From: Evgeny Tantsura (ivgen@castel.nl)
Date: Fri Jan 03 2003 - 10:05:07 GMT-3
bgp bestpath compare-routerid
To compare similar routes received from external BGP (eBGP) peers during
the best path selection process and switch the best path to the route with
the lowest router ID, use the bgp bestpath compare-routerid command in
router configuration mode. To return the router to the default setting,
use the no form of this command.
bgp bestpath compare-routerid
no bgp bestpath compare-routerid
Syntax Description
This command has no arguments or keywords.
Defaults
Border Gateway Protocol (BGP) does not compare similar paths received from
eBGP peers during the best path selection process and switch the best path
to the route with the lowest router ID.
> Hi, >
> you are right. If imagine there were the same router id's than it would
> choose the oldest on, so entry 2. But in the decision process the rid is
> coming before he looks at the age of the entry.
>
> Hope it helps !
>
> Bye,
> joerg
>
>
> -----Urspr|ngliche Nachricht-----
> Von: nobody@groupstudy.com [mailto:nobody@groupstudy.com] Im Auftrag von
> cebuano
> Gesendet: Freitag, 3. Januar 2003 06:46
> An: ccielab@groupstudy.com
> Betreff: 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).
> .
> .
With kind regards/ met vriendelijke groeten,
------------------------------------------------
E. Tantsura
Network Developer
Essent Kabelcom N.V.
Dr.van Deenweg 84
8025BN Zwolle, The Netherlands
Tel: +31-(0)38-850-7642
Fax: +31-(0)38-850-7410
Mob: +31-(0)6-290-80458
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