From: Mingzhou Nie (mnie@xxxxxxxxx)
Date: Wed Aug 28 2002 - 22:40:04 GMT-3
Nick,
Don't believe that's true. MED is always compared whether "bgp
deterministic med " enabled to not. the command simply changes the
comparison behavior a little bit by grouping routes from same neiborgh
and compare.
In the given example, if you change the order as below.
entry1: AS(PATH) 500, med 150, external, rid 172.16.13.1
entry2: AS(PATH) 500, med 100, internal, rid 172.16.8.4
entry3: AS(PATH) 100, med 200, external, rid 1.1.1.1
even w/o the command, entry2 will be chosen over entry1.
ming
--- Nick Shah <nshah@connect.com.au> wrote:
> > Can we say that by default (bgp deterministic-med is not enabled)
> > Metric(MED) is not compared between ANY (doesn't matter from the
> same AS
> or
> > not) routes with the same path lenghts ?
>
> Think it this way, whats the BGP path decision process, Weight,
> localpref,
> origin code, ASPATH, MED (stop here). Now unless bgp
> deterministic-med is
> enabled, MED comparison is skipped.
>
> So if all else be the same till here (till MED) and you really want
> MED to
> influence path, then turn on 'BGP DETERMINISTIC MED'
>
> > I.e. MED - is not taken into consideration in the
> > BGP best path selection alghorithm ? (by default) So forget about
> MED
> > completely until enable "bgp-deterministic-med" or "bgp
> > always-compare-med" ?
>
> Correct
>
> > Also this sentence:
> > (MED is used in this comparison by default because the winners of
> each
> group
> > are now from different ASs - enabling bgp always-compare-med
> changes this
> > default behavior).
> > from http://www.cisco.com/warp/public/459/37.html
> >
> > Is it mistake ?? Should I read (MED is NOT used in this
> comparision....)
> > because it doesn't compare MEDs from different ASs until "bgp
> > always-compare-med" is enabled ?
>
> That looks like a'convoluted statement' and a typo.
>
> The correct document is : (meaning, one which confirms the actual
> behaviour
> is )
> http://www.cisco.com/warp/public/459/bgp-med.html
>
> //snip//Enabling the bgp deterministic-med command ensures the
> comparison of
> the MED variable when choosing routes advertised by different peers
> in the
> same autonomous system (AS). Enabling the bgp always-compare-med
> command
> ensures the comparison of the MED for paths from neighbors in
> different
> autonomous systems. The bgp always-compare-med command is useful when
> multiple service providers or enterprises agree on a uniform policy
> for
> setting MED. Thus, for network X, if Internet Service Provider (ISP)
> A sets
> the MED to 10, and ISP B sets the MED to 20, then both ISPs agree
> that ISP A
> has the better performing path to X. //snip//
>
> rgds
>
> Nick
>
> >
> > > -----Original Message-----
> > > From: Nick Shah [mailto:nshah@connect.com.au]
> > > Sent: Wednesday, August 28, 2002 7:36 PM
> > > To: Jon O'Nan; ccielab@groupstudy.com
> > > Subject: Re: deterministic med
> > >
> > >
> > > 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,
> > > however it only compares MED, if prefix is received from same
> > > neighbor and
> > > the best man wins
> > > * Turning on bgp always-compare-med, starts comparing 'same
> > > prefix' recd.
> > > from 'different neighbors' and the best man wins
> > > * Turning on both of them may really not be required, but it
> basically
> > > compares if prefixes are recd. in groups of AS's (say same
> > > prefix from AS
> > > 200, via 2 neighbors, and same prefix from AS 100 via 1
> neighbor).
> > >
> > > rgds
> > > Nick
> > > ----- Original Message -----
> > > From: "Jon O'Nan" <jononan@columbus.rr.com>
> > > To: <ccielab@groupstudy.com>
> > > Sent: Thursday, August 29, 2002 8:58 AM
> > > Subject: deterministic med
> > >
> > >
> > > > Am I correct that:
> > > >
> > > > bgp deterministic med - compares med within AS
> > > >
> > > > VS.
> > > >
> > > > bgp always-compare-med - compares med from another AS
> > > >
> > > >
> > > > Thanks
> > > > Jon
> > > >
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