WB1 7.17 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.
 
Configuration
 
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 bgp 100
no network 150.1.1.0 mask 255.255.255.0

R3:
router bgp 100
no network 150.1.3.0 mask 255.255.255.0

R5:
router bgp 100
no network 150.1.5.0 mask 255.255.255.0

R6:
router bgp 100
no network 150.1.6.0 mask 255.255.255.0
auto-summary
network 150.1.0.0
network 155.1.0.0
 
R4:
router bgp 100
no network 150.1.4.0 mask 255.255.255.0
auto-summary
no network 150.1.0.0
redistribute connected route-map CONNECTED_TO_BGP
!
route-map CONNECTED_TO_BGP
match interface Loopback0
match interface FastEthernet 0/1

-------------------------------------------

Rack1R6#show ip bgp regexp ^$

the regex ^$ filter is used to select the routes locally advertised in this AS.

R6:
Now check the BGP table in R6 and confirm that auto-summarized prefixes are now in there.
In this router, both prefixes has the origin of “i” which mean IGP.

-------------------------------------------

Rack1R6#show ip bgp neighbors 54.1.1.254 advertised-routes
BGP table version is 34, 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 2172416 32768 i
*>i204.12.1.0 150.1.4.4 0 100 0 i

Total number of prefixes 4

-------------------------------------------

Rack1R4#show ip bgp regexp ^$

checking the BGP tables of R4 for auto-summarized prefixes.
the regex ^$ filter is used to select the routes locally advertised in this AS.

R4:
Notice the origin of “?” which means the prefix has been redistributed into BGP. The prefix 155.1.0.0 even has BGP metric assigned based on the IGP metric (EIGRP metric).

-------------------------------------------

Rack1R4# show ip bgp neighbors 204.12.1.254 advertised-routes
BGP table version is 42, 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

*>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 2172416 32768 ?
*> 204.12.1.0 0.0.0.0 0 32768 i

Total number of prefixes 4