From: Omer Ansari (omer@xxxxxxxxxx)
Date: Thu Aug 29 2002 - 08:32:58 GMT-3
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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