From: cebuano (cebu2ccie@cox.net)
Date: Fri Jan 03 2003 - 12:04:27 GMT-3
Well, if you haven't noticed yet, the Parkhurst book you are referring
to has MANY, MANY technical info NOT included, specially since it was
written pre-12.2 and does not present the topic from an ISSUES
perspective but simply to test the different commands. Don't forget to
also check its errata on Ciscopress because I've exchanged e-mails with
the author to confirm the errors I found both in this book and the OSPF
command reference book as well.
Regards.
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Jennifer Bellucci
Sent: Friday, January 03, 2003 1:38 AM
To: cebuano; ccielab@groupstudy.com
Subject: Re: Clarification needed on BGP and MEDs
get hold of BGP config and command handbook from ciscopress. Did wonders
for
me, after that flick through halabi and JD 2, those books are good for
the
concepts and stuff but the book above gives you what I wanted, to find
out
what all the commands actually do, plus reading the command guide can
sometimes damage your mental stability.
----- Original Message -----
From: "cebuano" <cebu2ccie@cox.net>
To: <ccielab@groupstudy.com>
Sent: Friday, January 03, 2003 5:46 AM
Subject: Clarification needed on BGP and MEDs
> Hi group.
> I need to clarify my reading of this CCO page.
>
> The following examples demonstrate how the bgp deterministic med and
bgp
> always-compare-med commands can influence MED-based path selection.
> Note: Cisco Systems recommends enabling the bgp deterministic med
> command in all new network rollouts. For existing networks, the
command
> must either be deployed on all routers at the same time, or
> incrementally, with care to avoid possible internal BGP (iBGP) routing
> loops.
> For example, consider the following routes for network 10.0.0.0/8:
> entry1: AS(PATH) 500, med 150, external, rid 172.16.13.1
> entry2: AS(PATH) 100, med 200, external, rid 1.1.1.1
> entry3: AS(PATH) 500, med 100, internal, rid 172.16.8.4
> The order in which the BGP routes were received is entry3, entry2, and
> entry1 (entry3 is the oldest entry in the BGP table and entry1 is the
> newest one).
> Note: When BGP receives multiple routes to a particular destination,
it
> lists them in the reverse order they were received, from the newest to
> the oldest. BGP then compares the routes in pairs starting with the
> newest entry and moving toward the oldest entry (starting at top of
the
> list and moving down). For example, entry1 and entry2 are compared.
The
> best of these two is then compared to entry3, and so on.
>
> Example 1: Both Commands Disabled
>
> Entry1 and entry2 are compared first. Entry2 is chosen as the best of
> these two because it has a lower router ID. The MED is not checked
since
> the paths are from a different neighbor AS.
> Shouldn't example 1 instead say, "Entry2 is chosen as the best of
these
> two because it has an OLDER received path"?
> Step 10 of the BGP Bestpath Selection states.
> "When both paths are EXTERNAL, prefer the OLDEST path."
> Isn't Entry2 "older" than Entry1?
> Or is something wrong with my understanding of this English?
>
> TIA.
> Elmer
>
> BTW - do you guys/gals have a recommendation on very good lab
scenarios
> to bring the true IOS behavior of these different BGP "knobs" to life?
> There are so many changes to the 12.2 release that it makes me not
want
> to dwell too much on Halabi's book. I wish Doyle had THREE chapters on
> this instead of two (pity he spent three chapters on Multicast
instead).
> .
.
.
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