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BGP /24 and ISP

I am just curious if every ISPs will accept a /24 subnet for BGP advertisement.  If not, is there a way to find out what they do accept and/or who doesn't accept this?

Secondly, we have a primary ISP that is advertise a /16 and on the secondary ISP, we advertised a /24 subnet (that is included in the /16 of the primary ISP) to test speed and throughout..etc to avoid having to bring out the primary circuit.

We're seeing some different throughput when testing with speedtest.net and when changing the server to a local city, we're getting latency error so not sure about this.

The /24 is a local subnet on the second ISP router and I Just advertised this under the BGP statement and we have a default route out with a next hop of our second ISP.  The second ISP has a static route for this /24 to us and a default route out to the internet.

Not sure what might be causing some latency errors when changing the speedtest to some local servers.   It seems to be fine if we change the server to Seattle, Washington for example and I'm located in SF.

thank you!
Watch Question

Most ISPs I've dealt with only allow /24 and bigger subnets, but the minimum seems to be /24. Only way to really find out is to ask them.

For the latency and speed differences, it depends again on the ISP and where you are at. If your ISP has a speed test server you're going to get a faster speed being on their network, but when you change servers to another provider or in a different city then it depends on the route you're taking. For example, I'm in El Paso, but my ISP traffic goes out through Phoenix, Dallas, or Denver, nothing is really sourced out of El Paso. So my speed tests are better when I'm testing to servers in Dallas. If I were to test a local El Paso server, my traffic has to go to Dallas then come back to El Paso which increases my round trip time and affects my speed.

When running the speed test check out the PING time. If a local server has a higher ping time than a remote one, you may be dealing with the same issue as me. All in all, it really doesn't matter too much, because you're testing with specific servers. If you're reporting good bandwidth to Seattle, then you should be fine, it means your link has the capacity to go that high. In the real world the webpage or download server could be located anywhere and the distance between you and that server and their bandwidth is what will determine actual throughput.
Distinguished Expert 2019

Adding to the above, you can test the route/path traceroute
When you have towpaths, the return might come back over the other

Much depends on what it is you are testing.
LateNaiteCEO and Founder


Thank you both for responding.  I'll be taking a look some more.
Top Expert 2011

The only way to find out what your ISP will accept is to ask them. You'd need to find out what prefix length they are matching on their inbound BGP filtering.

As for the primary and secondary ISP issue. Check out a looking glass site to see the how the internet is seeing your advertised BGP prefix such as https://www.us.ntt.net/support/looking-glass/
LateNaiteCEO and Founder


I'm still working on this but closing it for now.
CEO and Founder
See my previous update.