From: Howard C. Berkowitz (hcb@xxxxxxxxxxxx)
Date: Wed Jun 05 2002 - 08:53:31 GMT-3
At 8:19 AM +0200 6/5/02, GOLBERY Irhne wrote:
>Thanks for your answer and help Nick,Louis and Peter
>
>Workaround is route-map --> OK
>The med is the IGP metric in this Case --> OK
>
>The bahavior is now clear for me but however I am so surprised not to find
>any documentation about this way of working.
>
>
>Thanks again.
>
>Irene.
Using IGP metric as the MED source can be acceptable in small, flat
BGP networks, and I doubt it would cause a problem in the CCIE lab.
Substantial care is needed if you do this in more hierarchical iBGP
systems, especially involving hierarchical route reflectors, but also
with single-level route reflectors and confederations.
The basic rule is that you want to create an equivalent to the OSPF
rule always preferring intra-area routes. IGP metrics should be
selected that an intra-cluster, or hierarchical cluster-to-cluster,
or confederation route is always preferred over a non-hierarchical
one. You won't necessarily get this from automatic metric injection.
See
http://www.ietf.org/internet-drafts/draft-ietf-idr-route-oscillation-01.txt,
which is coauthored by a Cisco employee, Alvaro Retana. Alvaro is a
coauthor on my BMWG drafts and I know to be very good.
There have also been NANOG discussions on persistent route
oscillation, as well as on assorted routing design lists; it isn't
completely clear how widespread is the problem. Not all ISPs have
been collecting statistics that would identify it.
>
>-----Message d'origine-----
>De : Nick Shah [mailto:nshah@connect.com.au]
>Envoyi : mercredi 5 juin 2002 00:05
>@ : GOLBERY Irhne; ccielab@groupstudy.com
>Objet : Re: BGP and Metric
>
>
>BGP Med is generally derived from 2 sources, IGP metric & explicitly
>specifying via route-map. If the IGP metric is unacceptable, apply a
>route-map to the neighbor, and change the metric to 0 or anything else to
>your liking.
>
>The Med is converted/used as metric when the route is put into routing
>table, this can be tweaked via table-map command .
>
>So in general
>
>IGP (Metric ) ---->(tweak with route-map)----> BGP med ----> (tweak with
>table-map)----> Routing table.
>
>If none of the tweaks are applied, the IGP metric is carried right into the
>routing table.
>
>rgds
>Nick
>-----Original Message-----
>From: GOLBERY Irhne <irene.golbery@arche.fr>
>To: ccielab@groupstudy.com <ccielab@groupstudy.com>
>Date: Tuesday, 4 June 2002 11:08
>Subject: BGP and Metric
>
>
>>Hi all,
>>
>>I am trying to understand the Metric parameters in the "Show ip bgp"
>command
>>- see below.
>>
>>I am "not using MED"(no MED config. nowhere) and however for the same
>>network 195.110.250.40 we can see metric 71 and 70.
>>
>>Can you help me ?
>>
>>Thanks
>>
>>Irene.
>>
>>
>>
>>ROUTER#sh ip b
>>BGP table version is 760, local router ID is 3.3.3.8
>>Status codes: s suppressed, d damped, h history, * valid, > best, i -
>>internal
>>Origin codes: i - IGP, e - EGP, ? - incomplete
>>
>> Network Next Hop Metric LocPrf Weight Path
>>
>>* i195.110.250.40/29 3.3.3.9 71 180 0 i
> >*> 0.0.0.0 70 32768 i
-- "What Problem are you trying to solve?" ***send Cisco questions to the list, so all can benefit -- not directly to me*** ******************************************************************************* * Howard C. Berkowitz hcb@gettcomm.com Chief Technology Officer, GettLab/Gett Communications http://www.gettlabs.com Technical Director, CertificationZone.com http://www.certificationzone.com "retired" Certified Cisco Systems Instructor (CID) #93005
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