Information Technology Reference
In-Depth Information
defined by an
advertise-map
. Note that the aggregate's AS_SET is inherited only from the
components that are defined in the map.
A common route aggregation practice is to group as large an address space as possible into
as few prefix entries as possible. This is desirable in reducing the number of prefixes carried
by the Internet, but it's detrimental to adjacent networks that have multiple connections to
the aggregating network. One result of aggregation is that routing accuracy of neighbors is
lost. In this situation, more-specific routes can be generated to better identify a prefix's
address subsets across multiple connections. Deaggregation is a BGP feature that
reconstructs components from a received aggregate prefix.
Deaggregation is accomplished by using the conditional injection feature.
Conditional
injection
is the creation of more-specific components when an aggregate exists. These com-
ponents are injected into the local BGP RIB to provide more-specific routing information
in the local AS than the aggregate. These components can be installed in the IP RIB and
advertised to other BGP peers within the AS.
Conditional route injection is configured as follows:
bgp inject-map map1 exist-map map2 [copy-attributes]
BGP tracks the prefix (the aggregate) in the exist-map to determine whether to install a
prefix or prefixes as specified in the inject-map. The exist-map must have at least two match
clauses:
•
match ip address prefix-list
specifies the aggregate based on which to inject more
specifics. Only one exact match is allowed.
•
match ip route-source
specifies the neighbor that sent the aggregate. The component
inherits the attributes from the aggregate if the option
copy-attributes
is specified;
otherwise, they are treated as locally generated routes for some of the attributes. The
NEXT_HOP is always the eBGP peer that originated the aggregate. Additional
matches can be made for AS_PATH and community.
Within the inject map, use
set ip address prefix-list
to define the prefixes to be injected into
the local BGP RIB. The injected prefixes can be displayed with the
show ip bgp injected-
path
command.
Figure 4-4 shows a sample topology that takes advantage of conditional injection to achieve
deaggregation. Both AS 300 and AS 400 are customers of AS 200 and receive address blocks
assigned by AS 200. The prefix block is 172.16.1.0/24 for AS 300 and 172.16.2.0/24 for AS
400. When announcing to AS 100, border routers of AS 200 summarize their address space
to a single aggregate, 172.16.0.0/16.
Because AS 100 follows a best-exit policy (sometimes called
cold-potato routing
), it
attempts to optimize its exit points. With a single aggregate, however, traffic destined for
AS 300 might be exiting the AS via R3. If more-specific prefixes are available, you can
control the traffic flows with better granularity.
