From: VANGADASALAM,SURENDRAN (Non-HP-Singapore,ex4) (surendran_vangadasalam@xxxxxxxxxx)
Date: Wed Aug 21 2002 - 08:11:34 GMT-3
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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