From: Edison Ortiz (edisonmortiz@gmail.com)
Date: Sun Jun 24 2007 - 20:20:05 ART
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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