Re: [bess] draft-mohanty-bess-evpn-bum-opt-00 - clarification on problem description

"Rabadan, Jorge (Nokia - US/Mountain View)" <> Sat, 24 March 2018 10:58 UTC

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From: "Rabadan, Jorge (Nokia - US/Mountain View)" <>
To: Eric C Rosen <>, Sandy Breeze <>, "" <>
Thread-Topic: [bess] draft-mohanty-bess-evpn-bum-opt-00 - clarification on problem description
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Subject: Re: [bess] draft-mohanty-bess-evpn-bum-opt-00 - clarification on problem description
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Eric, as discussed and you point out, one can easily interpret that IMET is not mandatory in some cases where multi-destination traffic is not needed. In any case, whether this document is Informational or Standards Track is probably not that important.

If this had to be done, out of the options you list, I think omitting the PTA would not be backwards compatible since the use of PTA is a MUST in RFC7432, so RRs wouldn’t like it. Maybe label zero could cause issues too. So maybe a flag is the least disruptive one if the document has to modify something.

I still think it may be better to proceed with the IMET withdraw procedure and clarify that it is only valid for:
a) BUM traffic in IR cases
b) BDs with no igmp/mld/pim proxy
c) BDs with no OISM or IRBs
d) BDs with I-ES associated to overlay tunnels and no other ACs

And any other restrictions/caveats we may need to add.

My 2 cents.

From: Eric C Rosen <>
Date: Friday, March 23, 2018 at 5:00 PM
To: "Rabadan, Jorge (Nokia - US/Mountain View)" <>, Sandy Breeze <>, "" <>
Subject: Re: [bess] draft-mohanty-bess-evpn-bum-opt-00 - clarification on problem description

[Jorge] my interpretation of RFC7432 is that IMET routes are mandatory to enable the handling of multi-destination traffic in a BD. But in a non-DF PE for a given ES and with no other ACs in the BD, assuming Ingress Replication, there is no such multi-destination traffic (Tx or Rx). So one could interpret that RFC7432 is ok with withdrawing the IMET route in that case.

If we consider the case of all-active multi-homing, then there may well be Tx multi-destination traffic in the scenario under discussion, as multicast traffic from a given ES could arrive at any PE attached to the ES, whether or not that PE is the DF.

The relevant section from RFC 7432 is:

11.  Handling of Multi-destination Traffic

   Procedures are required for a given PE to send broadcast or multicast
   traffic received from a CE encapsulated in a given Ethernet tag
   (VLAN) in an EVPN instance to all the other PEs that span that
   Ethernet tag (VLAN) in that EVPN instance.  In certain scenarios, as
   described in Section 12 ("Processing of Unknown Unicast Packets"), a
   given PE may also need to flood unknown unicast traffic to other PEs.

   The PEs in a particular EVPN instance may use ingress replication,
   P2MP LSPs, or MP2MP LSPs to send unknown unicast, broadcast, or
   multicast traffic to other PEs.

   Each PE MUST advertise an "Inclusive Multicast Ethernet Tag route" to
   enable the above.  The following subsection provides the procedures
   to construct the Inclusive Multicast Ethernet Tag route.  Subsequent
   subsections describe its usage in further detail.

Interestingly, this says that the IMET route is mandatory to enable "the above", where "the above" is "send broadcast or multicast traffic received from a CE".  Note it says "send", not "receive".

If P2MP tunnels are used for the BUM traffic, the IMET route is certainly required to support all-active multi-homing.  If IR is used, or if single-active multi-homing is used, one could argue that RFC 7432 didn't really need to require the IMET route.  However, it does.

[John] Wouldn’t it be better to have this draft define a bit in the Multicast Flags extended community (<>) indicating that that the originating PE is neither DF nor backup DF for this broadcast domain on any ES to which it is attached?  This allows us to always advertise the IMET route and makes the situation explicit.  I think the consensus is that this situation is rare so the number of IMET route updates shouldn’t be excessive and we could also say that this bit is only set by EVPN DC GWs.
If it's worth doing at all, this would be a better method.  Alternatives would be omitting the PMSI Tunnel attribute, or setting the MPLS label in the PMSI Tunnel attribute to 0.
[Sandy] We’d considered alternative methods other than withdraw, such as extended community or something specific in PMSI Tunnel Attribute.  Withdraw/don’t advertise RT3 approach was chosen for the following reasons;
·         Requires no change to protocol
Since the proposal changes the conditions under which an IMET route is originated, it is certainly changing the protocol.  (It's obvious that the finite state machine is changed.)  Perhaps what is meant is that the protocol change is backwards compatible with systems that implement only RFC7432.  But it does not appear to be backwards compatible with systems that have IRB, and the draft has no analysis of the impact on all the various extensions and proposed extensions to RFC7432.
·         Is computationally easier on all participating PE’s, to deal with a simple withdraw than to look for something in an update.  For instance, on transition from BDF to NDF for example
These are of course not the only considerations.