From: Ademola Osindero (osindero@xxxxxxxxxxxxxxxxx)
Date: Wed Aug 21 2002 - 09:43:03 GMT-3
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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