Subject: RE: Review of draft-ietf-idr-bgp4-17.txt-advertise inactive

From: "Abarbanel, Benjamin" <[email-masked]>
Date: Thu, 19 Sep 2002 11:43:34 -0400

Jeff:

> -----Original Message-----
> From: Jeffrey Haas [mailto:[email-masked]]
> Sent: Thursday, September 19, 2002 11:25 AM
> To: Abarbanel, Benjamin
> Cc: [email-masked]
> Subject: Re: Review of draft-ietf-idr-bgp4-17.txt-advertise inactive
> 
> 
> On Wed, Sep 18, 2002 at 04:56:01PM -0400, Abarbanel, Benjamin wrote:
> > Then all other route types that win over the BGP routes in 
> the Routing table
> > should be automatically exported to BGP.
> 
> Probably the simplest way of dealing with this is dealing with
> the case when a route in LocRib and the Routing table conflict
> and the Routing Table has a non-BGP route installed in it is
> to note that (in almost all cases if not always) the route actually
> installed in the Routing Table is the one passed to Phase 3 of
> the selection process.
> 

My point is that the non-BGP route has to be configured for export to
BGP for this to be done. Sometimes it is not obvious and the operator
forgets to do so. Sometimes, he intentially does not want to do so.
Thus a potential inconsistency could occur. 

Remember non-BGP routes do not have attributes like AS-Path. If an
IGP/Static route is prefered over a IBGP route for a destination that is
outside this AS, it could create routing loops. Implying the route got
exported from EBGP to IGP by another IBGP router. Then that router flooded
this route as a regular IGP 
route to the current router, where the problem occured. Now that I have
twisted your thinking a bit, you see the problem.

> -- 
> Jeff Haas 
> NextHop Technologies

---

From: Curtis Villamizar <[email-masked]>
Date: Fri, 20 Sep 2002 13:53:12 -0400

In message <[email-masked]>, 
> twisted your thinking a bit, you see the problem.

Static routes can cause black holes and routing loops.  You don't need
BGP to help static routes do this but you can mess up BGP routing with
static routes.  That is inherent to a route that you make up from
nothing other than a configuration entry.  [btw - a static route will
only casue a route loop in BGP if the static route points outside your
AS to someone who is routing based on it, otherwise just a black
hole.]

An IGP route (that is not derived from a static route) will not cause
a route loop if injected into BGP and if the recommendations of the
BGP spec are followed, namely "don't advertise a route you don't use"
and apply the same route selection rules thrughout the AS.  If an
interface in your IGP is misnumbered, it could blackhole traffic to
the legitimate site but the BGP protocol can't prevent that.

Other uses of BGP that may cause a routing loop have so far been left
outside of the spec.  The spec goes further to specify the an order in
which route selection rules should be applied, which if not violate
will not form a route loop no matter the configuration of the AS and
will result in route loop free interoperability with other vendors.
We have so far left such things as modifying the route selection rules
within an AS, using alternate route selection rules or ordering which
can accidentally be configured to form persistent route loops, as
outside the spec.

Curtis