Overview

This section describes BGP auto-summary, which is different from auto-summary with RIP/EIGRP.

What is BGP auto-summary?

BGP supports auto-summary. However, the meaning is different from that of auto-sumamry in distance vector routing protocols such as RIP/EIGRP. If BGP auto-summary is enabled, routes redistributed to BGP are automatically summarised at the class boundary.

BGP auto-summary behavior

The following figure shows the redistribution of routes learned by OSPF to BGP. If auto-summary is enabled, routes redistributed from OSPF to BGP are automatically summarised at the class boundary and advertised to the neighbor.

Figure: BGP auto-summary
Figure: BGP auto-summary

Note that auto-summary is only performed on routes that have been redistributed to BGP. Auto-summary is also performed when they are redistributed and placed on the BGP table. In the figure above, the auto-summary will put 172.16.0.0 on the BGP table.

Although such auto-summary exists, it is disabled by default.

Specific examples of BGP auto-summary

Network diagram

Verify BGP auto-summary with the following network diagram

Configure and verify BGP auto-summary

Routing within AS100 is performed by OSPF. Redistribute 100.100.1.0/24, 100.100.2.0/24, and 100.100.3.0/24 learned by OSPF to BGP and perform auto-summary at that time.

R1 BGP auto-summary configuration

access-list 10 permit 100.100.0.0 0.0.255.255
!
route-map AUTO-SUMMARY permit 10
 match ip address 10
!
router bgp 100
 redistribute ospf 1 route-map AUTO-SUMMARY
 auto-summary

Then the BGP table on R1 would look like this.

R1 show ip bgp

R1#show ip bgp
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
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 100.0.0.0        0.0.0.0                  0         32768 ?
*> 100.1.1.0/24     172.16.1.11              0             0 1 i
*> 100.1.2.0/24     172.16.1.11              0             0 1 i
*> 100.1.3.0/24     172.16.1.11              0             0 1 i

The BGP table shows that three routes, 100.100.1.0/24, 100.100.2.0/24, and 100.100.3.0/24, redistributed from OSPF, are automatically summarised to 100.0.0.0/8.

Figure: BGP auto-summary configuration
Figure: BGP auto-summary configuration

In addition, if auto-summary is disabled in R1, the following will occur.

R1 disable auto-summary

R1(config)#router bgp 100
R1(config-router)#no auto-summary
R1(config-router)#end
R1#show ip bgp
BGP table version is 19, 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
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 100.1.1.0/24     172.16.1.11              0             0 1 i
*> 100.1.2.0/24     172.16.1.11              0             0 1 i
*> 100.1.3.0/24     172.16.1.11              0             0 1 i
*> 100.100.1.0/24   192.168.13.3             2         32768 ?
*> 100.100.2.0/24   192.168.13.3             2         32768 ?
*> 100.100.3.0/24   192.168.13.3             2         32768 ?

If auto-summary is disabled, you will see that the OSPF routes are placed as-is in the BGP table.

Summary

Points

  • Automatic BGP aggregation is for routes that have been redistributed to BGP.
  • In the current IOS, BGP auto-summary is disabled by default.