From: Scott Morris (swm@emanon.com)
Date: Mon Apr 24 2006 - 16:03:02 GMT-3
No problems! We sometimes get so hung up in looking at one thing that we
forget to see the other things around it! (grin)
It happens to everyone at some point! (grin)
Scott Morris, CCIE4 (R&S/ISP-Dial/Security/Service Provider) #4713, JNCIE
#153, CISSP, et al.
CCSI/JNCI
IPExpert CCIE Program Manager
IPExpert Sr. Technical Instructor
smorris@ipexpert.com
http://www.ipexpert.com
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
gscisco@xs4all.nl Cisco
Sent: Monday, April 24, 2006 2:24 PM
To: ccielab@groupstudy.com
Subject: Re: BGP IBGP vs EBGP path
Scott,
Thanks. Obvious indeed. It's funny since with the first solution by
prepending I didn't prepend even add an AS to one of the BGP advertising
routers because of the AS path. I think I must have gotten confused when
playing with the MED.
Anyway added a prepended AS to the redistributing router and then it does
start to look at MED.
Thanks again for making it clear :)
Vincent
On 4/23/06, Scott Morris <swm@emanon.com> wrote:
>
> Per my experience, that document is actually correct. You have other
> issues though! BGP paths (at step 7) are chosen in numerical order of
> origin code (0 = internal (IGP), 1 = external (EGP), 2 =
> Incomplete/Unknown) (Step 5 occurs first, which would indicate eBGP
> neighbor)
>
> Now, bear in mind that you say the route was redistributed... In the
> Cisco world, that will set the origin code as incomplete. This is
> irregardless of the iBGP or eBGP relationship (as this type code
> doesn't change in updates).
>
> The preference you are seeing is likely due to other information such
> as the router-id of the advertising neighbor being lower. Looking
> through the RFC,
> the spec lays this out as one of the tie-breaking mechanisms. On the
> other
> hand, I'd probably think the decision would take place even before
> that based on AS-path length! If your PE's are the same AS, then
> that's a damn short AS path. :) If you learn it from the CE, then
> your path would be longer, and this is done in step 4!!!
>
> So you aren't making it to step 7 in preferring your internal path to
> the other PE!
>
> HTH,
>
>
> Scott Morris, CCIE4 (R&S/ISP-Dial/Security/Service Provider) #4713,
> JNCIE #153, CISSP, et al.
> CCSI/JNCI
> IPExpert CCIE Program Manager
> IPExpert Sr. Technical Instructor
> smorris@ipexpert.com
> http://www.ipexpert.com
>
>
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
> Of gscisco@xs4all.nl Cisco
> Sent: Sunday, April 23, 2006 5:00 PM
> To: ccielab@groupstudy.com
> Subject: BGP IBGP vs EBGP path
>
> Hi,
>
> I hope you guys can help me understand this as IBGP and EBGP routes
> behave different than I have always understood them.
>
> From what I read in cisco's "BGP Best Path Selection Algorithm" -
>
>
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431
> .shtml#background
>
> 7 - Prefer eBGP over iBGP paths.
>
> However it is not the behavior I'm seeing myself
>
> I have the following setup
>
> CE1 === default route -> / <- static redistributed in BGP === PE 1
>
> CE2 ====== running BGP ====================================== PE 2
>
>
> PE1 and PE2 are within the same AS and run iBGP between them.
>
> I have a network 157.161.77.0/24 which PE1 has a static route for
> which it redistributes into BGP. PE2 gets this same network from CE2
> via (e)BGP. It however does not take this as the best BGP path as
> keeps prefering the internally learned one via iBGP
>
> PE3#sh ip b 157.161.77.0
> BGP routing table entry for 157.161.77.0/24, version 5
> Paths: (2 available, best #2, table Default-IP-Routing-Table)
> Advertised to non peer-group peers:
> 161.239.99.2
> Local
> 172.16.1.3 (metric 11) from 172.16.1.3 (213.239.99.3)
> Origin incomplete, metric 10, localpref 100, valid, internal,
> best <<<<< iBGP learned path
> 65000
> 161.239.99.2 from 161.239.99.2 (161.239.99.2)
> Origin IGP, localpref 100, valid, external <<<<<<<<<< eBGP
> learned
> path
>
>
> Am I misinterpreting cisco's path algorithm ? Furthermore I also see
> that it should have taken the the latter path as it's Origin is better
> as well (IGP goes before "incomplete") ?
>
> Can anyone shine a light on this for me as I'm quite confused.
>
> Thanks in advance,
>
> Vincent
>
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