From: Dan Pontrelli (dp595@xxxxxxxxxxxxx)
Date: Sun Jul 29 2001 - 19:54:32 GMT-3
That link explains the difference between "bgp deterministic med" and "bgp
always-compare-med" and doesn't mention "bgp bestpath med-confed."
I read about "bgp bestpath med-confed" on the doc cd, but it does not paint
a clear picture for me, so if someone can enlighten me as to whether or not
I understand it correctly that would be great.
I believe "bgp bestpath med-confed" works as follows:
First it is important to understand how "bgp deterministic med" and "bgp
always-compare-med" work:
Let's take the following scenario:
R1 learns the same exact route from two different external peers. All
decision making factors that are a higher priority than med comparison are
also equal. Each external peer belongs to a different AS.
----------------------------------------------------------------------------
----------------------------------------------------------------------------
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Default BGP action (bgp deterministic med):
R1 in its decision process will not compare the meds from two different
Autonomous Systems. This is the default BGP behavior; in other words you
don't have to enter "bgp deterministic med" unless it has been disabled.
----------------------------------------------------------------------------
----------------------------------------------------------------------------
-------
"bgp always-compare-med" (disabled by default):
R1 in its decision process 'will' compare the meds and choose the route with
the lowest med.
----------------------------------------------------------------------------
----------------------------------------------------------------------------
-------
bgp bestpath medconfed (disabled by default):
R1 will not compare the meds if the external peers are in different
Autonomous Systems but it 'will' compare the meds if the external peers are
in different Sub-Autonomous Systems (I 'believe' it is also required that
they are confederation peers, in other words they can't be Sub-ASs of two
different confederations, but it would be nice to get some reassurance on
this).
Dan Pontrelli
Customer Support Engineer - Verio/NTT
CCNP, MCSE, CNA
> Hi Steven,
> The doco on this command is less than good <grin>.
> According to the BGP Command and Configuration Handbook (p86) it basically
> forces the router to make a best path decision based on neighbor AS and
MED.
> This ensures that the same 'best path' is chosen every time. From what the
> book says, and I'm not sure of this myself, is that without this command
the
> order of paths can make a difference in path choice. Hmmmm not really
happy
> with this.
> Check out http://www.cisco.com/warp/customer/459/bgp-med.html. This
explains
> it really wll.
>
> bgp bestpath med-confed only compares MEDs for routes with an AS
> confederation sequence in their AS paths.
>
> HTH,
> Lachlan
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
> Steven Weber
> Sent: Tuesday, 7 August 2001 8:38 AM
> To: Groupstudy
> Subject: different types of MED.....
>
>
> Can someone please explain the difference between bgp deterministic
med
> and bgp bestpath med-confed.
>
> Thanks
> Steve
> **Please read:http://www.groupstudy.com/list/posting.html
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