From: Mingzhou Nie (mnie@xxxxxxxxx)
Date: Thu Aug 22 2002 - 00:07:43 GMT-3
guys,
if next hop is inaccessible, or route is not synced, it will be
specifically shown as follows.
r132#show ip bgp 150.10.1.0
BGP routing table entry for 150.10.1.0/24, version 2
Paths: (2 available, best #2, table Default-IP-Routing-Table)
Advertised to non peer-group peers:
172.16.5.1
10
150.10.13.1 (inaccessible) from 172.16.5.1 (172.16.5.1)
Origin IGP, metric 0, localpref 100, valid, internal, not
synchronized
In the original question, neither next hop is inaccessible nor route is
not synced, so the normal patch selection steps should follow. That's
why I don't understand why the route with higher metric is chosen give
all criteria before him are same.
Ming
--- Brian McGahan <brian@cyscoexpert.com> wrote:
> Omer,
>
> Actually, origin code is another completely unrelated attribute
> in this case. You're confusing the origin code with the status code
> of
> internal or external. Take the following BGP table output:
>
> router#sh ip bgp
> BGP table version is 1019, local router ID is 192.168.0.1
> 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
> *> 24.48.0.0/14 192.168.0.10 0 3549
> 7843 i
> *>i24.48.80.0/21 192.168.0.2 0 3549
> 7843 i
>
> Both of these prefixes have been originated via IGP (the
> "network" command in BGP), as the ORIGIN code on the right indicates.
> The first prefix is learned via an EBGP neighbor, and the second is
> learned via an iBGP neighbor. This is indicated by the status code on
> the left. The absence of an "i" in the status code implies that the
> prefix is learned via an EBGP neighbor.
>
> The BGP selection after MED is to prefer EBGP learned routes
> over iBGP learned routes. However, this selection still has nothing
> to
> do with administrative distance. Let's suppose that you change the
> distance of iBGP to 1, and EBGP to 254. Assuming that everything is
> equal up to this decision, BGP best path selection will still chose
> the
> EBGP learned route over the iBGP learned route.
>
> Now lets suppose that you have an iBGP learned prefix, and you
> also have an IGP route to this prefix. Assuming that this iBGP
> learned
> prefix is best in the BGP table, distance would be the limiting
> factor
> that would prevent it from making it to the IP routing table.
>
> This is another reason that redistribution of BGP into IGP is
> dangerous. Not only will your IGP not be able to handle the
> redistribution of a full BGP view, any iBGP learned routes will
> automatically be routing through IGP now, since the distances of all
> IGPs are lower than that of iBGP. IOS will try to prevent this by
> default by only letting EBGP learned prefixes be redistributed into
> IGP,
> however you can change this behavior with the "BGP
> redistribute-internal" command under the BGP process.
>
> My point being, don't confuse administrative distance with the
> BGP best-path selection process. They are completely unrelated.
>
>
> 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: Omer Ansari [mailto:omer@ansari.com]
> Sent: Wednesday, August 21, 2002 2:42 PM
> To: Brian McGahan
> Cc: 'Ademola Osindero'; 'Yadav, Arvind K (CAP, GECIS)';
> 'VANGADASALAM,SURENDRAN (Non-HP-Singapore,ex4)'; 'Omer Ansari';
> ccielab@groupstudy.com
> Subject: RE: BGP Metric
>
> 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
>
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