If they are all internal, all routes will be advertised if they have different RD. However, the topology you posted has ASBRs - routes advertised between them are external, not internal... Or am I missing out on something in your description of the problem?
Can you post some more information - configurations, "show" output, etc. -- Marko CCIE #18427 (SP) My network blog: http://cisco.markom.info/ On Sun, Oct 18, 2009 at 10:41, karim okasha <[email protected]> wrote: > 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
