From: Omer Ansari (omer@xxxxxxxxxx)
Date: Thu Aug 22 2002 - 06:52:49 GMT-3
Donny,
man I need sleep. works killing me...
localpref is 100 by default, and its not used for the best path decision
here.
(it just doesnt show up in the "show ip bgp" for ebgp routes; cosmetic
issue)
MED should be the guy which is used for best path selection in this case.
also i think the reason why the command below was not showing "NOT
SYNCHRONIZED" was because of this bug:
CSCdr90728 BGP: Paths are not marked as not synchronized
Ademola can confirm.
> > > > > > 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
> > > > > >
> > >
and now...
self#conf t
self(config)#int mouth0
self(config-if)#shut
On Thu, 22 Aug 2002, Omer Ansari wrote:
> Donny,
>
> actually in this case, the problem was lack of syncronization for the
> ibgp learnt routes as pointed out by Ademola.
>
> such paths are not considered as valid candidates.
> http://www.cisco.com/warp/public/459/25.shtml
>
> once the issue is fixed, along with the ather dotted pre-requisites, the
> router included these routes also while doing the best path algo, and
> as per the list above, i believe local preference decided the best path.
>
> [localpref is not Xchanged by EBGP peers and thus you don't see it below]
>
> correct me if i'm wrong.
>
> > > > > > 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
> >
>
> On Thu, 22 Aug 2002, Donny MATEO wrote:
>
> >
> > a bit confused here.
> > I thought the reason why the med didn't take precedence is that if the MED
value is 100 for IGP
> > which is the default, it will be ignored so the next tie breaker is whether
the router is learned
> > via EBGP / IBGP ?
> >
> > Best Regards
> > Donny
> >
> >
> >
> > Omer Ansari
> > <omer@ansari.com> To: Brian McGahan <bri
an@cyscoexpert.com>
> > Sent by: cc: "'Ademola Osindero
'" <osindero@lagos.sns.slb.com>, "'Yadav, Arvind K (CAP,
> > nobody@groupstudy GECIS)'" <Arvind.Yadav@geci
s.ge.com>, "'VANGADASALAM,SURENDRAN
> > .com (Non-HP-Singapore,ex4)'" <s
urendran_vangadasalam@non.hp.com>, "'Omer Ansari'"
> > <omer@ansari.com>, <ccielab
@groupstudy.com>
> > Subject: RE: BGP Metric
> > 22-08-2002 03:41
> > Please respond to
> > Omer Ansari
> >
> >
> >
> >
> >
> >
> > yes. and just to clarify my (and possibly other ppls) misunderstanding,
> > step 5 in the selection process is:
> >
> > 5.Prefer the path with the lowest origin type: IGP is lower than EGP,
> > and EGP is lower than INCOMPLETE.
> >
> > and not actually the AD , which is more for which route gets into the
> > route table.
> >
> >
> > On Wed, 21 Aug 2002, Brian McGahan wrote:
> >
> > > 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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