From: Peng Zheng (zpnist@xxxxxxxxx)
Date: Thu Aug 29 2002 - 13:43:18 GMT-3
I think enable deterministic med make paths from same
AS sub-sorted.
Correct me if I'm wrong.
--- Jon O'Nan <jononan@columbus.rr.com> wrote:
> Can you explain why cisco says to always enable
> deterministic med?
>
> Thanks
> Jon
>
> ----- Original Message -----
> From: "Omer Ansari" <omer@ansari.com>
> To: "Peng Zheng" <zpnist@yahoo.com>
> Cc: "Jon O'Nan" <jononan@columbus.rr.com>; "Volkov,
> Dmitry (Toronto - BCE)"
> <dmitry_volkov@ca.ml.com>; "Nick Shah"
> <nshah@connect.com.au>;
> <ccielab@groupstudy.com>
> Sent: Thursday, August 29, 2002 7:32 AM
> Subject: Re: deterministic med
>
>
> > I have to agree with Peng.
> >
> > if MED is the best selection criteria for the BGP
> path selection, yes the
> > smallest med should win with bgp always compare
> med.
> >
> > even if it is not the best criteria (say e.g. you
> have a better local
> > pref), then the ones before it should win,
> disqualifying med from
> > the selection criteria.
> >
> > thoughts?
> >
> >
> > On Wed, 28 Aug 2002, Peng Zheng wrote:
> >
> > > But still the smallest MED is chosen in both
> case.
> > > What's the difference?
> > >
> > >
> > > --- Jon O'Nan <jononan@columbus.rr.com> wrote:
> > > > It is true you will always compare med with
> just the
> > > > command bgp
> > > > always-compare-med, but it appears without the
> > > > deterministic med you may not
> > > > take the optimal route due to the fact it will
> > > > compare meds from newest to
> > > > oldest instead of grouping same AS's and
> comparing
> > > > them first.
> > > >
> > > >
> > > >
> > > > ----- Original Message -----
> > > > From: "Peng Zheng" <zpnist@yahoo.com>
> > > > To: "Volkov, Dmitry (Toronto - BCE)"
> > > > <dmitry_volkov@ca.ml.com>; "Nick Shah"
> > > > <nshah@connect.com.au>
> > > > Cc: <ccielab@groupstudy.com>
> > > > Sent: Thursday, August 29, 2002 12:21 AM
> > > > Subject: RE: deterministic med
> > > >
> > > >
> > > > > But I don't think so.
> > > > >
> > > > > Because when bgp always-compare-med enabled:
> > > > >
> > > > > AS(PATH) 500, med 100, internal, rid
> 172.16.8.4
> > > > >
> > > > > always is chosen as the best path although
> the
> > > > entry
> > > > > change from 3 to 2.
> > > > >
> > > > > That's what I said.
> > > > >
> > > > >
> > > > > --- "Volkov, Dmitry (Toronto - BCE)"
> > > > > <dmitry_volkov@ca.ml.com> wrote:
> > > > > > Peng,
> > > > > >
> > > > > > You are NOT right. Read it
> > > > > >
> > > >
> http://www.cisco.com/warp/customer/459/bgp-med.html
> > > > > > carefully:
> > > > > >
> > > > > > Example 2: bgp deterministic-med Disabled,
> bgp
> > > > > > always-compare-med Enabled
> > > > > > ENTRY1 IS COMPARED TO ENTRY2. These
> entries are
> > > > from
> > > > > > DIFFERENT neighbor ASs,
> > > > > > but since the bgp always-compare-med
> command is
> > > > > > enabled, MED is used in the
> > > > > > comparison. Entry1 is best of these two
> entries
> > > > > > because it has a lower MED.
> > > > > > Next, entry1 is compared to entry3. The
> MED is
> > > > > > checked again because the
> > > > > > entries are now from the same AS. Entry3
> is
> > > > chosen
> > > > > > as the best path.
> > > > > >
> > > > > > Example 3: bgp deterministic-med Enabled,
> bgp
> > > > > > always-compare-med Disabled
> > > > > > When the bgp deterministic-med command is
> > > > enabled,
> > > > > > routes from the
> > > > > > SAME AS are GROUPED TOGETHER !!!
> > > > > > There is a group for AS 100 and a group
> for AS
> > > > 500.
> > > > > > The best entries for
> > > > > > each group are compared
> > > > > >
> > > > > > Router compares FROM THE SAME AS FIRST (it
> > > > reorders
> > > > > > routes) !!!
> > > > > >
> > > > > > Example 4: Both Commands Enabled
> > > > > > The comparisons in this example would be
> the
> > > > same as
> > > > > > in Example 3, except
> > > > > > for the last comparison between entry2 and
> > > > entry1.
> > > > > > The MED is taken into
> > > > > > account for the last comparison because
> the bgp
> > > > > > always-compare-med command
> > > > > > is enabled
> > > > > >
> > > > > > Dmitry
> > > > > >
> > > > > >
> > > > > > > -----Original Message-----
> > > > > > > From: Peng Zheng
> [mailto:zpnist@yahoo.com]
> > > > > > > Sent: Wednesday, August 28, 2002 11:23
> PM
> > > > > > > To: Nick Shah
> > > > > > > Cc: ccielab@groupstudy.com
> > > > > > > Subject: Re: deterministic med
> > > > > > >
> > > > > > >
> > > > > > > But I think no matter how many ASs and
> paths,
> > > > the
> > > > > > > smallest MED will be chosen if bgp
> > > > > > always-compare-med
> > > > > > > enabled. Is it right?
> > > > > > >
> > > > > > >
> > > > > > >
> > > > > > >
> > > > > > >
> > > > > > > --- Nick Shah <nshah@connect.com.au>
> wrote:
> > > > > > > > Yes, only if using 2 paths, try it
> with 3 or
> > > > 4
> > > > > > > > paths, 2 recd. from ASX and 2
> > > > > > > > recd. from ASY.
> > > > > > > >
> > > > > > > > Nick
> > > > > > > > ----- Original Message -----
> > > > > > > > From: "Peng Zheng" <zpnist@yahoo.com>
> > > > > > > > To: "Nick Shah" <nshah@connect.com.au>
> > > > > > > > Cc: <ccielab@groupstudy.com>
> > > > > > > > Sent: Thursday, August 29, 2002 11:51
> AM
> > > > > > > > Subject: Re: deterministic med
> > > > > > > >
> > > > > > > >
> > > > > > > > > I know that. But it's same as I
> said.
> > > > Enable
> > > > > > bgp
> > > > > > > > > always-compare-med has the same
> result as
> > > > > > enable
> > > > > > > > both.
> > > > > > > > >
> > > > > > > > >
> > > > > > > > > --- Nick Shah <nshah@connect.com.au>
> > > > wrote:
> > > > > > > > > > Not sure what you mean here, but..
> > > > Consider
> > > > > > > > this..
> > > > > > > > > >
> > > > > > > > > > 192.168.1.0/24 recd. from AS500
> MED =
>
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