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
