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BGP Route Aggregation

1. Definition

  • Route Aggregation (RA) also known as BGP Route Summarization is a method to minimize the size of the routing table, announcing the whole address block received from the Regional Internet Registry (RIR) to other ASes.

  • RA is opposite to non-aggregation routing, where individual sub-prefixes of the address block are announced to BGP peers.

  • RA reduces the size of the global routing table, decreases routers’ workload and saves network bandwidth.

2. Network topology

image

3. Configuration

3.1. Initial configuration

Below is an initial configuration of all three routers.

  • R1
conf t
router bgp 65100
 bgp router-id 1.1.1.1
 no bgp default ipv4
 neighbor 192.168.12.2 remote-as 65200
 address-family ipv4 unicast
   neighbor 192.168.12.2 activate
   network 192.168.0.0 mask 255.255.255.0
  • R2
conf t
router bgp 65200
 bgp router-id 2.2.2.2
 no bgp default ipv4
 neighbor 192.168.12.1 remote-as 65100
 neighbor 192.168.23.3 remote-as 65300
 address-family ipv4 unicast
  neighbor 192.168.12.1 activate
  neighbor 192.168.23.3 activate
  • R3
conf t
router bgp 65300
 bgp router-id 3.3.3.3
 no bgp default ipv4
 neighbor 192.168.23.2 remote-as 65200
 address-family ipv4 unicast
  neighbor 192.168.23.2 activate

3.2. BGP Route Aggregation with Static Discard Route

  • R1: Create an aggregate address with a static discard route 192.168.0.0/20 pointing to a null interface.

  • The discard static route 192.168.0.0/20 configured on a router R1 makes the router to discard any packet that matches the route. However, as long as there are more specific (longer prefix) working routes in a routing table of the router R1, packets matching these routes are not discarded.

  • The BGP tables of R2 and R3 routers are injected with the network command configured on R1 router, matching the static discard route.

  • R1

router bgp 65100
 bgp router-id 1.1.1.1
 bgp log-neighbor-changes
 no bgp default ipv4-unicast
 neighbor 192.168.12.2 remote-as 65200
 !
 address-family ipv4
  network 192.168.0.0 mask 255.255.240.0
  neighbor 192.168.12.2 activate
 exit-address-family

ip route 192.168.0.0 255.255.240.0 Null0
  • The BGP table of the router R3 is shown in below
R3#show bgp ipv4 unicast 
BGP table version is 4, local router ID is 3.3.3.3
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, 
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *>  192.168.0.0/20   192.168.23.2                           0 65200 65100 i
R3#

3.3. BGP Route Aggregation with Aggregate-address Command

  • R1 advertise the aggregate prefix 192.168.0.0/20 to its BGP neighbor R2.
  • The aggregate address is advertised to a neighbor as long as it represents at least one part of the aggregate address in the BGP table of a router.
  • The parts are called components or the contributing routes and represent more specific matches for the aggregated route.
  • We will inject a single route 192.168.0.0/24 into the BGP table of R1 with the network command.
R1#show running-config | section bgp
router bgp 65100
 bgp router-id 1.1.1.1
 bgp log-neighbor-changes
 no bgp default ipv4-unicast
 neighbor 192.168.12.2 remote-as 65200
 !
 address-family ipv4
  network 192.168.0.0 mask 255.255.255.0
  aggregate-address 192.168.0.0 255.255.240.0
  neighbor 192.168.12.2 activate
 exit-address-family

R1# show bgp ipv4 uni
BGP table version is 7, local router ID is 1.1.1.1
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, 
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *>  192.168.0.0      0.0.0.0                  0         32768 i
 *>  192.168.0.0/20   0.0.0.0                            32768 i
R1#
  • The BGP table of the router R3
R3# show bgp ipv4 unicast 
BGP table version is 7, local router ID is 3.3.3.3
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, 
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *>  192.168.0.0      192.168.23.2                           0 65200 65100 i
 *>  192.168.0.0/20   192.168.23.2                           0 65200 65100 i
R3#

The BGP table of R3 contains the aggregated prefix along with a more-specific route 192.168.0.0/24.

3.3.1. Option summary-only
  • By default, all more-specific routes summarized by the aggregate route are advertised.
  • To advertise only the aggregate route and suppress the advertisement of all the component routes, the keyword summary-only can be used.
R1# show running-config | sec bgp
router bgp 65100
 bgp router-id 1.1.1.1
 bgp log-neighbor-changes
 no bgp default ipv4-unicast
 neighbor 192.168.12.2 remote-as 65200
 !
 address-family ipv4
  network 192.168.0.0
  aggregate-address 192.168.0.0 255.255.240.0 summary-only
  neighbor 192.168.12.2 activate
 exit-address-family
  • R1's bgp table is marking 192.168.0.0/24 suppressed s>
R1# show bgp ipv4 unicast        
BGP table version is 8, local router ID is 1.1.1.1
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, 
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 s>  192.168.0.0      0.0.0.0                  0         32768 i
 *>  192.168.0.0/20   0.0.0.0                            32768 i
 
R1#show bgp ipv4 uni 192.168.0.0
BGP routing table entry for 192.168.0.0/24, version 8
Paths: (1 available, best #1, table default, Advertisements suppressed by an aggregate.)
  Not advertised to any peer
  Refresh Epoch 1
  Local
    0.0.0.0 from 0.0.0.0 (1.1.1.1)
      Origin IGP, metric 0, localpref 100, weight 32768, valid, sourced, local, best
      rx pathid: 0, tx pathid: 0x0
R1#
  • R3's bgp table now contains only the aggregated route 192.168.0.0/20.
R3#show bgp ipv4 unicast 
BGP table version is 8, local router ID is 3.3.3.3
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, 
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *>  192.168.0.0/20   192.168.23.2                           0 65200 65100 i
R3#

