From: Edison Ortiz (edisonmortiz@gmail.com)
Date: Mon Jun 25 2007 - 17:18:44 ART
Actually, most documents I've read confirm what you stated,
the lack of metric attribute on the route defaults to zero.
However, during my labbing, that was not the case. On equal
routes, the one with a MED value broke the tie-breaker.
Play around and set a MED 0 from both eBGPs. You will see
the lowest RID will win.
__
Edison Ortiz
(Routing & Switching, CCIE # 17943)
----- Original Message -----
From: "Bit Gossip" <bit.gossip@chello.nl>
To: "Edison Ortiz" <edisonmortiz@gmail.com>
Cc: <ccielab@groupstudy.com>
Sent: Monday, June 25, 2007 4:07 PM
Subject: Re: BGP Best path selection
> 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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