BGP Auto-summary

Two-router lab

R1 and R2 form eBGP. R1 has a connected 10.255.0.1/32 loopback, but no 10.0.0.0/8 route. Start with no originated BGP prefixes.

hostname R1
ip cef
interface Loopback0
 ip address 10.255.0.1 255.255.255.255
exit
interface GigabitEthernet1
 ip address 192.0.2.1 255.255.255.252
 no shutdown
exit
router bgp 65010
 bgp router-id 10.255.0.1
 no bgp default ipv4-unicast
 neighbor 192.0.2.2 remote-as 65020
 address-family ipv4 unicast
  no auto-summary
  neighbor 192.0.2.2 activate
 exit-address-family
end

hostname R2
ip cef
interface Loopback0
 ip address 10.255.0.2 255.255.255.255
exit
interface GigabitEthernet1
 ip address 192.0.2.2 255.255.255.252
 no shutdown
exit
router bgp 65020
 bgp router-id 10.255.0.2
 no bgp default ipv4-unicast
 neighbor 192.0.2.1 remote-as 65010
 address-family ipv4 unicast
  no auto-summary
  neighbor 192.0.2.1 activate
 exit-address-family
end

Without auto-summary

On R1, try a classful network statement without a mask. For an address beginning with 10, the historical class A boundary is /8. This command requests 10.0.0.0/8, not the /32 interface prefix.

router bgp 65010
 address-family ipv4 unicast
  network 10.0.0.0
 exit-address-family

R1# show ip route 10.255.0.1
Routing entry for 10.255.0.1/32
  Known via "connected", distance 0, metric 0
  * directly connected, via Loopback0

R1# show ip bgp 10.0.0.0/8
% Network not in table

The installed /32 is not an exact match for the requested /8. With automatic summarization disabled, that classful network statement does not originate a route.

Enable auto-summary

Leave the classful network statement in place and enable the legacy behavior on R1.

router bgp 65010
 address-family ipv4 unicast
  auto-summary
 exit-address-family

R1# show ip bgp 10.0.0.0/8
BGP routing table entry for 10.0.0.0/8
Paths: (1 available, best #1, table default)
  Local
    0.0.0.0 from 0.0.0.0 (10.255.0.1)
      Origin IGP, metric 0, localpref 100, weight 32768, valid, sourced, local, best

R2# show ip bgp 10.0.0.0/8
     Network          Next Hop            Metric LocPrf Weight Path
*>   10.0.0.0/8       192.0.2.1            0             0 65010 i

The installed component /32 now satisfies the legacy classful origination rule, so R1 originates the covering /8 and R2 learns it. The enabling command alone does not export every connected route: this experiment also needs the classful network statement and a matching component route.

What the summary hides

A broad advertisement does not create real reachability for all covered destinations. R2 can direct traffic for uncovered addresses toward R1, where the component /32 does not provide a route to deliver it. A discard summary, if installed by the image, only drops such traffic. Summarization reduces prefix detail, but it must represent a deliberate forwarding design.

This behavior is different from explicitly creating an aggregate prefix. It also does not mean every route learned from an eBGP or iBGP neighbor is automatically rewritten into a classful summary.

Return to explicit prefixes

Remove the experimental classful statement, disable auto-summary, and originate only the installed host prefix.

router bgp 65010
 address-family ipv4 unicast
  no network 10.0.0.0
  no auto-summary
  network 10.255.0.1 mask 255.255.255.255
 exit-address-family

R2# show ip bgp 10.255.0.1/32
     Network          Next Hop            Metric LocPrf Weight Path
*>   10.255.0.1/32    192.0.2.1            0             0 65010 i

R2# show ip bgp 10.0.0.0/8
% Network not in table

R2# show ip route 10.255.0.1
Routing entry for 10.255.0.1/32
  Known via "bgp 65020", distance 20, metric 0
  * 192.0.2.1, via GigabitEthernet1

After the updates converge, R2 has the intended /32 and no exact /8 entry. An explicit subnet-and-mask network statement still requires its exact installed route; auto-summary is not a substitute for planning aggregates.

Final configurations

hostname R1
ip cef
interface Loopback0
 ip address 10.255.0.1 255.255.255.255
exit
interface GigabitEthernet1
 ip address 192.0.2.1 255.255.255.252
 no shutdown
exit
router bgp 65010
 bgp router-id 10.255.0.1
 no bgp default ipv4-unicast
 neighbor 192.0.2.2 remote-as 65020
 address-family ipv4 unicast
  no auto-summary
  neighbor 192.0.2.2 activate
  network 10.255.0.1 mask 255.255.255.255
 exit-address-family
end

hostname R2
ip cef
interface Loopback0
 ip address 10.255.0.2 255.255.255.255
exit
interface GigabitEthernet1
 ip address 192.0.2.2 255.255.255.252
 no shutdown
exit
router bgp 65020
 bgp router-id 10.255.0.2
 no bgp default ipv4-unicast
 neighbor 192.0.2.1 remote-as 65010
 address-family ipv4 unicast
  no auto-summary
  neighbor 192.0.2.1 activate
 exit-address-family
end