Hi, I might be being slow, or you might not understand my question - I am not sure it has been a long day. I want a filter that will automatically match the shorter prefixes that match any longer prefix, once I can match them I can drop them. I don't want to manually configure a static prefix list for lots and lots and lots of reasons. If the longer prefix disappears from the route table I want to stop filtering the shorter prefixes - automatically. -----Original Message----- From: Mike Walter [mailto:mwalter@3z.net] Sent: 15 January 2008 16:52 To: Ben Butler; nanog@merit.edu Subject: RE: BGP Filtering Ben, Look here. They show an example of prefix filtering on the 128.0.0.0/8 network. I would assume you could extrapolate and come up with your own rule. http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cbgp.h tml#wp7487 Mike Walter, MCP Systems Administrator 3z.net a PCD Company http://www.3z.net -----Original Message----- From: owner-nanog@merit.edu [mailto:owner-nanog@merit.edu] On Behalf Of Ben Butler Sent: Tuesday, January 15, 2008 11:45 AM To: nanog@merit.edu Subject: RE: BGP Filtering Hi Jason, Fantastic news, it is possible. We are using Cisco - would you be so kind as to give me a clue into which bit of Cisco's website you would like me to read as I have already read the bits I suspected might tell me how to do this but have guessed wrong / the documentation hasn't helped - so a handy pointer would be appreciated. Kind Regards Ben -----Original Message----- From: Jason Dearborn [mailto:jasondearborn@gmail.com] Sent: 15 January 2008 16:35 To: Ben Butler Subject: Re: BGP Filtering That's typically a function of your router software. Juniper, Force10, and Cisco all have support for this. Check your manual. On Jan 15, 2008 8:11 AM, Ben Butler <ben.butler@c2internet.net> wrote:
Hi,
Considering:
Total number of prefixes smaller than registry allocations:
113220
!!!!!
/20:17046 /21:16106 /22:20178 /23:21229 /24:126450
That is saying to me that a significant number of these smaller prefixes are due to de-aggregation of PA and not PI announcements.
My question is - how can I construct a filter / route map that will filter out any more specific prefixes where a less specific one exists
in the BGP table.
If my above conclusion is correct a significant portion ~47% of the number of the prefixes in the table could be argued to be very unnecessary at one level or another.
Is such a filter possible easily or would it have to be explicitly declared, any chance of a process the automatically tracks and publishes a list of offending specifics similar to Team Cymru's Bogon BGP feed.
As a transit consumer - why would I want to carry all this cr*p in my routing table, I would still be getting a BGP route to the larger prefix anyway - let my transit feeds sort out which route they use & traffic engineering.
Thoughts anyone?
Kind Regards
Ben