From: Carlos (cchorao@xtra.co.nz)
Date: Sat Jan 04 2003 - 02:22:45 GMT-3
Elmer ,
I agree with you. Entry 2 is choosen becuase it is the older. The age test
precedes the router-id test unless the "bgp bestpath compare-routerid"
command is issued.
Carlos
Telecom New Zealand - Advanced Solutions Group : Network Design and Security
----- Original Message -----
From: "cebuano" <cebu2ccie@cox.net>
To: <ccielab@groupstudy.com>
Sent: Friday, January 03, 2003 6:46 PM
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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