Lab 111 - BGP Aggregation with AS-SET

Prerequisites: CCNP level skills.

Personal Note!
Three day trip to Galway (Sun-Tue). Nice distraction but I'm tired like hell today. Fell deeply in love with the town. It must be these 17 thousands young students on the streets. The fountain of Youth!


Note!
R4 loopbacks as per Lab 108.

Topology

Pic. 1 - Topology Diagram.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

Task 1
Configure R4 to advertise loopbacks to 40.x.x.x/16 and 44.4.x.x/19 into BGP. Do not use the network statement. Make sure the prefixes are advertised with the IGP origin code ('i').


Task 2
Configure R1 and R2 aggregation for the prefixes advertised on R4 (in Task 1). Make sure that R5 can see the all AS numbers to reach these two aggregates.


Solution

Task 1

Configure R4 to advertise loopbacks to 40.x.x.x/16 and 44.4.x.x/19 into BGP. Do not use the network statement. Make sure the prefixes are advertised with the IGP origin code ('i').

R4 Configuration:

!

ip prefix-list NET_40 seq 5 permit 40.0.0.0/8 le 16
ip prefix-list NET_44 seq 5 permit 44.4.0.0/16 le 19
!

route-map CONN_TO_BGP permit 10
 match ip address prefix-list NET_40

 set origin igp
!
route-map CONN_TO_BGP permit 20
 match ip address prefix-list NET_44

 set origin igp
!
router bgp 40
 no synchronization
 bgp router-id 172.16.104.4
 bgp log-neighbor-changes
 network 172.16.104.0 mask 255.255.255.0
 redistribute connected route-map CONN_TO_BGP
 neighbor 10.1.14.1 remote-as 10
 neighbor 10.1.24.2 remote-as 20
 no auto-summary

!

Verification:

Pic. 2 - BGP Prefixes Originated by R4.

Task 2

Configure R1 and R2 aggregation for the prefixes advertised on R4 (in Task 1). Make sure that R5 can see the all AS numbers to reach these two aggregates.

R1 Configuration:

!

router bgp 10
 no synchronization
 bgp router-id 172.16.101.1
 bgp log-neighbor-changes
 network 172.16.101.0 mask 255.255.255.0
 aggregate-address 40.0.0.0 255.252.0.0 as-set summary-only
 aggregate-address 44.4.0.0 255.255.128.0 as-set summary-only
 neighbor 10.1.13.3 remote-as 30
 neighbor 10.1.14.4 remote-as 40
 no auto-summary

!

R2 Configuration:

!

router bgp 20
 no synchronization
 bgp router-id 172.16.102.2
 bgp log-neighbor-changes
 network 172.16.102.0 mask 255.255.255.0
 aggregate-address 40.0.0.0 255.252.0.0 as-set summary-only
 aggregate-address 44.4.0.0 255.255.128.0 as-set summary-only
 neighbor 10.1.23.3 remote-as 30
 neighbor 10.1.24.4 remote-as 40
 no auto-summary

!

Pic. 3 - BGP Table on R5.

Notice!
R1 and R2 being aggregating routers remove AS 40 before they advertise the the summary routes. They set two attributes: aggregator and atomic-aggregate. Below picture shows the aggregate route WITHOUT the 'as-set' option.

Pic. 4 - R5's Aggregate without 'AS-SET'.

The result is loosing the AS 40 in the path. R5 knows that some information has been lost.

Pic. 5 - R5's Aggregate with 'AS-SET'.

Notice!
The 'atomic-aggregate' no longer shows. AS 40 is prepended to aggregate.

Pic. 6 - BGP Table on R5.

Notice!
Remember that losing information about AS number can cause loops. BGP routers reject prefixes if their own AS shows in the path.