From: Omer Ansari (omer@xxxxxxxxxx)
Date: Thu Aug 29 2002 - 19:51:54 GMT-3
That's exactly what I wanted to know.
thanks Brian!
On Thu, 29 Aug 2002, Brian McGahan wrote:
> Okay, now I understand the confusion. Locally originated doesn't have
> anything to do with the AS-Path. When you "show ip bgp", locally
> originated routes have a next-hop of "0.0.0.0". A local aggregate with
> an AS-SET still counts as locally originated.
>
> And yes, local-preference and locally originated are two separate
> decisions. Local-preference is first. I just group them together due
> to the mnemonic I use to remember the best path decision process.
>
> 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 4:36 PM
> > To: Brian McGahan
> > Cc: ccielab@groupstudy.com
> > Subject: RE: how does as-set change the bgp best path selection?
> >
> > Brian,
> >
> > two things:
> >
> > 1.
> >
> > > Weight
> > > Local-Pref/Locally Originated <---
> > > AS-Path
> > > Origin
> > > MED
> > > EBGP over iBGP
> > > Shortest IGP path
> > > Router-ID (lowest)
> >
> > I thought there was a distinction b/w the local-pref and the locally
> > originated criteria (steps2 and 3) as in:
> >
> > http://www.cisco.com/warp/public/459/25.shtml
> >
> > 2. this is the reasoning behind my question:
> >
> > without as-set the aggregate address bgp route shows up as a locally
> > originated via igp (just like one generated using the network command)
> and
> > all is well in the best path selection.
> >
> > but, with the as-set, show ip bgp would show this aggregate address
> with
> > all the ASes appended [am sure this is how the neighbors would see it,
> not
> > sure how the local router would see it]
> >
> > if indeed the local router would see the route with all the ASes
> > appended, wouldn't that disqualify this aggregate address as being a
> > locally generated route then?
> >
> >
> >
> > > > -----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
> > > > > >
This archive was generated by hypermail 2.1.4 : Sat Sep 07 2002 - 19:48:42 GMT-3