For a long time we had /24 from our ISP with a static route. Now we would like to have our own routers and do BGP on our own. Our ISP says your block is on our 16 so it may create routing loop. How do generally people solve this. Does it mean customers having /24 from the bigger segment cannot do routing?

Another question, we have 2 /24 blocks not contiguous. Now, we advertise both on the WAN interface in BGP and having one IP from one /24 on the LAN interface. Now what is the best possible to recommended way to take the traffic in to LAN from the other IP subnet.


  • Could you please say why you want to do this. That would make it a lot easier to figure out what could work. The description doesn't mention any of the reasons for actually needing to run BGP, and the global routing table already has enough pollution, so if you don't absolutely need to do this, I'd recommend against it.
    – MAP
    Aug 4, 2016 at 5:08
  • we have various security reasons to do it
    – user88975
    Aug 5, 2016 at 10:39
  • Except BGP will not help your security in any way. It even makes you vulnerable to some additional attack vectors.
    – MAP
    Aug 7, 2016 at 21:23
  • yes, we are fully aware of it and have things in place to secure the upstream.
    – user88975
    Aug 8, 2016 at 10:55

1 Answer 1


Routing a more specific prefix (the /24 in this case) taken from a larger block normally wouldn't be a problem, since a more specific route will win over a less specific one.

However, there may be another challenge here, if the /16 is assigned to your current ISP and you're planning to announce the /24 from that /16 via other networks. In that case, those networks must accept that you're announcing a prefix not assigned to you. I can imagine that your ISP will not allow you to do so if you would want to stop using their services (since they own the aggregated prefix from which you're using a /24), but if you remain a customer but decide to connect your network via them and possible another network, there's no technical drawback.

Possibly you'll need to create valid route objects in a routing database like RIPE or RADB (a bit depending on where you're located) so networks will accept your prefix. Your current ISP will most likely need to coorperate to create these objects. As long as you keep the ISP owning the /16 as an upstream network, even networks not accepting your /24 would still be able to reach you via your ISP.

So to sum it up I don't think there are any unsolvable technical issues here (unless there are more details I'm not aware of), there may be policise or commercial reasons why your ISP says this is not possible.

Your second question is hard to answer without more details on the current network topology. You may consider asking this in a seperate question and providing some additional details there.

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