BGP Auto Summary

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.

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.

Related articles

BGP route summary is explained in detail in the following articles.

N-Study

BGP Route Summary : network command | N-Study

https://www.n-study.com/en/bgp-detail/bgp-aggregation-by-network-command/

The network command can be used to generate BGP routes for known network addresses in the routing table. This section describes the generation of BGP aggregate route by the network command.

N-Study

BGP Route Summary aggregate-address command | N-Study

https://www.n-study.com/en/bgp-detail/aggregate-address/

The aggregate-address command can be used to generate BGP aggregate routes.This section describes the aggregate-address command and the conditions for the command to work.

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.

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
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.

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

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