Information Technology Reference
In-Depth Information
Aggregation and Deaggregation
Aggregation of prefix information reduces the number of entries BGP has to carry and
store. There are two common ways prefixes can be aggregated in BGP:
•
Using the
network
command to enter an aggregate address and a static route to Null0
•
Using the
aggregate-address
command to create an aggregate
Because the first method is straightforward, this section focuses on the second method—
using the
aggregate-address
command. Here is the full command with its various options:
aggregate-address address mask [as-set] [summary-only] [suppress-map map1]
[advertise-map map2] [attribute-map map3]
The creation of an aggregate in the BGP RIB is dependent on the existence of at least one
component route in the local BGP RIB. Without any options specified, BGP attributes of
the individual components are not included in the aggregate. The aggregate prefix has the
following default attributes:
•
NEXT_HOP
—0.0.0.0 (local)
•
AS_PATH
—i (blank AS_PATH; origin code IGP)
•
MED
—Not set
•
LOCAL_PREF
—100
•
WEIGHT
—32768
•
AGGREGATOR
—Local
•
ATOMIC_AGGREGATE
—Tagged to the aggregate
By default, both the aggregate and its components are advertised. When
summary-only
is
enabled for the aggregate, only the aggregate is advertised, and all the specific component
routes are suppressed. The aggregate still maintains the default attributes just listed. If only
a subset of the components are to be suppressed, you can define the subset with
suppress-
map
. If a subset of suppressed routes needs to be made available, you can unsuppress those
routes on a per-neighbor basis using the
neighbor unsuppress-map
command.
The option
as-set
allows AS path loop detection for the aggregate. Additionally, some of
the attributes of components are included additively with the aggregate, even if they
conflict. For example, if one component prefix has community set to 100:200 and another
has it set to
no-export
, the community of the aggregate is 100:200 and
no-export
. The
aggregate is not advertised to an eBGP peer.
The option
attribute-map
(a form of route map for setting BGP attributes) is used to clean
up the aggregate's attributes. Using the previous community example, if an attribute map
resets the community to 100:300, the previous two community values are replaced with
100:300, and the aggregate is advertised to an eBGP peer with 100:300. If only a subset of
components are to be used to form the aggregate's attributes, these components can also be
