(OT?) Anyone wanna address my ISP's issues? [CIDR/BGP question]
efinleywork at efinley.com
Thu Jun 14 20:05:25 UTC 2007
On Thu, 14 Jun 2007 14:44:56 -0500, you wrote:
>Elliot Finley wrote:
>> On Thu, 14 Jun 2007 14:07:07 -0500, you wrote:
>>> The DSL modem's outside (static) IP is n.n.n.70, the gw
>>> is n.n.n.69, and the mask is 255.255.255.252. From
>>> inside, I can ping .70, but not .69 (and, needless to say,
>>> nothing else, either). From the outside, it's the
>>> other way 'round. Traceroute (from outside) shows different
>>> endpoints for the two addresses (that is, the last hop
>>> before .69 is one router, and, when looking for .70, it's
>>> another router (but not the one that leads to .69)).
>>> If I did my CIDR homework correctly, the net is n.n.n.68/30.
>>> Using "BGPlay" (http://bgplay.routeviews.org/bgplay/), I get
>>> the message: "The selected data sources have no information on
>>> prefix n.n.n.68/30. Please check that this prefix is globally
>>> My question: shouldn't it be 'announced', if the ISP intends
>>> to route me TCP/IP traffic? I apologize for my ignorance,
>>> but BGP isn't something I figured to need to know at this
>>> point in my life (although, it doesn't hurt to learn, usually)....
>> anything smaller than a /24 will be filtered. The ISP would announce
>> the larger block that your /30 lives in.
>Thank you very much, Elliot; You wouldn't believe how hard it's been
>to get anyone at, err, "tech support", to even address the issue.
>It makes sense, I suppose, otherwise the global routing table
>would be much larger than it is (?)
>Anyone up for further questions? The .70 --> .69 route on the
>modem has a metric of "5", but with the .252 mask, shouldn't it
>be required to be one hop away?
We really need further information to debug/diagnose this problem.
I'll give you a diagnosis for two different scenarios.
#1) you are using private addresses on your LAN and your DSL
modem/router is NATting for you:
Your modem/router isn't routing. ( this is more common than it should
be. we replace customers' routers because of this problem regularly.)
Your ISP has fat fingered a netmask - most likely changing a .252 to a
#2) you are using public addresses on your LAN and your DSL
modem/router is just routing for you:
Same possibilities as above with the addition of:
Your ISP has *not* put the route in for your public block of IPs.
Your ISP *HAS* put the route in for your public block of IPs, but for
whatever reason, that route isn't propagating through their network.
Those will be the most likely problems. I'm betting on your modem
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