Re: [bess] FW: WG Last Call (including implem status & shepherd) for draft-ietf-bess-evpn-vpws-03

["Rabadan, Jorge (Nokia - US)" <[e-mail]>]

HI Jeffrey,


On 5/12/16, 8:13 PM, "BESS on behalf of EXT Jeffrey (Zhaohui) Zhang" <[e-mail] on behalf of [e-mail]> 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
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