nested prefixes in Internet

Luke Guillory lguillory at reservetele.com
Tue Oct 25 03:33:43 UTC 2016


Misread the email. Ignore my ignorance. 

Sent from my iPad

> On Oct 24, 2016, at 9:48 PM, Owen DeLong <owen at delong.com> wrote:
> 
> How do you figure that?
> 
> Owen
> 
>> On Oct 24, 2016, at 06:22 , Luke Guillory <lguillory at reservetele.com> wrote:
>> 
>> That only works if the /24 isn't coming from the middle of the /20 block and is on the front end or back end.
>> 
>> 
>> 
>> Luke Guillory
>> Network Operations Manager
>> 
>> Tel:    985.536.1212
>> Fax:    985.536.0300
>> Email:  lguillory at reservetele.com
>> 
>> Reserve Telecommunications
>> 100 RTC Dr
>> Reserve, LA 70084
>> 
>> _________________________________________________________________________________________________
>> 
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>> 
>> -----Original Message-----
>> From: NANOG [mailto:nanog-bounces at nanog.org] On Behalf Of Martin T
>> Sent: Monday, October 24, 2016 7:06 AM
>> To: Matt Buford; Baldur Norddahl
>> Cc: nanog
>> Subject: Re: nested prefixes in Internet
>> 
>> Thank you all for the replies! I'll go with the solution where "ISP A"
>> announces both /19 prefix and /24 prefix.
>> 
>> 
>> Martin
>> 
>>> On Thu, Oct 20, 2016 at 1:16 AM, Matt Buford <matt at overloaded.net> wrote:
>>> On Mon, Oct 10, 2016 at 2:44 PM, Baldur Norddahl
>>> <baldur.norddahl at gmail.com>
>>> wrote:
>>> 
>>> 
>>>> Is that a real problem? In my experience a /24 is honoured almost
>>>> universially.
>>>> 
>>> 
>>> Here's a real-world issue I ran into with this.  In this case, it
>>> isn't that someone filtered /24s, but that they didn't have a full
>>> table (peering routes only plus a default).  This resulted in them
>>> following the less specific route for traffic destined for the /24.
>>> 
>>> A customer of mine was advertising only a /20 to me and then only a
>>> more specific /24 was advertised from a remote site of theirs to a
>>> different ISP.  The customer did have a connection between their two
>>> sites, so in theory it was OK if the traffic arrived on their "wrong"
>>> link, except that the customer strongly didn't want this to happen,
>>> thus the inconsistent routes.
>>> 
>>> This customer found that the remote /24 was unable to access a large
>>> CDN provider.  This CDN told them that a traceroute to the /24 went to
>>> my network (we peered at an exchange) and was then dropped.
>>> 
>>> This seemed odd at first, as I confirmed I was not advertising the /24
>>> to them so why were they routing it to me?  It turned out that the CDN
>>> provider was running a peer-routes-only network with a default to
>>> their transit.  This meant that they saw the /20 from their peer (me)
>>> but never saw the /24, since they carried no transit routes.  This
>>> resulted in them routing the entire /20 to me.
>>> 
>>> My peering router was not willing to route traffic from a peering
>>> exchange towards transit I had to pay for, so it was dropped.
>>> 
>>> The customer's split advertisements didn't seem particularly
>>> unreasonable or invalid, though perhaps they were not the preferred
>>> setup.  It wasn't unreasonable for me to not route from a peering
>>> exchange to transit.  It wasn't unreasonable of the CDN to have a
>>> peering-and-default routing table.  But, those three things together were not compatible.
>>> 
>>> I called the customer and presented them with my findings and some
>>> potential options to consider, and consistent advertisements was one
>>> of those options.  The customer strongly wanted incoming traffic to
>>> arrive directly to the correct location so he didn't want to do that.
>>> I suggested a possible compromise was for him to advertise both the
>>> /24 and /20 to me, but use communities so that I never advertised his
>>> /24 to any upstreams or peering exchanges.  That way, if traffic for
>>> the /24 accidentally hit my network like we were running into, I would
>>> route it to him and he could pass it to the correct site.  It meant
>>> that a little traffic would arrive at the wrong site in his network
>>> and have to pass over his back-end link, but at least it would be
>>> fairly limited.  He didn't like this either.  He didn't want to split
>>> the /20 advertisement up to no longer cover the /24 either, I think just because "I shouldn't have to do that!"
>>> 
>>> The option the customer chose in the end was to use a community on his
>>> advertisement to instruct my routers to no longer advertise his /20 to
>>> any peering exchanges at all.  That ensured the CDN didn't directly
>>> send me anything for him (not the /24 or the /20), and the transit
>>> networks in-between took care of making sure traffic to the /24 didn't
>>> accidentally end up on my network.  While I didn't find it very
>>> elegant to be shifting traffic from peering exchanges to transit, it
>>> wasn't a significant amount of traffic and it got him off my back.
> 



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