From CT3

Assume we want to change the origin of the aggregate route generated with the as-set option from unknown ("?" in the BGP table listing) to IGP. To do, this we'll need to specify an attribute map and reference it from the aggregate-address command:

R3(config)# route-map SetAttributes
R3(config-route-map)# set origin igp

R3(config)# router bgp 30
R3(config-router)# aggregate-address 172.16.0.0 255.255.248.0 as-set suppress-map
 MySuppressMap attribute-map SetAttributes

After applying the attribute map, we can see that the aggregate route is now advertised with an origin of IGP ("i"):

R4# show ip bgp

   Network          Next Hop            Metric LocPrf Weight Path
*> 172.16.0.0/21    10.0.0.9                 0             0 30 {10,20} i
*> 172.16.4.0/24    10.0.0.9                               0 30 20 ?
*> 172.16.5.0/24    10.0.0.9                               0 30 20 ?