Re: [isp-bgp] BGP and policy based routing question

["Marshall Eubanks" <tme@...>]

A number of large ISP's will aggregate any prefix longer than a /24; some
will even to it /24's if they are not in the old class C address range.

Of course, if ISP 1 "gave" you the /26, then any time BGP routes you through
an aggregating ISP that only sees you through ISP 2, then you will be black holed.
(This happened to me at Verio a while back, back when they were very rigourous about
aggregation.) If traffic for the block arrives at ISP 2, then, unless you have set up static
routing, you can be black holed there as well.

Do not count on getting a warning about this, generally it is silent and
only found by accident.

I would recommend that you try and qualify an obtain a microassignment  address space
from ARIN under 4.2.2.2 of the policy manual and policy proposal 2002-3

http://www.arin.net/policy/2002_3.html

That was set up for exactly this purpose.

As one of the instigators of 2002-3, I would be curious to hear of any experiences
with it, good or bad.

Regards
Marshall Eubanks


On Mon, 15 Nov 2004 06:35:46 -0600
 Pete Templin <petelists@...> wrote:
> David Enns wrote:
> > We just multihomed our network and set up BGP. When we started our ISP 7
> > years ago we were assigned a small block of 64 ip addresses from ISP 1,
> > 192.168.1.0/26, for our use. We put our first name server on this network.
> > Now that we have set up BGP this block is unreachable for some networks from
> > the internet. After doing some troubleshooting we believe that our router is
> > sending traffic over ISP 2. Because this network is less than a class C it
> > does not now what to do with the traffic and drops it.
> 
> What you have looks good.  The IP policy route-map ns1 statement on 
> S2/0/1 is unnecessary and may impact router CPU, but won't hurt 
> anything.  Is it not working for you?
> 
> The /26 is probably reachable just fine from the Internet, but ISP2 may 
> be filtering your traffic because its routing table doesn't show 
> 192.168.1.0/26 pointing towards you.  You may want to check with them, 
> and ask if they can install a static route for the /26 to you, and/or 
> manually update their filters.
> 
> pt
> 




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