host2(config-router)#aggregate-address 172.24.0.0 255.255.224.0
As configured above, router SanJose advertises the more specific routes as well as
the aggregate route to router Boston. Alternatively, you can use the summary-only
option to configure router SanJose to suppress the more specific routes and advertise
only the aggregate route:
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 summary-only
Each of these configurations sets the atomic-aggregate attribute in the aggregate
route. This attribute informs recipients that the route is an aggregate and must not
be deaggregated into more specific routes.
Aggregate routes discard the path information carried in the original routes. To
preserve the paths, you must use the as-set option. This option creates an AS-Set
that consists of all the AS numbers traversed by the summarized paths. The AS-Set
is enclosed within curly brackets; for example, {3, 2}. Each AS number appears only
once, even if it appears in more than one of the original paths. If you use the as-set
option, the atomic-aggregate attribute is not set for the aggregated route. The following
commands configure router SanJose to aggregate the routes while preserving the
path information:
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 summary-only
as-set
If you do not want to aggregate all more specific routes, you can use a route map to
limit aggregation. Consider Figure 17 on page 59 again. Suppose you do not want
router SanJose to aggregate prefix 172.24.48.0/20. The following commands show
how you can configure a route map on router SanJose to match this prefix, and how
to invoke the route map with the advertise-map option:
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)#neighbor 10.2.2.3 route-map lmt_agg in
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 advertise-map
lmt_agg
host2(config-router)#exit
host2(config)#route-map lmt_agg permit 10
host2(config-route-map)#match ip address 1
host2(config-route-map)#exit
host2(config)#access-list 1 permit 172.24.48.0 0.240.255.255
You can use the attribute-map option to configure attributes for the aggregated
route. In Figure 17 on page 59, suppose that router LA has been configured to set
the community attribute for route 172.24.160.0/19 to no-export. This attribute is
60 â– Advertising Routes
JUNOSe 11.1.x BGP and MPLS Configuration Guide
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