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| 9.6 |  | BGP Attributes |
| 9.6.7 |  | The Atomic Aggregate attribute |

The Atomic Aggregate attribute is a well-known 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.

BGP can be manually configured to summarize routes by using the

aggregate-address command.

Using the

aggregate-address command with no arguments will create an aggregate entry, a supernet route, in the BGP routing table. This happens as long as the router knows at least one specific BGP route that belongs to that supernet. Therefore, if the router knows just one route, it can claim to know hundreds of others. This feature should be used with caution. The aggregate route will be advertised as coming from the AS on the router and has the Atomic Aggregate attribute set to True. This shows that information might be missing. By default, the Atomic Aggregate attribute is set to True unless the as-set keyword is specified.

Using the

as-set keyword creates an aggregate entry. However, 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. This route must be continually withdrawn and updated as autonomous system path reachability information for the summarized route changes.

If the router is to propagate the supernet route only, and not 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 RTA is to suppress more specific routes and to update other BGP routers only about the supernet 160.0.0.0 /8, issue this command:

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

| Web Links Configuring BGP http://www.cisco.com/univercd/cc/td/doc/product/ software/ios122/122cgcr/fipr_c/ipcprt2/1cfbgp.htm