LAB: BGP Auto Summary

- Configure R4 and R6 to originate classful auto-summaries for all of your internally assigned address space.
-  BB1 and BB3 should not see any of the subnet advertisements that make up this summary.
- Ensure full reachability from your internal network to all routes learned from AS 54.
- Do not use the aggregate-address command to accomplish this and use different methods to originate routes at R4 and R6.

BGP auto-summarization is the legacy feature that automatically summarizes network prefixes to their classful boundaries when the prefixes are advertised into BGP. The automatic summarization starts working when you enable it using the command auto-summary under BGP process configuration. It only applies in
the following two cases:

1) A network command is configured with a classful subnet, e.g. network 54.0.0.0 or network 155.1.0.0 or network 192.168.1.0. In this case, the classful aggregate is installed into BGP table if there is a prefix in the IGP table that is a subnet to the classful network. For example, if you advertise network 150.1.0.0 than it would work if any of the prefixes 150.1.2.0/24 or 150.1.3.0/24 etc is in the IGP table. This is in contrary with the regular exact match requirement imposed by the BGP network statements.
 

2) Prefixes are advertised into BGP using route redistribution. All redistributed networks are subject to auto-summarization, i.e. only the major classful subnets are installed in the BGP table.
 

Since the feature is legacy, you wont seem much use of it nowadays. However, it may become handy in some tricky CCIE scenario that verifies your knowledge of BGP advertisement methods. This scenario uses both methods of route origination with prefix auto-summarization: classful network statement and route
redistribution.

R1

router eigrp 100
 network 150.1.1.1 0.0.0.0
 network 155.1.0.0
 no auto-summary
!
router bgp 100
 no synchronization
 bgp log-neighbor-changes
 neighbor IBGP_PEERS peer-group
 neighbor IBGP_PEERS remote-as 100
 neighbor IBGP_PEERS update-source Loopback0
 neighbor IBGP_PEERS route-reflector-client
 neighbor 150.1.3.3 peer-group IBGP_PEERS
 neighbor 150.1.4.4 peer-group IBGP_PEERS
 neighbor 150.1.5.5 peer-group IBGP_PEERS
 neighbor 150.1.6.6 peer-group IBGP_PEERS
 no auto-summary

R3

router eigrp 100
 network 150.1.3.3 0.0.0.0
 network 155.1.0.0
 no auto-summary
!
router bgp 100
 no synchronization
 bgp log-neighbor-changes
 neighbor 150.1.1.1 remote-as 100
 neighbor 150.1.1.1 update-source Loopback0
 neighbor 155.1.23.2 remote-as 200
 no auto-summary

R4

router eigrp 100
 network 150.1.4.4 0.0.0.0
 network 155.1.0.0
 network 204.12.1.0
 no auto-summary
!
router bgp 100
 no synchronization
 bgp log-neighbor-changes
 network 204.12.1.0
 redistribute connected route-map CONNECTED_TO_BGP
 neighbor 150.1.1.1 remote-as 100
 neighbor 150.1.1.1 update-source Loopback0
 neighbor 204.12.1.254 remote-as 54
 auto-summary
!
route-map CONNECTED_TO_BGP permit 10
 match interface Loopback0 FastEthernet0/1

R6

router eigrp 100
 passive-interface Serial0/0
 network 54.0.0.0
 network 150.1.6.6 0.0.0.0
 network 155.1.0.0
 no auto-summary
!
router bgp 100
 no synchronization
 bgp log-neighbor-changes
 network 54.1.1.0 mask 255.255.255.0
 network 150.1.0.0
 network 155.1.0.0
 neighbor 54.1.1.254 remote-as 54
 neighbor 150.1.1.1 remote-as 100
 neighbor 150.1.1.1 update-source Loopback0
 auto-summary

verification 

the routes via redistribution in bgp will appear as incomplete (?).

Rack1R4#show ip bgp regexp ^$
BGP table version is 72, local router ID is 150.1.4.4
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
r>i54.1.1.0/24      150.1.6.6                0    100      0 i
* i150.1.0.0        150.1.6.6                0    100      0 i
*>                  0.0.0.0                  0         32768 ?
* i155.1.0.0        150.1.6.6                0    100      0 i
*>                  0.0.0.0                  0         32768 ?
*> 204.12.1.0       0.0.0.0                  0         32768 i

Now look up those prefixes in the BGP table. Notice that both prefixes appear as if they were NOT summarized in classic BGP sense. That is, prefixes don’t have any information about the aggregator or the atomic aggregate attribute. This is due to the fact that summarization was performed on the IGP prefixes, not the BGP networks.

Rack1R4#show ip bgp 150.1.0.0
BGP routing table entry for 150.1.0.0/16, version 70
Paths: (2 available, best #2, table Default-IP-Routing-Table)
Flag: 0x820
  Advertised to update-groups:
     1          2        
  Local
    150.1.6.6 (metric 156160) from 150.1.1.1 (150.1.1.1)
      Origin IGP, metric 0, localpref 100, valid, internal
      Originator: 150.1.6.6, Cluster list: 150.1.1.1
  Local
    0.0.0.0 from 0.0.0.0 (150.1.4.4)
      Origin incomplete, metric 0, localpref 100, weight 32768, valid, sourced, best

Rack1R4#show ip bgp 155.1.0.0 
BGP routing table entry for 155.1.0.0/16, version 69
Paths: (2 available, best #2, table Default-IP-Routing-Table)
  Advertised to update-groups:
     1          2        
  Local
    150.1.6.6 (metric 156160) from 150.1.1.1 (150.1.1.1)
      Origin IGP, metric 0, localpref 100, valid, internal
      Originator: 150.1.6.6, Cluster list: 150.1.1.1
  Local
    0.0.0.0 from 0.0.0.0 (150.1.4.4)
      Origin incomplete, metric 0, localpref 100, weight 32768, valid, sourced, best

Rack1R4#
Rack1R4#show ip bgp neighbors 204.12.1.254 advertised-routes
BGP table version is 76, local router ID is 150.1.4.4
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
r>i54.1.1.0/24      150.1.6.6                0    100      0 i
*> 150.1.0.0        0.0.0.0                  0         32768 ?
*> 155.1.0.0        0.0.0.0                  0         32768 ?
*> 204.12.1.0       0.0.0.0                  0         32768 i

Total number of prefixes 4 

the routes via network statement in bgp will appear as igp.

Rack1R6#show ip bgp regexp ^$
BGP table version is 67, local router ID is 150.1.6.6
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
*> 54.1.1.0/24      0.0.0.0                  0         32768 i
*> 150.1.0.0        0.0.0.0                  0         32768 i
*> 155.1.0.0        0.0.0.0                  0         32768 i
r>i204.12.1.0       150.1.4.4                0    100      0 i

Rack1R6#show ip bgp neighbors 54.1.1.254 advertised-routes
BGP table version is 67, local router ID is 150.1.6.6
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
*> 54.1.1.0/24      0.0.0.0                  0         32768 i
*> 150.1.0.0        0.0.0.0                  0         32768 i
r>i150.1.2.0/24     155.1.23.2               0    100      0 200 i
*> 155.1.0.0        0.0.0.0                  0         32768 i
r>i204.12.1.0       150.1.4.4                0    100      0 i
*>i205.90.31.0      155.1.23.2               0    100      0 200 254 ?
*>i220.20.3.0       155.1.23.2               0    100      0 200 254 ?
*>i222.22.2.0       155.1.23.2               0    100      0 200 254 ?

Total number of prefixes 8