From: Brian McGahan (brian@xxxxxxxxxxxxxxx)
Date: Thu Aug 29 2002 - 22:15:44 GMT-3
Omer,
> if local_pref of both the local and learnt aggregate are the same and
no
> weights have been applied on routes, the bgp best path algo should put
> the [local] aggregate address going to Null0 in the route table
instead
> of the "learnt 12.0.0.0/8" route.
Correct. Although it may not necessarily be in the routing
table, it will at least be the best path in the BGP table.
> e.g if the "learnt 12.0.0.0/8" route has a low bgp metric, and that
turns
> out to be the next criteria for path selection would that route be put
in
> the table or the aggregate-address for 12.0.0.0/8 with as-set..
MED is much lower in the decision process. The locally
originated aggregate will always be preferred unless you change the
weight. The decision process is:
Weight
Local-Pref/Locally Originated
AS-Path
Origin
MED
EBGP over iBGP
Shortest IGP path
Router-ID (lowest)
HTH
Brian McGahan, CCIE #8593
Director of Design and Implementation
brian@cyscoexpert.com
CyscoExpert Corporation
Internetwork Consulting & Training
http://www.cyscoexpert.com
Voice: 847.674.3392
Fax: 847.674.2625
> -----Original Message-----
> From: Omer Ansari [mailto:omer@ansari.com]
> Sent: Thursday, August 29, 2002 3:31 PM
> To: Brian McGahan
> Cc: ccielab@groupstudy.com
> Subject: RE: how does as-set change the bgp best path selection?
>
> Brian,
> thanks that makes sense.
>
> however, my question was targetting more the bgp best path selection
than
> the eventual routing of the packets.
>
> say we have a bgp route 12.1.2.0/24, and we are also learning a
12.0.0.0/8
> network from a neighbor.
>
> we also have "aggregate-address 12.0.0.0 255.0.0.0" command on the
> router.
>
> if local_pref of both the local and learnt aggregate are the same and
no
> weights have been applied on routes, the bgp best path algo should put
> the [local] aggregate address going to Null0 in the route table
instead
> of the "learnt 12.0.0.0/8" route.
>
> this entry ofcourse was never competing with the 12.1.2/24 entry in
the
> best path algo.
>
> ..
>
> if the above is true, my question was if the aggregate address was
used
> with as-set, does this drop the aggregate address further down in the
best
> path algo selection?
>
> e.g if the "learnt 12.0.0.0/8" route has a low bgp metric, and that
turns
> out to be the next criteria for path selection would that route be put
in
> the table or the aggregate-address for 12.0.0.0/8 with as-set..
>
> regards,
> Omer
>
>
> On Thu, 29 Aug 2002, Brian McGahan wrote:
>
> > Omer,
> >
> > You will never route traffic to a locally generated aggregate,
> > since the outgoing interface is null. Besides, if the local router
is
> > doing the aggregation, it should always have a longer match to the
> > destination network in the first place. This is the reason the null
> > route is used, to in fact prevent the local router from routing
traffic
> > for which the longest match is your locally generated aggregate.
> >
> > HTH
> >
> > Brian McGahan, CCIE #8593
> > Director of Design and Implementation
> > brian@cyscoexpert.com
> >
> > CyscoExpert Corporation
> > Internetwork Consulting & Training
> > http://www.cyscoexpert.com
> > Voice: 847.674.3392
> > Fax: 847.674.2625
> >
> >
> > > -----Original Message-----
> > > From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On
Behalf
> > Of
> > > Omer Ansari
> > > Sent: Thursday, August 29, 2002 3:39 AM
> > > To: ccielab@groupstudy.com
> > > Subject: how does as-set change the bgp best path selection?
> > >
> > > All,
> > >
> > > we know that local routes from the bgp router are 3rd in the
selection
> > > list:
> > > http://www.cisco.com/warp/public/459/25.shtml
> > >
> > > this includes bgp routes generated using the aggregrate-address
> > command.
> > >
> > > however if the as-set option is used, does that route still
qualify as
> > a
> > > locally generated route and if not, does that drop this route
further
> > down
> > > in the bgp path selection list?
> > >
> > > thanks
> > > Omer
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