From: Volkov, Dmitry (Toronto - BCE) (dmitry_volkov@xxxxxxxxx)
Date: Thu Aug 29 2002 - 00:52:43 GMT-3
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,
> > > > > > 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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