I have discovered weird routing from our ISP all related to BGP. Here is the scenario (real-one). - there is a /19 prefix bought about 10 years ago, which was registered with ARIN authority. - as bussiness grew each site got it /24.

The trouble starts in APNIC region in ASIA. We export one /24 prefix the regular way using two local ISPs. When trying to reach this prefix from different ASIA sites like India or China all traffic is sent to US West Coast then back to our site. Except additional hops, this also adds about 150ms latency.

What I already tried: - AS prepend each ISP at a time so the other left would be preffered for all inbound traffic - both tests went similar as in both scenarios traffic hit US before reaching this trouble site.

As there is no other local special config I can think of like communities, how can I explain this?

Are both local ISP taking in consideration the ARIN info and based on that send traffic to US?


  • 1
    The best thing would be to go to a asia based looking glass and check your prefix and see if your /24 is making it. You did not include your prefix so I would not check my self. My guess is that people is taking the /19 summary back to the US. Did you get this prefix from someone or from ARIN.
    – fredpbaker
    Nov 22, 2013 at 16:33
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    Can you provide your prefix? It will make it easier for us to see what is going wrong here. Without that it won't be able to answer your question. The number of possibly correct answers is just too big :-) Nov 22, 2013 at 17:21
  • Thanks for the input, guys! Unfortunately I cannot provide the prefix, because of the current NDA I have signed.... I will see what I can do around this.
    – laf
    Nov 26, 2013 at 12:10
  • Did you call the ISPs and discuss the sub-optimal routing with them ? What was their feedback ? Did they re-route your traffic ?
    – aseaudi
    Jan 4, 2014 at 20:50
  • Yes, I tried, still no good feedback! Thanks for the comment!
    – laf
    Jan 22, 2014 at 15:18

1 Answer 1


I guess I should start by saying that IP prefixes are not technically bought. When an IP prefix is allocated to a provider from an RIR like ARIN, there is no transfer of ownership. The IP range is always allocated to the RIR, and the allocation to the provider is only valid so long as the original terms of the allocation are upheld.

As for the routing part of the question, it is hard to give a specific answer without details on the actual prefixes used; however the most likely explanation is that some providers have policies in place which are not accepting the more specific /24 announcements.

I assume that the site in the US is announcing the entire /19 range, so any ISP that does not carry the specific /24 prefix will instead route towards the covering /19.

As for why some of the ISPs are not accepting the specific /24 announcements, this can have a number of reasons, and to be honest you are only going to be able to get a definitive answer from the ISP doing the filtering; however there are a few things you can try to improve your chances of the announcements being accepted.

Some ISPs will build their policies dynamically from information in routing registries. My experience is strongest in the RIPE service region, where routing policy can be registered using RPSL syntax in the RIPE database. ARIN also have a routing registry, but for the ARIN service region it is more common to register policy with the Merit RADb at ra.net. RPKI is still in it's early days and not many people are using RPKI in their policies; however it is an area to consider for the future which may allow for more reliable route distribution.

As you have seen, if the route is not in the table of these other ISPs then you are not going to be able to influence their routing by means of attributes on your announcements (i.e. as path length, communities, etc). You are only going to fix things by getting them to accept your announcement.

  • One tricky scenario is if a router is running out of memory an ISP may choose to filter out some routes and add a default gateway. If that filter hits the more-specific but not the less-specific then the traffic will end up routed towards the less-specific route you announced rather than towards the default gateway. Aug 30, 2016 at 22:31

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