HI Jeffrey,
On 5/12/16, 8:13 PM, "BESS on behalf of EXT Jeffrey (Zhaohui) Zhang" <bess-boun...@ietf.org on behalf of zzh...@juniper.net> wrote: >Hi Jorge, > >> >If a PE sends a route w/o setting P-bit, wouldn't that indicate it is a >> backup? Why would it bother sending the route if it does not want to be >> the backup? >> >> [JORGE] Let’s assume you have 3 PEs in ES-1 (PE1/2/3) that is a single- >> active ES. PE4 is a remote PE doing backup function. > >I assume PE4 is not "doing backup function" but is just one end of the PW. Or >perhaps you mean it needs to decide which of PE1/2/3 is the primary/backup. [JORGE2] by “doing backup function” I mean that PE4 sets up a destination to PE1 with a backup to PE2. Aliasing and Backup functions are effectively done by the remote PEs based on the information advertised by the PEs in the ES, right? > >> - PE1 is the primary - the winner of the DF election. PE1 sends P=1 >> - PE2 is the backup - the winner of the DF election (without PE1). PE2 >> sends B=1. > >Hmm ... DF election is for BUM in RFC 7432 and never mentioned in this draft. >If it is used for VPWS, it needs to be specified. [JORGE2] In VPWS DF election is needed for single-active. The NDF will block the AC for the service. For all-active there is no DF, of course, since there is no BUM ;-) > >The draft says multiple PEs could all set the P/B bits: > > In multihoming single active scenario, a remote PE receiving P=1 from > more than one PE will select only one primary PE when forwarding > traffic. A remote PE receiving B=1 from more than one PE will select > only one backup PE. A remote PE MUST receive P=1 from at least one PE > before forwarding traffic. > >And the decision is based on the receiving PE, not on DF election? [JORGE] This is BGP, there are always transient situations, so it may happen that even in single active you get more than one P=1 or B=1, so you need to say what to do in that case, right? In a steady situation, in single-active MH, only the DF MUST send the P=1 since it is only the DF the one unblocking the AC in the ES. > >> - PE3 then sends P=B=0. This indicates that PE3 is neither primary nor >> backup, but the AC is active (if it was oper-down, it would withdraw the >> route). > >What purpose does the route from PE3 serve? [JORGE] since the AC is oper-up on PE3, PE3 needs to send the route. At PE4 you know that PE3’s AC in the ES is good, but simply neither primary nor backup. > >> - In case of a failure on PE1, PE2 will activate its AC on ES-1 since he >> wins the new DF election. > >What does "activate" involve exactly? Just re-advertise the route with P-bit >set and B-bit cleared? [JORGE] unblock Tx/Rx on the AC… and later readvertise the route with the new bits. > >> At the same time, PE4 can immediately send >> traffic to PE2 as soon as PE1 withdraws the AD routes. PE4 does not even >> need to wait for the confirmation that PE2 is the new DF. This backup >> mechanism speeds up convergence big time. >> - The mechanism above does not work if PE2 and PE3 send both the same >> P=B=0, since it case of PE1 failure, PE4 could not send any traffic (it >> does not know who is the active one in the ES) and would need to wait for >> PE2 to send P=1. > >I don't see any use of the route from PE3. The draft seems to say that both >PE1 and PE2 can set the P bit initially and PE4 will pick one to send traffic >to. If it picks PE1 and then PE1 withdraws the route, PE4 will then pick PE2? [JORGE] This is single active, so only one can transmit/receive to/from the CE. So only that one should send P=1. > >Jeffrey >_______________________________________________ >BESS mailing list >BESS@ietf.org >https://www.ietf.org/mailman/listinfo/bess _______________________________________________ BESS mailing list BESS@ietf.org https://www.ietf.org/mailman/listinfo/bess
