[101672] in North American Network Operators' Group
Re: BGP Filtering
daemon@ATHENA.MIT.EDU (Dave Israel)
Tue Jan 15 13:05:07 2008
Date: Tue, 15 Jan 2008 12:51:28 -0500
From: Dave Israel <davei@otd.com>
To: Ben Butler <ben.butler@c2internet.net>
CC: nanog@merit.edu
In-Reply-To: <F9181128E9584B40B5A04C43800604B406DCDE@anyanka.c2internet.net>
X-OTD-MailScanner-From: davei@otd.com
Errors-To: owner-nanog@merit.edu
This is a multi-part message in MIME format.
--------------010903070005050506090500
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Ben,
I think I understand what you want, and you don't want it. If you
receive a route for, say, 204.91.0.0/16, 204.91.0.0/17, and
204.91.128.0/17, you want to drop the /17s and just care about the /16.
But a change in topology does not generally result in a complete update
of the BGP table. Route changes result in route adds and draws, not a
flood event. So if you forgot about the /17s and just kept the /16, and
the /16 was subsequently withdrawn, your router would not magically
remember that it had /17s to route to as well. You'd drop traffic,
unless you had a default, in which case you'd just route it suboptimally.
-Dave
Ben Butler wrote:
> Hi,
>
> Agreed that is why I have lots of RAM - doesn't mean I should carry on
> upgrading my tower of babble though to make it ever higher and higher if
> there is a better way of doing things.
>
> I still don't see how a default route to a portioned pop is going to
> help in the slightest - you are saved by getting the prefixes from an
> alternate transit and the default doesn't get used. Where is does help
> is to capture anything which has been filtered out completely and then
> there is no prefix from the alternate transit provider anyway - so
> whichever default gets used and takes its chances.
>
> Bogons - obviously.
>
> My question was if what I was asking was possible.
>
> Kind Regards
>
> Ben
>
>
> -----Original Message-----
> From: Joe Abley [mailto:jabley@ca.afilias.info]
> Sent: 15 January 2008 17:07
> To: Ben Butler
> Cc: nanog@merit.edu
> Subject: Re: BGP Filtering
>
>
> On 15-Jan-2008, at 11:40, Ben Butler wrote:
>
>
>> Defaults wont work because a routing decision has to be made, my
>> transit originating a default or me pointing a default at them does
>> not guarantee the reachability of all prefixes..
>>
>
> Taking a table that won't fit in RAM similarly won't guarantee
> reachability of anything :-)
>
> Filter on assignment boundaries and supplement with a default. That
> ought to mean that you have a reasonable shot at surviving de-peering/
> partitioning events, and the defaults will pick up the slack in the
> event that you don't.
>
> For extra credit, supplement with a bunch of null routes for bogons so
> packets with bogon destination addresses don't leave your network, and
> maybe make exceptions for "golden prefixes".
>
>
>> I am struggling to see a defensible position for why just shy of 50%
>> of all routes appears to be mostly comprised of de-aggregated routes
>> when aggregation is one of the aims RIRs make the LIRs strive to
>> achieve. If we cant clean the mess up because there is no incentive
>> than cant I simply ignore the duplicates.
>>
>
> You can search the archives I'm sure for more detailed discussion of
> this. However, you can't necessarily always attribute the presence of
> covered prefixes to incompetence.
>
>
> Joe
>
--------------010903070005050506090500
Content-Type: text/html; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<meta content="text/html;charset=ISO-8859-1" http-equiv="Content-Type">
<title></title>
</head>
<body bgcolor="#ffffff" text="#000000">
<br>
Ben,<br>
<br>
I think I understand what you want, and you don't want it. If you
receive a route for, say, 204.91.0.0/16, 204.91.0.0/17, and
204.91.128.0/17, you want to drop the /17s and just care about the
/16. But a change in topology does not generally result in a complete
update of the BGP table. Route changes result in route adds and draws,
not a flood event. So if you forgot about the /17s and just kept the
/16, and the /16 was subsequently withdrawn, your router would not
magically remember that it had /17s to route to as well. You'd drop
traffic, unless you had a default, in which case you'd just route it
suboptimally.<br>
<br>
-Dave<br>
<br>
<br>
Ben Butler wrote:
<blockquote
cite="mid:F9181128E9584B40B5A04C43800604B406DCDE@anyanka.c2internet.net"
type="cite">
<pre wrap="">Hi,
Agreed that is why I have lots of RAM - doesn't mean I should carry on
upgrading my tower of babble though to make it ever higher and higher if
there is a better way of doing things.
I still don't see how a default route to a portioned pop is going to
help in the slightest - you are saved by getting the prefixes from an
alternate transit and the default doesn't get used. Where is does help
is to capture anything which has been filtered out completely and then
there is no prefix from the alternate transit provider anyway - so
whichever default gets used and takes its chances.
Bogons - obviously.
My question was if what I was asking was possible.
Kind Regards
Ben
-----Original Message-----
From: Joe Abley [<a class="moz-txt-link-freetext" href="mailto:jabley@ca.afilias.info">mailto:jabley@ca.afilias.info</a>]
Sent: 15 January 2008 17:07
To: Ben Butler
Cc: <a class="moz-txt-link-abbreviated" href="mailto:nanog@merit.edu">nanog@merit.edu</a>
Subject: Re: BGP Filtering
On 15-Jan-2008, at 11:40, Ben Butler wrote:
</pre>
<blockquote type="cite">
<pre wrap="">Defaults wont work because a routing decision has to be made, my
transit originating a default or me pointing a default at them does
not guarantee the reachability of all prefixes..
</pre>
</blockquote>
<pre wrap=""><!---->
Taking a table that won't fit in RAM similarly won't guarantee
reachability of anything :-)
Filter on assignment boundaries and supplement with a default. That
ought to mean that you have a reasonable shot at surviving de-peering/
partitioning events, and the defaults will pick up the slack in the
event that you don't.
For extra credit, supplement with a bunch of null routes for bogons so
packets with bogon destination addresses don't leave your network, and
maybe make exceptions for "golden prefixes".
</pre>
<blockquote type="cite">
<pre wrap="">I am struggling to see a defensible position for why just shy of 50%
of all routes appears to be mostly comprised of de-aggregated routes
when aggregation is one of the aims RIRs make the LIRs strive to
achieve. If we cant clean the mess up because there is no incentive
than cant I simply ignore the duplicates.
</pre>
</blockquote>
<pre wrap=""><!---->
You can search the archives I'm sure for more detailed discussion of
this. However, you can't necessarily always attribute the presence of
covered prefixes to incompetence.
Joe
</pre>
</blockquote>
</body>
</html>
--------------010903070005050506090500--
