From: Brian McGahan (brian@xxxxxxxxxxxxxxx)
Date: Thu Aug 29 2002 - 23:34:56 GMT-3
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