Cisco NCS 5500 Series Configuration Manuals page 163

Implementing Routing Policy
Aggregation
The aggregation attach point generates an aggregate route to be advertised based on the conditional presence
of subcomponents of that aggregate. Policies attached at this attach point are also able to set any of the valid
BGP attributes on the aggregated routes. For example, the policy could set a community value or a MED on
the aggregate that is generated. The specified aggregate is generated if any routes evaluated by the named
policy pass the policy. More specifics of the aggregate are filtered using the suppress-route keyword. Any
actions taken to set attributes in the route affect attributes on the aggregate.
In the policy language, the configuration is controlled by which routes pass the policy. The suppress map was
used to selectively filter or suppress specific components of the aggregate when the summary-only flag is not
set. In other words, when the aggregate and more specific components are being sent, some of the more
specific components can be filtered using a suppress map. In the policy language, this is controlled by selecting
the route and setting the suppress flag. The attribute-map allowed the user to set specific attributes on the
aggregated route. In the policy language, setting attributes on the aggregated route is controlled by normal
action operations.
In the following example, the aggregate address 10.0.0.0/8 is generated if there are any component routes in
the range 10.0.0.0/8 ge 8 le 25 except for 10.2.0.0/24. Because summary-only is not set, all components of
the aggregate are advertised. However, the specific component 10.1.0.0 are suppressed.
route-policy sample
end-policy
router bgp 2
address-family ipv4
The effect of aggregation policy on the attributes of the aggregate is cumulative. Every time an aggregation
policy matches a more specific route, the set operations in the policy may modify the aggregate. The aggregate
in the following example has a MED value that varies according to the number of more specific routes that
comprise the aggregate.
route-policy bumping-aggregation
end-policy
If there are three matching more specific routes, the MED of the aggregate is the default plus 15; if there are
seventeen more specific routes, the MED of the aggregate is the default plus 85.
The order that the aggregation policy is applied to prefix paths is deterministic but unspecified. That is, a
given set of routes always appears in the same order, but there is no way to predict the order.
if destination in (10.0.0.0/8 ge 8 le 25) then
set community (10:33)
endif
if destination in (10.2.0.0/24) then
drop
endif
if destination in (10.1.0.0/24) then
suppress-route
endif
aggregate-address 10.0.0.0/8 route-policy sample
.
.
.
set med +5
Routing Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.3.x
Aggregation
151