From: enginedrive2002 (enginedrive2002@yahoo.ca)
Date: Thu Nov 14 2002 - 09:32:35 GMT-3
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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