From: Khalid Siddiq (khalid@sys.net.pk)
Date: Mon Sep 23 2002 - 07:31:49 GMT-3
steve,
I agree with u, but by advertising the routes on link2 only and filtering it on link 1 can effect the policy of As20,
As20 has set the high local preference for this route on link2 and if their is no routes recieved on that link, the traffic in AS20 can take the path to link2.
As u say that,to influence outgoing traffic you need to affect the incoming routes, in AS10 by route filtering and advertising only the aggregate route on link 1 and both aggregate, more specific route on link 2 will definately effect the outgoing policy in AS20.
clarify me if i am wrong.
khalid
-----Original Message-----
From: steven.j.nelson@bt.com [mailto:steven.j.nelson@bt.com]
Sent: Monday, September 23, 2002 1:31 PM
To: Khalid Siddiq; ccielab@groupstudy.com
Subject: RE: Basic BGP decision process
Khalid
Brian is right, to influence outgoing traffic you need to affect the
incoming routes and to influence incoming traffic you need to affect the
outgoing routes, but in this case AS20 will have the ultimate say in how
traffic is routed inbound.
Thanks
Steve
CCIE#10055
-----Original Message-----
From: Khalid Siddiq [mailto:khalid@sys.net.pk]
Sent: 23 September 2002 10:07
To: ccielab
Subject: FW: Basic BGP decision process
Brian,
I dont think their is no way for Administrator of As10 to Force As 20 to use
link 1.
If redundency is not required for the loopback address 1.1.1.1/32, the
administrator of As10 can filter that route on link 2 and advertise that
only to link 1, this effect the policy of As 20.
i have another option if redundency is required,the administrator of As10
can aggregate this loopback address, and send only the aggrgate route to
link 2 and its more specific and aggregate route on link 1.
please clarify if i am wrong.
regards,
khalid
-----Original Message-----
From: Brian McGahan [mailto:brian@cyscoexpert.com]
Sent: Monday, September 23, 2002 1:14 AM
To: 'Joe Higgins'; 'Asim Khan'; ccielab@groupstudy.com
Subject: RE: Basic BGP decision process
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
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