From: cebuano (cebu2ccie@cox.net)
Date: Sun Jan 05 2003 - 03:54:56 GMT-3
Hi Gang.
I am enclosing the response I got from the feedback I made to the URL on
CCO. I hope this helps those reviewing BGP now. This is what I
received...
Hello Elmer,
Thanks for your feedback. In this document the paths are ordered from
newest to oldest is to explain how deterministic med influences the path
selection. The way BGP path selection is implemented is way more
complicated that the simplified way it is explained the path selection
doc. The reason the oldest path selection ( point 10 in path selection
doc) was introduced was to reduce the flapping and that a new route
should not displace the old stable route and create instabilities in the
network because of route flapping. It also depends on number of other
reasons like if you already have a bgp best path in the bgp table and
that if you have just two paths to select from or more than two.
Hope this helps
Regards
Vivek Baveja
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Carlos
Sent: Saturday, January 04, 2003 12:23 AM
To: cebuano; ccielab@groupstudy.com
Subject: Re: Clarification needed on BGP and MEDs
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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