From: Omer Ansari (omer@xxxxxxxxxx)
Date: Thu Aug 22 2002 - 04:26:50 GMT-3
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 va
lue is 100 for IGP
> which is the default, it will be ignored so the next tie breaker is whether t
he router is learned
> via EBGP / IBGP ?
>
> Best Regards
> Donny
>
>
>
> Omer Ansari
> <omer@ansari.com> To: Brian McGahan <brian
@cyscoexpert.com>
> Sent by: cc: "'Ademola Osindero'"
<osindero@lagos.sns.slb.com>, "'Yadav, Arvind K (CAP,
> nobody@groupstudy GECIS)'" <Arvind.Yadav@gecis.
ge.com>, "'VANGADASALAM,SURENDRAN
> .com (Non-HP-Singapore,ex4)'" <sur
endran_vangadasalam@non.hp.com>, "'Omer Ansari'"
> <omer@ansari.com>, <ccielab@g
roupstudy.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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