From: Brian McGahan (brian@xxxxxxxxxxxxxxx)
Date: Mon Jul 29 2002 - 18:24:12 GMT-3
Ted,
As Chris stated, synchronization is the first requirement in the
BGP decision process. After that, the process is as follows:
Next-hop reachability
Weight
Local Preference
AS-Path (shortest)
Origin Code (EGP > IGP > Unknown)
MED
EBGP over iBGP routes
Shortest internal path
Router-ID (lowest)
However, there is another criterion that is worth mentioning.
Default local-preference for iBGP learned routes on a Cisco router is
100. Although local preference is higher in the decision process than
EBGP over IBGP, this is not the default case. You must have a
local-preference greater than the default (100) to choose the iBGP route
over the EBGP route. Therefore if everything (except for the default
local-pref of the iBGP route) is equal for two routes up to the EBGP
over iBGP decision, the EBGP route will be chosen. Even though the iBGP
route has a local-pref of 100, it chooses the EBGP route. Setting the
iBGP route to have a local-pref of at least 101 will make it chose the
iBGP route first.
I have not seen this documented, and it may be version specific,
so someone please correct me if I'm wrong.
HTH
Brian McGahan, CCIE #8593
Director of Design and Implementation
brian@cyscoexpert.com
CyscoExpert Corporation
Internetwork Consulting & Training
http://www.cyscoexpert.com
Voice: 847.674.3392
Fax: 847.674.2625
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Chris Hugo
Sent: Monday, July 29, 2002 3:53 PM
To: Chris Hugo; Ted McDermott
Cc: ccielab@groupstudy.com
Subject: Re: BGP Route Preference Decision
BTW,
The word assumption was not to be meant as a put-down. I was trying to
fit a reply post in my schedule and I was doing some other tasks and not
thinking right.
sincerely stated,
chris hugo
Chris Hugo <chrishugo@yahoo.com> wrote: Hi Ted,
Before you make that assumption fix the sync issue :). Also take a look
at Halabi's BGP (pg.152-159) bible which states their are three RIBs
(RIB-IN, LOC-RIB, and RIB-OUT). BGP's input policy engine runs thru the
Decision Process before the routes even touch the Loc-RIB (IP Routing
Table).
HTH and have fun with BGP,
chris hugo
Ted McDermott wrote: I'm listing a BGP and IP route table below for
network
128.213.0.0. It appears that before the BGP
decision-making process is entered, (i.e. next-hop
reachable, then weight, then local-preference, etc.)
the first criteria in the decision-making process is
whether the route is learned via EBGP or IBGP, because
of the administrative distance difference. Thus, if
you have one route learned via EBGP and the other via
IBGP, it will always take the EBGP, regardless of the
other BGP parameters. Is that correct?
Note the equal weight, local-preference, and the
longer as-path of the preferred route.
rtb#sho ip route 128.213.0.0
Routing entry for 128.213.0.0/16
Known via "bgp 100", distance 20, metric 0
Tag 300, type external
Last update from 192.208.10.5 13:55:00 ago
Routing Descriptor Blocks:
* 192.208.10.5, from 192.208.10.5, 13:55:00 ago
Route metric is 0, traffic share count is 1
AS Hops 4
rtb>sho ip bgp 128.213.0.0
BGP routing table entry for 128.213.0.0/16, version 8
Paths: (2 available, best #1)
Advertised to non peer-group peers:
203.250.13.41
300 500 400 200
192.208.10.5 from 192.208.10.5 (192.208.10.174)
Origin IGP, localpref 100, valid, external,
best, ref 2
200
128.213.63.2 from 203.250.13.41 (203.250.13.41)
Origin IGP, metric 0, localpref 100, valid,
internal, not synchronized, ref 2
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