BGP Route Aggregation with AS-SET

This little scenario involves 3 routers (R1, R2 and R3)  R1 (AS 1) and R2 (AS 2) are directly connected via Frame Relay.  R2 is directly connected to R3 (AS 3) via Frame Relay as well but R3 is an uncontrolled device (in the CCIE Lab this could be a Backbone router) and we plan on aggregating routes R2 learns from R3 so R1 has a more manageable routing table.

The configs are pretty simple (R3's config wont be displayed until the end but it can be inferred as we go along)

R1

hostname R1
interface Serial0/0
 no ip address
 encapsulation frame-relay
 no frame-relay inverse-arp
interface Serial0/0.12 point-to-point
 ip address 10.1.12.1 255.255.255.0
 frame-relay interface-dlci 102
router bgp 1
 no synchronization
 neighbor 10.1.12.2 remote-as 2
 no auto-summary

R2

hostname R2
interface Serial0/0
 no ip address
 encapsulation frame-relay
 no frame-relay inverse-arp
interface Serial0/0.21 point-to-point
 ip address 10.1.12.2 255.255.255.0
 frame-relay interface-dlci 201
interface Serial0/0.23 point-to-point
 ip address 10.1.23.2 255.255.255.0
 frame-relay interface-dlci 203
router bgp 2
 no synchronization 
 neighbor 10.1.12.1 remote-as 1
 neighbor 10.1.23.3 remote-as 3
 no auto-summary

So let's start off with with R2 sees from R3


R2#sh ip bgp | b Network
   Network          Next Hop            Metric LocPrf Weight Path
*> 172.168.0.0/24   10.1.23.3                0             0 3 ?
*> 172.168.1.0/24   10.1.23.3                0             0 3 ?
*> 172.168.2.0/24   10.1.23.3                0             0 3 ?
*> 172.168.3.0/24   10.1.23.3                0             0 3 ?
*> 172.199.0.0/24   10.1.23.3                0             0 3 ?
*> 172.199.1.0/24   10.1.23.3                0             0 3 ?
*> 172.199.2.0/24   10.1.23.3                0             0 3 ?
*> 172.199.3.0/24   10.1.23.3                0             0 3 ?

So let's aggregate these for R1 so it only sees the summary routes

R2#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R2(config)#router bgp 2
R2(config-router)#aggregate-address 172.199.0.0 255.255.252.0 summary-only
R2(config-router)#aggregate-address 172.168.0.0 255.255.252.0 summary-only
R2(config-router)#end

Let's see what R1 now sees

R1>sh ip bgp | b Network
   Network          Next Hop            Metric LocPrf Weight Path
*> 172.168.0.0/22   10.1.12.2                0             0 2 i
*> 172.199.0.0/22   10.1.12.2                0             0 2 i

Alright, we can see the two aggregate routes, however we see that the AS-PATH shows R2 as the originator.  What if we wanted to see that these originated from R3?

Within the aggregate-address command, we can include the AS-SET option, which will show us the  Autonomous Systems in the path which contribute to this route

R2#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R2(config)#router bgp 2
R2(config-router)#aggregate-address 172.168.0.0 255.255.252.0 summary-only as-set
R2(config-router)#aggregate-address 172.199.0.0 255.255.252.0 summary-only as-set
R2(config-router)#end

So now let's see what R1 sees:

R1>sh ip bgp | b Network
   Network          Next Hop            Metric LocPrf Weight Path
*> 172.199.0.0/22   10.1.12.2                0             0 2 3 ?

Now this is a little odd, we can see that the route that is present does now indeed include the full AS-Path, however we have lost the route for 172.168.0.0/22 even though we previously had it.  Maybe R2 is doing something wrong?

R2#show ip bgp neighbor 10.1.12.1 advertised-routes
BGP table version is 21, local router ID is 10.1.23.2
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
*> 172.199.0.0/22   0.0.0.0                       100  32768 3 ?

Hello 172.168.0.0/22? Have we lost it?


