From: Jon O'Nan (jononan@xxxxxxxxxxxxxxx)
Date: Thu Aug 29 2002 - 13:01:48 GMT-3
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 =
> > > 100
> > > > > > > > > 192.168.1.0/24 recd. from AS400 MED = 50
> > > > > > > > >
> > > > > > > > > if BGP deterministic-med is turned on,
> > > these
> > > > > > > both
> > > > > > > > > paths will NOT be chosen
> > > > > > > > > based on MED, because they are not from
> > > same
> > > > > AS.
> > > > > > > > >
> > > > > > > > > but if BGP always compare MED is turned
> > > on,
> > > > > then
> > > > > > > the
> > > > > > > > > path with 50 MED will
> > > > > > > > > be selected.
> > > > > > > > >
> > > > > > > > > rgds
> > > > > > > > > ----- Original Message -----
> > > > > > > > > From: "Peng Zheng" <zpnist@yahoo.com>
> > > > > > > > > To: "Nick Shah" <nshah@connect.com.au>
> > > > > > > > > Sent: Thursday, August 29, 2002 10:42 AM
> > > > > > > > > Subject: Re: deterministic med
> > > > > > > > >
> > > > > > > > >
> > > > > > > > > > But turn on bgp always-compare-med
> > > have
> > > > > the
> > > > > > > same
> > > > > > > > > best
> > > > > > > > > > path no matter if turn on bgp
> > > > > deterministic
> > > > > > > med.
> > > > > > > > > Is
> > > > > > > > > > that right?
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > > --- Nick Shah <nshah@connect.com.au>
> > > > > wrote:
> > > > > > > > > > > Jon
> > > > > > > > > > >
> > > > > > > > > > > I hope you have already read this
> > > > > paper..
> > > > > > > > > > >
> > > > > > > > >
> > > > > > >
> > > > >
> > > http://www.cisco.com/warp/customer/459/bgp-med.html
> > > > > > > > > > >
> > > > > > > > > > > regardless of that. I will list down
> > > the
> > > > > > > effect
> > > > > > > > > of
> > > > > > > > > > > these 2 commands.
> > > > > > > > > > >
> > > > > > > > > > > * By default (with none of these
> > > > > commands
> > > > > > > > > entered)
> > > > > > > > > > > MED is NOT compared when
> > > > > > > > > > > selecting the best path.
> > > > > > > > > > > * Turning on bgp deterministic-med,
> > > > > turns on
> > > > > > > the
> > > > > > > > > > > feature to compare MED,
> > >
> > === message truncated ===
> >
> >
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