passed along to router SanJose and preserved when the aggregate route is created.
As a result, the aggregate route is not advertised outside the AS. The following
commands demonstrate how to configure router SanJose to prevent the aggregate
from not being advertised:
host2(config)#router bgp 873
host2(config-router)#neighbor 10.2.2.3 remote-as 873
host2(config-router)#neighbor 10.5.5.1 remote-as 17
host2(config-router)#network 172.24.24.0 mask 255.255.248.0
host2(config-router)#aggregate-address 172.24.0.0 255.255.224.0 attribute-map
conf_agg_att
host2(config-router)#exit
host2(config)#route-map conf_agg_att permit 10
host2(config-route-map)#set community no-export
aggregate-address
■ Use to create an aggregate entry in a BGP routing table that summarizes more
specific routes.
■ For IPv4 routes, you must specify an aggregate IP address (address) and aggregate
IP mask (mask). For IPv6 routes, you must specify an aggregate IPv6 prefix
(ipv6Prefix).
■ The optional as-set keyword preserves path information by creating an AS-Set
that contains all the AS numbers traversed by the aggregated routes.
NOTE: Do not use the as-set keyword when you have many paths to aggregate. If
you do, the aggregated route is continually withdrawn and reupdated as AS-path
reachability information changes for the summarized routes.
■ The summary-only keyword advertises only the aggregate route; it suppresses
the advertisement of all more specific routes. Contrast with the suppress-map
keyword.
■ The suppress-map keyword enables you to specify a route map to filter particular
routes covered by the aggregate that will be suppressed. Contrast with the
summary-only keyword.
NOTE: If you want to suppress advertisements only to certain neighbors, you
can—with caution—use the neighbor distribute-list command. If a more specific
route leaks out, all BGP speakers will prefer that route over the less specific aggregate
you are generating (using longest-match routing).
■ The advertise-map keyword enables you to specify the advertise-map-tag, a
string of up to 32 characters that identifies the route map that sets the routes to
create AS-Set origin communities.
■ The attribute-map keyword enables you to specify the attribute-map-tag, a
string of up to 32 characters that identifies the route map that sets the attributes
of the aggregate route.
Advertising Routes ■ 61
Chapter 1: Configuring BGP Routing
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