From: enginedrive2002 (enginedrive2002@yahoo.ca)
Date: Thu Nov 14 2002 - 19:31:57 GMT-3
Thank you very much, Brian!
These answer my question.
E.D.
----- Original Message -----
From: "Brian McGahan" <brian@cyscoexpert.com>
To: "'enginedrive2002'" <enginedrive2002@yahoo.ca>; <ccielab@groupstudy.com>
Sent: November 14, 2002 11:28 AM
Subject: RE: BGP route selection criteria.
> E.D.
>
> This doesn't have anything to do with what prefix shows up in
> the routing table. BGP best path selection, as the name implies, is
> merely used to select the best path in the BGP table. A BGP router only
> advertises its best path. Therefore best path selection will determine
> which prefixes in your BGP table get propagated to other BGP neighbors.
>
> HTH
>
> Brian McGahan, CCIE #8593
> Director of Design and Implementation
> brian@cyscoexpert.com
>
> CyscoExpert Corporation
> Internetwork Consulting & Training
> Voice: 847.674.3392
> Fax: 847.674.2625
>
>
> > -----Original Message-----
> > From: enginedrive2002 [mailto:enginedrive2002@yahoo.ca]
> > Sent: Thursday, November 14, 2002 6:33 AM
> > To: Brian McGahan; ccielab@groupstudy.com
> > Subject: Re: BGP route selection criteria.
> >
> > Hi Brian,
> >
> >
> > I haven't test this scenario in the lab yet. But I think even this BGP
> > route
> > (3.3.3.3/32) win in the BGP path selection, it will not show up in the
> > routing table, due to IGP has lower AD.
> >
> > So, what's the point we have this "locally-originated" criteria?
> >
> > Regards,
> >
> >
> > E.D.
> >
> >
> > ----- Original Message -----
> > From: "Brian McGahan" <brian@cyscoexpert.com>
> > To: "'enginedrive2002'" <enginedrive2002@yahoo.ca>;
> > <ccielab@groupstudy.com>
> > Sent: November 13, 2002 5:10 PM
> > Subject: RE: BGP route selection criteria.
> >
> >
> > > E.D.
> > >
> > > "Locally originated" in BGP means that the local router is
> > > originating the prefix in question into the BGP domain via the
> "network"
> > > statement under BGP, "aggregate-address" statement under BGP, or the
> > > "redistribute" statement under BGP. Suppose we have the following
> > > scenario:
> > >
> > > ISP_A ISP_B
> > > | |
> > > R1-------------R2
> > > __|______________|__
> > > |
> > > R3
> > >
> > > R1, R2, & R3 are running some IGP together.
> > > R1, R2, ISP_A, & ISP_B are running BGP.
> > > R1 peers EBGP with ISP_A, and iBGP with R2.
> > > R2 peers EBGP with ISP_B, and iBGP with R1.
> > > R3 advertises a prefix 3.3.3.3/32 into IGP.
> > >
> > > R1 and R2 both use the "network 3.3.3.3 mask 255.255.255.255"
> statement
> > > under their BGP processes. Assume that the synchronization rule has
> > > been met, next-hop reachability is maintained, and all attributes
> are
> > > left to default.
> > >
> > > The BGP decision process is as follows:
> > >
> > > Weight
> > > Local-preference
> > > Locally-originated
> > > AS-PATH
> > > Origin
> > > MED
> > > EBGP over iBGP
> > > Shortest Internal Path (IGP metric to next-hop)
> > > Router-ID (lowest)
> > >
> > > The decision that will be the tie breaker in this case is
> > > locally-originated. R1 will select it's own locally-originated
> prefix
> > > as best, as will R2. When you "show ip bgp 3.3.3.3" you will see
> > > something like this:
> > >
> > > BGP routing table entry for 3.3.3.3/32, version 3
> > > Paths: (2 available, best #2, table Default-IP-Routing-Table)
> > > Advertised to non peer-group peers:
> > > 150.5.24.2 150.5.111.5
> > > Local
> > > 150.5.12.1 (metric 1) from 150.5.24.2 (150.5.2.2)
> > > Origin IGP, metric 1, localpref 1, weight 32768, valid,
> internal,
> > > synchronized
> > > Local
> > > 150.5.14.1 from 0.0.0.0 (150.5.4.4)
> > > Origin IGP, metric 1, localpref 1, weight 32768, valid,
> sourced,
> > > local, best
> > >
> > > "from 0.0.0.0" means locally originated, hence it is the best path.
> > >
> > > For more info on the BGP best path selection process:
> > >
> > >
> http://www.cisco.com/en/US/tech/tk648/tk365/technologies_tech_note09186a
> > > 0080094431.shtml
> > >
> > >
> > > HTH
> > >
> > > Brian McGahan, CCIE #8593
> > > Director of Design and Implementation
> > > brian@cyscoexpert.com
> > >
> > > CyscoExpert Corporation
> > > Internetwork Consulting & Training
> > > Voice: 847.674.3392
> > > Fax: 847.674.2625
> > >
> > >
> > > > -----Original Message-----
> > > > From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On
> Behalf
> > > Of
> > > > enginedrive2002
> > > > Sent: Wednesday, November 13, 2002 2:45 PM
> > > > To: ccielab@groupstudy.com
> > > > Subject: BGP route selection criteria.
> > > >
> > > > The BGP route selection criteria after the tie at "weight" and
> "local
> > > > preference" is "Prefer the route that was locally originated".
> > > >
> > > > Could anyone give me a scenario that the BGP route selection is
> break
> > > base
> > > > on
> > > > this?
> > > >
> > > > Thank you!
> > > >
> > > >
> > > > E.D.
> >
> > ______________________________________________________________________
> > Post your free ad now! http://personals.yahoo.ca
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