Changing the Attributes of the Aggregate

Changing the attributes of aggregate could be useful and necessary in some situations.
With the AS_SET option configured, an aggregate prefix can carry information about its component more-specific prefixes. If a component prefix forming the AS_SET aggregate is configured with the no-export community attribute, the aggregate will carry the same attribute and therefore prevent it from being advertised to EBGP peers.
The solution is modifies the community attribute of the aggregate using an attribute map, a form of route maps that applies only upon aggregates.

Network Setup for BGP Aggregation

RT1 tags 192.168.1.0/24 with a community value of NO_EXPORT.
When RT3 aggregates 192.168.1.0/24 into the AS_SET aggregate 192.168.0.0/16, the aggregate will contain the NO_EXPORT community, and therefore not being advertised to RT4.

RT3#sh ip bgp
BGP table version is 8, local router ID is 34.34.34.3
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 192.168.0.0/16   0.0.0.0                       100  32768 {100,200} i
s> 192.168.1.0      13.13.13.1               0             0 100 i
s> 192.168.2.0      23.23.23.2               0             0 200 i
RT3#
RT3#sh ip bgp 192.168.0.0[/16| 255.255.0.0]
BGP routing table entry for 192.168.0.0/16, version 8
Paths: (1 available, best #1, table Default-IP-Routing-Table, not advertised to EBGP peer)
  Not advertised to any peer
  {100,200}, (aggregated by 300 34.34.34.3)
    0.0.0.0 from 0.0.0.0 (34.34.34.3)
      Origin IGP, localpref 100, weight 32768, valid, aggregated, local, best
      Community: no-export
RT3#
======================================================================
RT4#sh ip bgp

RT4#

Below shows how to implement an attribute map on RT3 to manipulate the aggregate attributes – setting the community to NONE, and allow the aggregate to be advertised to EBGP peers – RT4.

RT3#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
RT3(config)#route-map SET-ATTRIB permit 10
RT3(config-route-map)#set community none
RT3(config-route-map)#exit
RT3(config)#
RT3(config)#router bgp 300
RT3(config-router)#aggregate-address 192.168.0.0 255.255.0.0 as-set summary-only attribute-map SET-ATTRIB
RT3(config-router)#end
RT3#
RT3#sh ip bgp 192.168.0.0[/16| 255.255.0.0]
BGP routing table entry for 192.168.0.0/16, version 9
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x880
  Advertised to non peer-group peers:
  13.13.13.1 23.23.23.2 34.34.34.4
  {100,200}, (aggregated by 300 34.34.34.3)
    0.0.0.0 from 0.0.0.0 (34.34.34.3)
      Origin IGP, localpref 100, weight 32768, valid, aggregated, local, best
RT3#

RT4#sh ip bgp
BGP table version is 6, local router ID is 34.34.34.4
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 192.168.0.0/16   34.34.34.3               0             0 300 {100,200} i
RT4#
RT4#sh ip bgp 192.168.0.0[/16| 255.255.0.0]
BGP routing table entry for 192.168.0.0/16, version 6
Paths: (1 available, best #1, table Default-IP-Routing-Table)
  Not advertised to any peer
  300 {100,200}, (aggregated by 300 34.34.34.3)
    34.34.34.3 from 34.34.34.3 (34.34.34.3)
      Origin IGP, metric 0, localpref 100, valid, external, best
RT4#