When you use the BGP aggregate-address command on Cisco IOS without any parameters, then all information of individual route attributes such as AS_PATH is lost.

This can cause issues since the AS_PATH is used for loop prevention. For example, it’s possible that an AS installs a summary that it shouldn’t. With the AS-SET parameter, you can optionally include AS information in the summary. In this lesson, I’ll show you how to do this.

Configuration

Here is the topology we’ll use:

We have four routers, all in a different AS. R2 and R3 have a loopback with an IP address that are advertised in BGP. R1 will send an aggregate to R4.

hostname R1
!
ip cef
!
interface GigabitEthernet0/1
 ip address 192.168.12.1 255.255.255.0
!
interface GigabitEthernet0/2
 ip address 192.168.13.1 255.255.255.0
!
interface GigabitEthernet0/3
 ip address 192.168.14.1 255.255.255.0
!
router bgp 1
 neighbor 192.168.12.2 remote-as 2
 neighbor 192.168.13.3 remote-as 3
 neighbor 192.168.14.4 remote-as 4
!
end

hostname R2
!
ip cef
!
interface Loopback0
 ip address 172.16.2.2 255.255.255.255
!
interface GigabitEthernet0/1
 ip address 192.168.12.2 255.255.255.0
!
router bgp 2
 network 172.16.2.2 mask 255.255.255.255
 neighbor 192.168.12.1 remote-as 1
!
end

hostname R3
!
ip cef
!
interface Loopback0
 ip address 172.16.3.3 255.255.255.255
!
interface GigabitEthernet0/1
 ip address 192.168.13.3 255.255.255.0
!
router bgp 3
 network 172.16.3.3 mask 255.255.255.255
 neighbor 192.168.13.1 remote-as 1
!
end

hostname R4
!
ip cef
!
interface GigabitEthernet0/1
 ip address 192.168.14.4 255.255.255.0
!
router bgp 4
 neighbor 192.168.14.1 remote-as 1
!
end

Right now, there is no aggregate so R4 sees two separate prefixes with the correct AS path information:

R4#show ip bgp 
BGP table version is 3, local router ID is 192.168.14.4
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, 
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter, 
              x best-external, a additional-path, c RIB-compressed, 
              t secondary path, 
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *>   172.16.2.2/32       192.168.14.1                           0 1 2 i
 *>   172.16.3.3/32       192.168.14.1                           0 1 3 i

Each prefix has the correct AS path.

Without AS-SET

Let’s create a summary/aggregate. We’ll start without the AS-SET parameter so that we have a before and after example:

R1(config)#router bgp 1
R1(config-router)#aggregate-address 172.16.0.0 255.255.0.0 summary-only

Here’s what we get on R4:

R4#show ip bgp 
BGP table version is 10, local router ID is 192.168.14.4
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, 
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter, 
              x best-external, a additional-path, c RIB-compressed, 
              t secondary path, 
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *>   172.16.0.0       192.168.14.1             0             0 1 i

We see the 172.16.0.0/16 prefix but all AS path information is lost. This prefix seems to come from AS 1 only.

If R4 was connected to R2 or R3 then those routers would install this prefix without hesitation since they don’t see their own AS number in the summary route. This could cause routing loops.

With AS-SET

Let’s add the as-set parameter on R1 now:

R1(config)#router bgp 1
R1(config-router)#aggregate-address 172.16.0.0 255.255.0.0 summary-only as-set

Here’s what we get on R4: