From: Brian McGahan (brian@xxxxxxxxxxxxxxx)
Date: Wed Aug 21 2002 - 11:16:13 GMT-3
A side note on MED:
http://www.cisco.com/warp/public/459/bgp-med.html
And BTW, distance does not have anything to do with BGP best
path selection. The BGP decision process is used to determine which
path is best. This does not necessarily mean that best routes will make
it to the IP routing table. Distance is part of this second decision,
but not the first.
http://www.cisco.com/warp/public/459/25.shtml
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
Ademola Osindero
Sent: Wednesday, August 21, 2002 7:43 AM
To: Yadav, Arvind K (CAP, GECIS); VANGADASALAM,SURENDRAN
(Non-HP-Singapore,ex4); 'Omer Ansari'
Cc: 'ccielab@groupstudy.com'
Subject: RE: BGP Metric
Arvind,
Omer's explanation is quite right. The issue of admin distance does not
come into play here . Infact on going thru some old posts, I found out
that the topic was touched in detail. The key is SYNCHRONIZATION. I
either
turn off synchronization or ensure the routes are synchronized.
I did ensured the routes were fully synchronized and the rule was
followed.
But this leads me to another question, how do i deal with my DMZ ?
Regards,
Ademola
At 08:18 AM 8/21/2002 -0400, Yadav, Arvind K (CAP, GECIS) wrote:
>I think EBGP routes are always prefered over IBGP because of
>Administrative Distance, By default med metric is set to 0 to all
routers
>and router always compare med by default. The
>bgp always-compare-med will be usefull when router learns router form
two
>different ASs.
>
>Arvind
>
>-----Original Message-----
>From: VANGADASALAM,SURENDRAN (Non-HP-Singapore,ex4)
>[mailto:surendran_vangadasalam@non.hp.com]
>Sent: Wednesday, August 21, 2002 4:42 PM
>To: 'Omer Ansari'; 'Ademola Osindero'
>Cc: 'ccielab@groupstudy.com'
>Subject: RE: BGP Metric
>
>
>Hi,
> I think Omer is somehow right. The bgp always-compare-med
should be
>used for it to choose the lower med value. If this not done then the
>decision process will be skipped to EBGP is better than IBGP.
>
>Cheers!!
>
>-----Original Message-----
>From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
>Omer Ansari
>Sent: 21 August 2002 09:09
>To: Ademola Osindero
>Cc: ccielab@groupstudy.com
>Subject: Re: BGP Metric
>
>
>I should have explained a little more before shooting my mouth off..
>
>quoting the great Mr McGahan himself!
>
>----
> 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.
>----
>
>hope that helps.
>
>On Wed, 21 Aug 2002, Omer Ansari wrote:
>
> > Ademola,
> >
> > looks like an ebgp route; AD = 20 better than IBGP right?
> >
> > On Mon, 19 Aug 2002, Ademola Osindero wrote:
> >
> > > Group,
> > >
> > > I thought MED is meant to take preference over internal or
external
>routes
> > > but i can't see it happen in the case below. R6 prefers
172.16.20.2 to
> > > 192.68.6.1 to reach network 172.16.10.0 despite the fact that the
latter
>as
> > > a lower metric of 0.
> > >
> > > Can anyone explain this?
> > >
> > > r6#sh ip bgp
> > > BGP table version is 7, local router ID is 192.168.11.1
> > > 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
> > > *> 172.16.1.0/24 172.16.20.2 100 0 3 i
> > > * i 192.68.6.1 50 100 0 3 i
> > > *> 172.16.10.0/24 172.16.20.2 100 0 3 i
> > > * i 192.68.6.1 0 100 0 3 i
> > >
> > > r6#sh ip bgp 172.16.10.0
> > > BGP routing table entry for 172.16.10.0/24, version 7
> > > Paths: (2 available, best #1, table Default-IP-Routing-Table)
> > > Advertised to non peer-group peers:
> > > 192.68.6.1
> > > 3
> > > 172.16.20.2 from 172.16.20.2 (172.16.220.1)
> > > Origin IGP, metric 100, localpref 100, valid, external,
best
> > > 3
> > > 192.68.6.1 from 192.68.6.1 (192.68.10.2)
> > > Origin IGP, metric 0, localpref 100, valid, internal
> > >
> > >
> > > Osindero Ademola
> > > Schlumberger Network Solutions
> > > Tel: 234 1 261 0446 Ext 5427
> > > Fax 234 1 262 1034
> > > email:osindero@lagos.sns.slb.com
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