From: Brian McGahan (brian@cyscoexpert.com)
Date: Mon Sep 23 2002 - 10:49:14 GMT-3
Conditional advertisement is probably the best solution proposed
so far. Khalid's solution of only advertising the route out one link
will obviously work, but that defeats the purpose of the problem.
Khalid's solution of aggregation will also work, but it's not really a
valid solution to aggregate just because. You shouldn't be aggregating
prefixes that you don't actually have.
In response to the AS-Path prepending and MED, yes this would
work under normal conditions. However in this case, the administrator
of AS 20 disagrees with AS 10 on their routing policy. Since Weight and
Local-Preference are higher in the decision process than AS-Path and
MED, AS 20 always gets the final word in their outbound routing policy.
"Configuring and Verifying the BGP Conditional Advertisement Feature"
http://www.cisco.com/warp/public/459/cond_adv.html
Be aware that there are some bugs in it though.
CSCdp20320 Bug Details
BGP:conditional advertisement state is Uninitialized
http://www.cisco.com/cgi-bin/Support/Bugtool/onebug.pl?bugid=CSCdp20320&
cco_product=IOS&fset=&swver=&keyw=conditional%20advertisement&target=&tr
ain=
CSCdp18563 Bug Details
BGP:adv sent even it non-exist-map is satisfied (conditional adv)
http://www.cisco.com/cgi-bin/Support/Bugtool/onebug.pl?bugid=CSCdp18563&
cco_product=IOS&fset=&swver=&keyw=conditional%20advertisement&target=&tr
ain=
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
> Carlos G Mendioroz
> Sent: Monday, September 23, 2002 5:12 AM
> To: Brian McGahan
> Cc: 'Joe Higgins'; 'Asim Khan'; ccielab@groupstudy.com
> Subject: Re: Basic BGP decision process
>
> There is a way out... and it is actually a good practice to use it.
> It is called conditional advertisement, and it makes you advertise
> the path ONLY to the neighbour via which you want the traffic back.
>
> (Conditional because as soon as this neighbour goes down, you do
> advertise
> in the other link...)
>
> Brian McGahan wrote:
> >
> > Joe,
> >
> > Actually it would be the other way around. Your inbound BGP
> > policy affects your outbound routing policy, and your outbound BGP
> > policy affects your inbound routing policy. You ultimately have
control
> > of how traffic leaves your AS, but not necessarily how traffic
returns.
> >
> > In this particular case, AS 20 always has the final say in
> > traffic engineering its outbound packets. Since weight and
> > local-preference are higher in the decision process than AS-Path and
> > MED, AS 20 can always set the policy of how traffic leaves its AS.
> >
> > Therefore to answer the question "If Link 2 is used then is
> > there any way for Administrator of AS10 to force AS20 to use Link
1," no
> > there is not.
> >
> > For more information on the BGP best-path selection process:
> >
> > http://www.cisco.com/warp/public/459/25.shtml
> >
> > 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
> > > Joe Higgins
> > > Sent: Sunday, September 22, 2002 2:38 PM
> > > To: Asim Khan; ccielab@groupstudy.com
> > > Subject: Re: Basic BGP decision process
> > >
> > > AS10 can do AS prepends but AS20 can change the default of AS path
> > > length controlling route selection thereby negating AS 10s action.
In
> > > the final word the guy that is sending the route can always
overrule
> > the
> > > guy that is receiving the routes as far as path selection is
> > concerned.
> > >
> > > Asim Khan wrote:
> > >
> > > > thats fine, but the question here is AS20 wants to
> > > > send its traffic through Link 2 and AS10 wants all
> > > > traffic from AS20 comes through Link 1.
> > > >
> > > > --- "Steven A. Ridder" <saridder@attbi.com> wrote:
> > > > > Incoming and outgoing traffic are treated
> > > > > separately.
> > > > >
> > > > > -----Original Message-----
> > > > > From: nobody@groupstudy.com
> > > > > [mailto:nobody@groupstudy.com] On Behalf Of
> > > > > Asim Khan
> > > > > Sent: Sunday, September 22, 2002 11:59 AM
> > > > > To: ccielab@groupstudy.com
> > > > > Subject: Basic BGP decision process
> > > > >
> > > > >
> > > > > Hi all,
> > > > >
> > > > > Suppose we have 4 routers Ra, Rb, Rc and Rd. Ra is
> > > > > in
> > > > > AS10 and Rb, Rc and Rd are in AS20 as shown below:
> > > > >
> > > > > AS10
> > > > > | loopback 1.1.1.1/32
> > > > > |
> > > > > Ra
> > > > > / \
> > > > > Link 1 / \Link 2
> > > > > -------------------------------
> > > > > / \
> > > > > Rc Rb
> > > > > AS20 \ /
> > > > > \ /
> > > > > \ /
> > > > > \ /
> > > > > Rd
> > > > > Now suppose the Administrator of AS20 wants all
> > > > > outgoing traffic to take Link 2, so he set the local
> > > > > preferance on Rb for incoming routes (1.1.1.1) from
> > > > > Ra
> > > > > as 200 and on Rc as 100(default). Rb,Rc and Rd are
> > > > > in
> > > > > full meshed. It means to reach 1.1.1.1 router Rd
> > > > > will
> > > > > take the path of Link 2. Now the administrator of
> > > > > AS10
> > > > > wants to influence the incoming traffic from AS20
> > > > > and
> > > > > force them to use Link 1. So he set the MED 5 for
> > > > > all
> > > > > the outgoing routes via Link 1 and the MED 10 for
> > > > > all
> > > > > the outgoing routes via Link 2.
> > > > >
> > > > > I know in BGP decision process local preference
> > > > > comes
> > > > > first then MED and in general BGP policies establish
> > > > > after mutual consultation of both the Administrator.
> > > > >
> > > > >
> > > > > But in this particular case, when both the
> > > > > administrator are in complete disagreement, which
> > > > > link
> > > > > will be used for traffic going from AS20 to AS10.
> > > > >
> > > > > If Link 2 is used then is there any way for
> > > > > Administrator of AS10 to force AS20 to use Link 1.
> > > > >
> > > > > Regards.
> > > > >
> > > > > Asim Khan
> > > > >
> > > > >
> > > > >
> > > > > __________________________________________________
> > > > > Do you Yahoo!?
> > > > > New DSL Internet Access from SBC & Yahoo!
> > > > > http://sbc.yahoo.com
> > > > >
> > > > >
> > > >
> > > > __________________________________________________
> > > > Do you Yahoo!?
> > > > New DSL Internet Access from SBC & Yahoo!
> > > > http://sbc.yahoo.com
>
>
> --
> Carlos G Mendioroz <tron@huapi.ba.ar> LW7 EQI Argentina
This archive was generated by hypermail 2.1.4 : Mon Oct 07 2002 - 07:44:00 GMT-3