R2#sh ip bgp | b Network
   Network          Next Hop            Metric LocPrf Weight Path
s> 172.168.0.0/24   10.1.23.3                0             0 3 ?
*> 172.168.0.0/22   0.0.0.0                       100  32768 3 ?
s> 172.168.1.0/24   10.1.23.3                0             0 3 ?
s> 172.168.2.0/24   10.1.23.3                0             0 3 ?
s> 172.168.3.0/24   10.1.23.3                0             0 3 ?
s> 172.199.0.0/24   10.1.23.3                0             0 3 ?
*> 172.199.0.0/22   0.0.0.0                       100  32768 3 ?
s> 172.199.1.0/24   10.1.23.3                0             0 3 ?
s> 172.199.2.0/24   10.1.23.3                0             0 3 ?
s> 172.199.3.0/24   10.1.23.3                0             0 3 ?

No, something must have happened to stop us sending things it on to R1 though.

R2#show ip bgp 172.168.0.0/22
BGP routing table entry for 172.168.0.0/22, version 20
Paths: (1 available, best #1, table Default-IP-Routing-Table, not advertised to EBGP peer)
  Not advertised to any peer
  3, (aggregated by 2 10.1.23.2)
    0.0.0.0 from 0.0.0.0 (10.1.23.2)
      Origin incomplete, localpref 100, weight 32768, valid, aggregated, local, best
      Community: no-export

When the AS-SET parameter is used, the communities of the contributing routes are also attached to the aggregate, at least one of these contributing routes must have had the no-export community attached to it.

R2#show ip bgp community no-export
BGP table version is 21, local router ID is 10.1.23.2
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
s> 172.168.0.0/24   10.1.23.3                0             0 3 ?
*> 172.168.0.0/22   0.0.0.0                       100  32768 3 ?
s> 172.168.2.0/24   10.1.23.3                0             0 3 ?

So if we would like to ignore R3's instruction of no-export to R1 while maintaining the AS-Path, we have to use our favourite IOS swiss-army knife also known as the route-map which in this form will be used as an attribute map to remove any communities on the aggregate which would normally be inherited from contributing routes:

R2#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R2(config)#route-map NoAttributes
R2(config-route-map)#set community none
R2(config-route-map)#router bgp 2
R2(config-router)#aggregate-address 172.168.0.0 255.255.252.0 summary-only as-set attribute-map NoAttributes
R2(config-router)#end

So let's check that this now works

R2#show ip bgp community no-export
BGP table version is 22, local router ID is 10.1.23.2
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
s> 172.168.0.0/24   10.1.23.3                0             0 3 ?
s> 172.168.2.0/24   10.1.23.3                0             0 3 ?

172.168.0.0/22 is no longer in this list, so it appears that this should be resolved:



R2#sh ip bgp nei 10.1.12.1 advertised-routes
BGP table version is 22, local router ID is 10.1.23.2
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
*> 172.168.0.0/22   0.0.0.0                       100  32768 3 ?
*> 172.199.0.0/22   0.0.0.0                       100  32768 3 ?

Total number of prefixes 2

R1>sh ip bgp | b Network
   Network          Next Hop            Metric LocPrf Weight Path
*> 172.168.0.0/22   10.1.12.2                0             0 2 3 ?
*> 172.199.0.0/22   10.1.12.2                0             0 2 3 ?

Success!

R3 Config for reference:

hostname R3
interface Serial0/0
 no ip address
 encapsulation frame-relay
 no frame-relay inverse-arp
interface Serial0/0.32 point-to-point
 ip address 10.1.23.3 255.255.255.0
 frame-relay interface-dlci 302
router bgp 3
 no synchronization
 redistribute static
 neighbor 10.1.23.2 remote-as 2
 neighbor 10.1.23.2 send-community
 neighbor 10.1.23.2 route-map R2-OUT out
 no auto-summary
route-map R2-OUT permit 10
 match ip address 1
 set community no-export
route-map R2-OUT permit 20
access-list 1 permit 172.168.0.0 0.0.254.255