Dear Marko, Thanks a lot for your support but the rule that you mention apply only for external prefix and from the topology that i mentioned both routes that are received by the PE are internal routes also as i mentioned i tried flapping the best route and it is always chosen so the oldest is better doesn't apply here.
I hope you get my point and if you think there is something wrong with what is said kindly illustrate. Thanks a lot On Sun, Oct 18, 2009 at 11:48 AM, Marko Milivojevic <[email protected]>wrote: > Hello Karim. You may have misunderstood the tie break process just a > little bit. Nothing to blame you for, this is an obscure case. > Usually, tie break will happen on RR, as ti will advertise only the > best prefix and not all it has. On initial RR, not all parameters are > the same, so it can make a decision... > > ...the exception is if the same prefix is in VPNv4 with different RD. > In this case, indeed, RR will advertise the "same" prefix to its > neighbors. RD can't be used as tie breaker, as it separates the > prefixes. Indeed, this is the situation that you may be having. So, > decision is left where it is now and where not all information is > useful or available. > > In this case, very unusual tie break rule will be used - prefix that > was received first, stays. See here (step 10): > > > http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml > > Just remember that nothing in BGP is random :-). > > -- > Marko > CCIE #18427 (SP) > My network blog: http://cisco.markom.info/ > > On Sun, Oct 18, 2009 at 04:15, karim okasha <[email protected]> wrote: > > Dear Bryan, > > > > Thanks a lot for your support. But i wanted to know exactly on what basis > > would the router choose one path over the other and let me explain. > > > > If the 2 paths are completely equal in every thing the last three > tiebrakers > > will be as follows. > > > > 1- Prefer the path from the lowest BGP router ID > > (The Router ID is the same as the route came from a RR so both > routes > > have the same originator) > > > > 2- prefer the path with the minimum cluster length. > > (again the 2 paths have the same cluster length) > > > > 3- Prefer the path with the lowest neighbor address. > > (I think that both will have the same value as the 2 routes again > are > > originated from the same Router which is the RR). > > > > That's is why i wanted to know what is the exact criteria to prefer one > > route over the other as in this specific example in the WB i tried > flapping > > the session between the CE and R3 but every time once the session go up > the > > router is the other AS starts prefering the route from R3 again. > > > > Again thanks a lot for your help. > > > > On Sat, Oct 17, 2009 at 11:42 PM, Bryan Bartik <[email protected]> > wrote: > >> > >> Karim, > >> > >> The path selection process is the same. Not sure what the tie-breaker is > >> in this case but it looks like it's pretty far down the list... > >> > >> On Fri, Oct 16, 2009 at 11:13 AM, karim okasha <[email protected]> > wrote: > >>> > >>> Dear all, > >>> > >>> I have a problem with task 7.4 in this Lab. this task states that we > need > >>> to configure R9 so that traffic from R8 always favors R2. > >>> > >>> The solution is understood but my problem is why in the first place > does > >>> R8 favors the route coming from R3 instead of the Router coming from > R2. > >>> > >>> I need to understand how BGP selects the best route in the following > >>> situation. > >>> > >>> ---- PE1 > >>> CE1 -- ---- ASBR ---- ASBR --- PE3 > >>> --- PE2 (RR) (RR) > >>> > >>> How is PE3 going to choose the best path towards CE1. > >>> > >>> Your help is very much appreciated. > >>> > >>> > >>> _______________________________________________ > >>> For more information regarding industry leading CCIE Lab training, > please > >>> visit www.ipexpert.com > >>> > >> > >> > >> > >> -- > >> Bryan Bartik > >> CCIE #23707 (R&S), CCNP > >> Sr. Support Engineer - IPexpert, Inc. > >> URL: http://www.IPexpert.com > > > > > > _______________________________________________ > > For more information regarding industry leading CCIE Lab training, please > > visit www.ipexpert.com > > > > >
_______________________________________________ For more information regarding industry leading CCIE Lab training, please visit www.ipexpert.com
