8.6 BGP Attributes
8.6.8
The Atomic Aggregate attribute
The Atomic Aggregate attribute is a well-know discretionary attribute (type code 6). The Atomic Aggregate attribute is set to either True or False. If True, this attribute alerts BGP routers that multiple destinations have been grouped into a single update. In other words, the BGP router that sent the update had a more specific route to the destination but did not send it. Because this can lead to routing problems, the Atomic Aggregate attribute warns receiving routers that the information they are receiving is not necessarily the most complete route information available.

You can manually configure BGP to summarize routes by using the aggregate-address command, which has the syntax shown in the figure.

Using the aggregate-address command with no arguments will create an aggregate entry (that is, a supernet route) in the BGP routing table, as long as the router knows at least one specific BGP route that belongs to that supernet. Thus, if your router knows just one route, it can claim to know hundreds of others (which is why this feature should be used with caution). The aggregate route will be advertised as coming from your router's AS and has the Atomic Aggregate attribute set to True, showing that information might be missing. By default, the Atomic Aggregate attribute is set to True unless you specify the as-set keyword.

Using the as-set keyword creates an aggregate entry, but the path advertised for this route will be an AS_Set consisting of all elements contained in all paths that are being summarized. Do not use this form of aggregate-address when aggregating many paths because this route must be continually withdrawn and updated as autonomous system path reachability information for the summarized route changes.

If you want your router to propagate the supernet route only, and you do not want it to propagate any more specific routes, use the summary-only keyword. When configured using this keyword, the router will send the supernet route and suppress the more specific routes known to BGP.

The following example shows the commands needed to configure a simple supernet advertisement, which will be sent with the Atomic Aggregate attribute set to True.

RTA(config)# router bgp 300
RTA(config-router)# neighbor 3.3.3.3 remote-as 200
RTA(config-router)# neighbor 2.2.2.2 remote-as 100
RTA(config-router)# network 160.10.0.0
RTA(config-router)# aggregate-address 160.0.0.0 255.0.0.0

If you wanted RTA to suppress more specific routes and to update other BGP routers only about the supernet 160.0.0.0 /8, we could issue this command:

RTA(config-router)# aggregate-address 160.0.0.0 255.0.0.0 summary-only