In order to reduce the amount of routing information within the routing table, routing summarization is used.

Cisco IOS provides 3 types of BGP summarization:

  • Aggregate Address - BGP is configured to advertise an aggregate network via the aggregate-address command.
  • Manual - A summarized network is added to RIB via a null route, the network is then advertised via BGP using the network command.
  • Auto-Summarization - If the RIB contains a network that is part of a classful network then advertise the classful network.

Aggregate Address

Let us look at the first way to create a summary route for a group of networks. This can be achieved via the aggregate-address command. Below shows an example of summarizing the networks:

router bgp 64520
  network mask
  network mask
  network mask
  aggregate-address summary-only

Once configured, you can then see the routes summarized by looking at the BGP table, using the status code s>. As shown below:

r1#show ip bgp         
BGP table version is 36, local router ID is
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, 
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter, 
              x best-external, a additional-path, c RIB-compressed, 
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
     Network          Next Hop            Metric LocPrf Weight Path
 *>                            32768 i
 s>                  0         32768 i
 s>                  0         32768 i
 s>                  0         32768 i

Manual Summarization

Manual summarization works by simply adding a static null route for the summarized network so that it is present within the RIB. This summarized network can then be adding to BGP via the network command.

When traffic comes into the router, the router will select the more precise route, ensuring that the null route is not hit.

ip route null0

router bgp 64520
  network mask

For example, let's say we have through to locally configured. Based on this we want to advertise a summarized route of . However as we do not have this network within our RIB, when we add the network to BGP, it is not added to the BGP table and in turn not advertised.


Even though Cisco disables auto-summarization by default (shown below), it is worth mentioning how the command works.

router bgp 64520
  no auto-summary

In short, when auto-summary is enabled BGP will automatically advertise the classful network, if you have the classful network or a subnet of this network in your routing table.[1]

For example, let's say I add network to BGP (with no auto-summary enabled). Unless I have the classful network of in the RIB it will not be advertised. However with auto-summary enabled, the fact that I have in the RIB, the network will be advertised.


  1. "BGP Auto-Summary |" 8 Jun. 2015, Accessed 11 Jun. 2019. ↩︎

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