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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withdrawn and updated as autonomous system path reachability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
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