BGP - Aggregation with advertise-map

R1,R2 and R3 connect to R5 via Frame-relay:


These 3 spokes are EBGP peers with R5.
These routes are advertised into bgp:

R1, loopback 150.1.1.1 AS1
R2, loopback 150.1.2.2 AS2
R3, loopback 150.1.3.3 AS3
R5, loopback 150.1.5.5 AS5

Here are R3's and R5's BGP table before any aggregation:

R5#show ip bgp | begin Network
Network Next Hop Metric LocPrf Weight Path
*> 150.1.1.0/24 155.1.0.1 0 0 1 i
*> 150.1.2.0/24 155.1.0.2 0 0 2 i
*> 150.1.3.0/24 155.1.0.3 0 0 3 i
*> 150.1.5.0/24 0.0.0.0 0 32768 i

R3#show ip bgp | begin Network
Network Next Hop Metric LocPrf Weight Path
*> 150.1.1.0/24 155.1.0.1 0 5 1 i
*> 150.1.2.0/24 155.1.0.2 0 5 2 i
*> 150.1.3.0/24 0.0.0.0 0 32768 i
*> 150.1.5.0/24 155.1.0.5 0 0 5 i

Suppose R5 wants to advertise a summary-only aggregate to R3:

R5(config)#router bgp 5
R5(config-router)#aggregate-address 150.1.0.0 255.255.248.0 as-set summary-only

R3 will deny the route because of the as-set option which forces R5 to include the AS numbers as an unordered set in the AS PATH:

R3#show ip bgp | begin Network
Network Next Hop Metric LocPrf Weight Path
*> 150.1.3.0/24 0.0.0.0 0 32768 i

R3#debug ip bgp updates
00:37:37: BGP(0): 155.1.0.5 rcv UPDATE w/ attr: nexthop 155.1.0.5, origin i, aggregated by 5 150.1.5.5, originator 0.0.0.0, path 5 {1,2,3}, community , extended community
00:37:37: BGP(0): 155.1.0.5 rcv UPDATE about 150.1.0.0/21 -- DENIED due to: AS-PATH contains our own AS;
R3#

We can have R5 remove R3's attributes (AS PATH) in the aggregate by using an advertise-map. This will allow R3 to recieve the aggregate.

First we create a prefix-list to match the route:

R5(config)#ip prefix-list R3 permit 150.1.3.0/24

The we create a route-map, note that we are denying the prefix. This means any matches will NOT have their attributes populated to the aggregate's attributes:

R5(config)#route-map DENY3 deny 10
R5(config-route-map)#match ip address prefix-list R3
R5(config-route-map)#exit
R5(config)#route-map DENY3 permit 20
R5(config-route-map)#exit

Finally, we apply the advertise-map to the aggregate-address command under the bgp process:

R5(config)#router bgp 5
R5(config-router)#aggregate-address 150.1.0.0 255.255.248.0 as-set summary-only advertise-map DENY3

Here are the final results:

R5#show ip bgp | begin Network
Network Next Hop Metric LocPrf Weight Path
*> 150.1.0.0/21 0.0.0.0 32768 {1,2} i
s> 150.1.1.0/24 155.1.0.1 0 0 1 i
s> 150.1.2.0/24 155.1.0.2 0 0 2 i
s> 150.1.3.0/24 155.1.0.3 0 0 3 i
s> 150.1.5.0/24 0.0.0.0 0 32768 i
R5#

R3#
00:46:26: BGP(0): 155.1.0.5 update run completed, afi 0, ran for 4ms, neighbor v
ersion 35, start version 38, throttled to 38
00:46:26: BGP(0): 155.1.0.5 rcvd UPDATE w/ attr: nexthop 155.1.0.5, origin i, at
omic-aggregate, aggregated by 5 150.1.5.5, path 5 {1,2}
00:46:26: BGP(0): 155.1.0.5 rcvd 150.1.0.0/21
00:46:26: BGP(0): Revise route installing 150.1.0.0/21 -> 155.1.0.5 to main IP t
able
R3#

R3#show ip bgp | begin Network
Network Next Hop Metric LocPrf Weight Path
*> 150.1.0.0/21 155.1.0.5 0 5 {1,2} i
*> 150.1.3.0/24 0.0.0.0 0 32768 i
R3#

Notice that the aggregate only has AS1 and AS2 in the AS PATH. This allows R3 to install the aggregate in it's BGP table.