Hey Karim, ok I understand this better now. It looks it is a tie all the way through the process. The algorithm definitely doesn't seem to take into account having multiple paths through the same neighbor. The only way this is possible of course is because the RD keeps the routes unique through each device. I will have to dig into this some more...I tried seeing if lowest RD or RT values may impact it, but it doesn't appear so.
On Sat, Oct 17, 2009 at 10:36 PM, Bryan Bartik <[email protected]> wrote: > I see what you are saying. Is R9 even getting two routes from R2? It's been > a few weeks since I did this lab and don't remember what the output was. Can > you give a "sho ip bgp vpnv4 all x.x..x" for the route in question on R2, R9 > and R8? > > > On Sat, Oct 17, 2009 at 9:15 PM, 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 >>> >> >> > > > -- > Bryan Bartik > CCIE #23707 (R&S), CCNP > Sr. Support Engineer - IPexpert, Inc. > URL: http://www.IPexpert.com > -- Bryan Bartik CCIE #23707 (R&S), CCNP Sr. Support Engineer - IPexpert, Inc. URL: http://www.IPexpert.com
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