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In a BGP routing context, giving these networks address:

192.168.0.0 /28
192.168.0.32 /28

For me, the summary address will be :

192.168.0.0 /26

But someone tells me that this is impossible because at least 192.168.0.16/28 is missing, so i'm wondering if this is possible?

Another example where this is impossible (in his point of view):

192.168.0.0/28
192.168.0.16/28
192.168.0.32/28

For him, this is impossible to summarize because of the lack of 192.168.0.48/28

192.168.0.0000|0000
192.168.0.0001|0000
192.168.0.0010|0000
192.168.0.0011|0000   --because this one is missing

if 192.168.0.48/28 was present, then the summary address will be:

192.168.0.0 /26

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2 Answers 2

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Generally speaking, route summarization hides information about more specific routes. It also hides the fact that you may not have a route to all addresses in your summary. So yes, you can summarize. Whether or not you should depends on what you are doing with your summary.

In the case of BGP, you are advertising that you can reach all networks in your summary - but in fact you can't. You will "blackhole" the missing networks because your router will attract traffic for 192.168.0.16/28 (in your example), but you may not actually have a route to that network and you will end up dropping the traffic. If that is OK in your network, then go ahead and summarize.

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  • So doing a summarization of a non-existant network means using more traffic.. and this is useless
    – Dimareal
    Dec 28, 2016 at 15:46
  • @Dimareal What are you trying to accomplish? It all depends on your situation. My organization advertises a /16 to the Internet, but not all those addresses are reachable. That is fine with us.
    – Ron Trunk
    Dec 28, 2016 at 15:47
  • @Trunk : Nothing, I just want to better understand bgp and aggregation by doing some gns3 simulations. By the way when the router realise that one of those addresses isn't reachable he will drop packets related to it. isn't it?
    – Dimareal
    Dec 28, 2016 at 15:59
  • Otherwise, you first response is totally suitable for me. thank you
    – Dimareal
    Dec 28, 2016 at 16:10
  • @Dimareal if the router has no route for the packet destination address, it will drop the packet.
    – Ron Trunk
    Dec 28, 2016 at 16:51
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I think what the question is about is if BGP will install the summarized route, if that middle subnet does not existing in the IP Route table. If that's the case, then the answer is yes, it will as long as one of the routes is in the BGP table:

show ip bgp | i 192.168.0.

You'd want to look to see if the 192.168.0.0/28 or 192.168.0.32/28 routes exist there. 

router bgp 64999
network 192.168.0.0 mask 255.2555.255.240
network 192.168.0.32 mask 255.255.255.240
aggregate-address 192.168.0.0 255.255.255.192 summary-only

More info can be seen at Troubleshooting When BGP Routes Are Not Advertised.

Routes Announced Using the aggregate-address Command BGP allows the aggregation of specific routes into one route using the aggregate-address address mask command.

Aggregation applies to routes that exist in the BGP routing table. This is in contrast to the network command, which applies to the routes that exists in IP routing table. Aggregation can be performed if at least one or more of the specific routes of the aggregate address exists in the BGP routing table. Refer to Understanding Route Aggregation in BGP for more information on BGP aggregation and associated attributes.

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