From: Nick Shah (nshah@xxxxxxxxxxxxxx)
Date: Wed Aug 28 2002 - 22:18:57 GMT-3
> 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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