  Set attributes of aggregate
nlri           Nlri aggregate applies to
route-map      Set parameters of aggregate
summary-only   Filter more specific routes from updates
suppress-map   Conditionally filter more specific routes from updates

A summary of each option is as follows

  • advertise-map
    Specify the pre-aggregation route for which you want the aggregate route to inherit attributes
  • as-set
    Inherit the AS number and COMMUNITY attribute of the route before aggregation
  • attribute-map
    Set attributes for aggregate routes
  • route-map
    Same as attribute-map
  • summary-only
    Suppress all routes before aggregation
  • suppress-map
    Selective Aggregation. Specify the route to be suppressed among the routes before aggregation.

These options should be considered from two perspectives.

  • What to do with the attributes of the aggregate route
  • How to handle pre-aggregation routes

The options for “what to do with aggregate route attributes” are as-set, advertise-map, attribute-map, and route-map.The options for “how to handle pre-aggregation routes” are summary-only and suppress-map.

What to do with the attributes of the aggregate route as-set/advertise-map/atrribute-map/route-map
How to handle pre-aggregation routessummary-only/suppress-map
表 aggregate-addressコマンドのオプションの分類

Related article

The options for the aggregate-address command are described in detail in the following article.