R3#show bgp ipv4 unicast 192.168.0.0 255.255.240.0
BGP routing table entry for 192.168.0.0/20, version 7
Paths: (1 available, best #1, table default)
  Not advertised to any peer
  Refresh Epoch 1
  65200 65100, (aggregated by 65100 1.1.1.1)
    192.168.23.2 from 192.168.23.2 (2.2.2.2)
      Origin IGP, localpref 100, valid, external, atomic-aggregate, best
      rx pathid: 0, tx pathid: 0x0
R3#
3.3.2. Option suppress-map
  • Suppress map defines components that should not be advertised.
ip prefix-list sup-list seq 10 permit 192.168.0.0/24
route-map sup-map permit 10
 match ip address prefix-list sup-list
 
R1# show run | sec bgp
router bgp 65100
 bgp router-id 1.1.1.1
 bgp log-neighbor-changes
 no bgp default ipv4-unicast
 neighbor 192.168.12.2 remote-as 65200
 !
 address-family ipv4
  network 192.168.0.0
  network 192.168.1.0
  aggregate-address 192.168.0.0 255.255.240.0 suppress-map sup-map
  neighbor 192.168.12.2 activate
 exit-address-family
  • Now 192.168.0.0/24 is marked as suppressed.
R1#show bgp ipv4 unicast 
BGP table version is 15, local router ID is 1.1.1.1
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, 
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 s>  192.168.0.0      0.0.0.0                  0         32768 i
 *>  192.168.0.0/20   0.0.0.0                            32768 i
 *>  192.168.1.0      0.0.0.0                  0         32768 i
R1#
  • R3's bgp table
R3# show bgp ipv4 unicast 
BGP table version is 13, local router ID is 3.3.3.3
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, 
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *>  192.168.0.0/20   192.168.23.2                           0 65200 65100 i
 *>  192.168.1.0      192.168.23.2                           0 65200 65100 i
R3#
3.3.3. Option unsuppress-map
  • If a subset of the suppressed routes needs to be made available, we can unsuppress these routes on a per neighbor basis using the neighbor unsuppress-map command.
R1#
ip prefix-list unsup-list seq 10 permit 192.168.0.0/24
route-map unsup-map permit 10
 match ip address prefix-list unsup-list
 
router bgp 65100
 bgp router-id 1.1.1.1
 bgp log-neighbor-changes
 no bgp default ipv4-unicast
 neighbor 192.168.12.2 remote-as 65200
 !
 address-family ipv4
  network 192.168.0.0
  network 192.168.1.0
  aggregate-address 192.168.0.0 255.255.240.0 summary-only
  neighbor 192.168.12.2 activate
  neighbor 192.168.12.2 unsuppress-map unsup-map
 exit-address-family

The component routes 192.168.0.0/24 and 192.168.1.0/24 are suppressed by the aggregate route 192.168.0.0/20. The route 192.168.0.0/24 is unsuppressed for neighbor 192.168.12.2.

R1# show bgp ipv4 unicast        
BGP table version is 6, local router ID is 1.1.1.1
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, 
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 s>  192.168.0.0      0.0.0.0                  0         32768 i
 *>  192.168.0.0/20   0.0.0.0                            32768 i
 s>  192.168.1.0      0.0.0.0                  0         32768 i
R1#
  • R3's BGP table displays both the aggregate route 192.168.0.0/20 and a component route 192.168.0.0/24.
R3# show bgp ipv4 uni 
BGP table version is 19, local router ID is 3.3.3.3
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, 
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *>  192.168.0.0      192.168.23.2                           0 65200 65100 i
 *>  192.168.0.0/20   192.168.23.2                           0 65200 65100 i
R3#
3.3.4. Option attribute-map
  • Attributes are inherited from component routes. If we need to remove attributes or set our own attributes to the aggregate route, we will use the attribute-map.
  • The configuration below on R1 router advertises the aggregate route 192.168.0.0/20 with the community 65100:500 towards R2 router.
R1#show running-config | section bgp
router bgp 65100
 bgp router-id 1.1.1.1
 bgp log-neighbor-changes
 no bgp default ipv4-unicast
 neighbor 192.168.12.2 remote-as 65200
 !
 address-family ipv4
  network 192.168.0.0
  network 192.168.1.0
  aggregate-address 192.168.0.0 255.255.240.0 as-set summary-only attribute-map set-attribute
  neighbor 192.168.12.2 activate
  neighbor 192.168.12.2 send-community
 exit-address-family
ip bgp-community new-format
  • R2's bgp table
R2#show bgp ipv4 unicast  192.168.0.0
BGP routing table entry for 192.168.0.0/20, version 22
Paths: (1 available, best #1, table default)
  Advertised to update-groups:
     2         
  Refresh Epoch 4
  65100, (aggregated by 65100 1.1.1.1)
    192.168.12.1 from 192.168.12.1 (1.1.1.1)
      Origin IGP, metric 0, localpref 100, valid, external, best
      Community: 65100:500
      rx pathid: 0, tx pathid: 0x0

Full of this Lab just re-implement from below link:

https://www.noction.com/knowledge-base/bgp-route-aggregation