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RE: BGP Filtering

  • From: Ben Butler
  • Date: Wed Jan 16 02:09:01 2008


That is where I got to last night with my cogitations before I feel


-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of
Robert Bonomi
Sent: 16 January 2008 01:26
To: [email protected]
Subject: Re: BGP Filtering

> Date: Tue, 15 Jan 2008 15:16:04 -0500
> From: "William Herrin" <[email protected]>
> Subject: Re: BGP Filtering
> On Jan 15, 2008 12:51 PM, Dave Israel <[email protected]> wrote:
> >    I think I understand what you want, and you don't want it.  If 
> > you receive a route for, say,,, and 
> >, 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 was subsequently withdrawn, your router would not magically remember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.
> Dave,
> That's half-true.
> The "routing table" is comprised of two components: the Routing 
> Information Base (RIB) and the Forwarding Information Base (FIB). The 
> RIB sits in slow, cheap memory and contains routes and metrics for 
> every route as announced by every neighbor. The FIB sits in fast, 
> expensive memory and contains the currently "best" route for each 
> destination. The FIB is built by choosing the best routes from the 
> RIB. Packet-forwarding decisions are made by consulting the FIB.
> Opportunistically filtering routes from the RIB would have exactly the

> problem you point out: routing updates are incremental. The knowledge 
> that the /16 has been withdrawn may not accompany the knowledge that 
> the /17s are available.
> Opportunistically filtering more-specific routes from the FIB, 
> however, could be very valuable at the edge of the DFZ. If Cisco 
> supported such filtering, those Sup2's could have another few years of

> life left in them. With 512m ram in a two-transit provider scenario a
> Sup2 could handle upwards of 1M routes in the RIB. Unfortunately, they

> can only handle 244k routes in the FIB.
> Ben, coming back to your question: I don't think there is a way to 
> make the software filter the routes inserted into the FIB. I don't see

> a reason why it couldn't be programmed to do that. But the fine folks 
> at Cisco didn't see fit to write that software. Its a pity 'cause it 
> would be very useful.

This leads to one 'obviuous' approach to the matter. a smart BGP 'proxy'
that accepts 'full' data from all the peers, manages the RIB  and
outputs =only= the current 'most desired' subset of routes to the target
In _that_ router the RIB and FIB are going to be equivalent,

This gets the 'hog' part of things off into 'commodity' hardware, and
where you can afford to burn CPU cycles implementing 'smarts' to
for other people's 'dumbs'.   

At a quick glance it looks like this would not be terribly difficult to
build, using Quagga/Zebra as a base platform.