From: Bit Gossip (bit.gossip@chello.nl)
Date: Mon Jun 25 2007 - 17:07:46 ART
Vvvvvvvvvvery interesting: I was not aware of that. Do you have any
reference to the doc for this?
So this is the sequence of events:
- each of the 2 routers in AS54 originates only one of the prefixes and
in doing so attaches MED=0
- they pass the prefix to eachother via iBGP preserving MED=0
- both of them advertise both prefixes to the eBGP peer but the prefix
locally originated maintains MED=0 while for the prefix learned via iBGP
the MED is scrapped out
- the eBGP peer receives both prefixes from both routers and prefers the
one with MED=0 because that one is announced by the router originating
it.
Is this correct?
Thanks,
Bit
On Sun, 2007-06-24 at 19:20 -0400, Edison Ortiz wrote:
> The routes are identical except for the missing MED attribute.
>
> MED is use as a hint to external neighbors about preferred path into an AS
> that has multiple entry points.
>
> A route with a MED attribute will be chosen over a route without a MED
> attribute
> since it's more trustworthy.
>
> By default, locally generated routes will add MED 0 as one of its
> attributes, again
> this is to prevent any loops within iBGP.
>
> If you were to advertise MED with a value of 0 from both routers, the one
> with the
> lowest RID will win.
>
> HTH,
>
> __
>
> Edison Ortiz
> (Routing & Switching, CCIE # 17943)
>
>
>
> ----- Original Message -----
> From: "Bit Gossip" <bit.gossip@chello.nl>
> To: "Edison Ortiz" <edisonmortiz@gmail.com>; <ccielab@groupstudy.com>
> Sent: Sunday, June 24, 2007 3:03 PM
> Subject: Re: BGP Best path selection
>
>
> > This is true; but how can this influence the selction?
> > AFAIK: the router running the selection prefers locally originated routes
> > over learned routes, with locally meaning on the same router performing
> > the selection.
> > In this case none of the prefixes is local and both are learned !
> >
> > Bit.
> >
> > ----- Original Message -----
> > From: "Edison Ortiz" <edisonmortiz@gmail.com>
> > To: "Bit Gossip" <bit.gossip@chello.nl>; <ccielab@groupstudy.com>
> > Sent: Sunday, June 24, 2007 8:34 PM
> > Subject: Re: BGP Best path selection
> >
> >
> >>I should've read the whole message before replying :)
> >>
> >> 28.119.16.0/24 was locally originated from 204.12.1.254
> >> while
> >> 112.0.0.0/8 was locally originated from 54.1.3.254
> >>
> >> __
> >>
> >> Edison Ortiz
> >> (Routing & Switching, CCIE # 17943)
> >>
> >>
> >>
> >> ----- Original Message -----
> >> From: "Bit Gossip" <bit.gossip@chello.nl>
> >> To: <ccielab@groupstudy.com>
> >> Sent: Sunday, June 24, 2007 11:41 AM
> >> Subject: BGP Best path selection
> >>
> >>
> >>> maybe I should take a break....
> >>> I can not understand the BGP selection process for these 2 prefixes:
> >>> -for 28.119.16.0/24 the best next-hop is 204.12.1.254
> >>> -for 112.0.0.0 instead it is 54.1.3.254
> >>> Any idea why? The difference seems to be that the path with MED=0 is
> >>> preferred
> >>> over the path with missing MED, but I have NOT configured 'bgp bestpath
> >>> med
> >>> missing-as-worst' and so missing MED should be treated as MED=0
> >>>
> >>> Rack1R6#show ip bgp 28.119.16.0/24
> >>> BGP routing table entry for 28.119.16.0/24, version 17
> >>> Paths: (2 available, best #2, table Default-IP-Routing-Table)
> >>> Flag: 0x820
> >>> Advertised to update-groups:
> >>> 1 2
> >>> 54
> >>> 54.1.3.254 from 54.1.3.254 (212.18.3.1)
> >>> Origin IGP, localpref 100, valid, external
> >>> 54
> >>> 204.12.1.254 from 204.12.1.254 (31.3.0.1)
> >>> Origin IGP, metric 0, localpref 100, valid, external, best
> >>>
> >>>
> >>>
> >>> Rack1R6#show ip bgp 112.0.0.0
> >>> BGP routing table entry for 112.0.0.0/8, version 4
> >>> Paths: (2 available, best #2, table Default-IP-Routing-Table)
> >>> Advertised to update-groups:
> >>> 1 2
> >>> 54 50 60
> >>> 204.12.1.254 from 204.12.1.254 (31.3.0.1)
> >>> Origin IGP, localpref 100, valid, external
> >>> 54 50 60
> >>> 54.1.3.254 from 54.1.3.254 (212.18.3.1)
> >>> Origin IGP, metric 0, localpref 100, valid, external, best
> >>>
> >>> _______________________________________________________________________
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> >>
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