From: Brian McGahan (brian@cyscoexpert.com)
Date: Thu Nov 14 2002 - 13:28:15 GMT-3
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.
>
> ______________________________________________________________________
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This archive was generated by hypermail 2.1.4 : Tue Dec 03 2002 - 07:23:00 GMT-3