Re: [bess] A question regarding Single-Active ES redundancy mode and DF election in RFC 7432

"Ali Sajassi (sajassi)" <> Tue, 04 September 2018 05:00 UTC

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From: "Ali Sajassi (sajassi)" <>
To: Alexander Vainshtein <>
CC: "" <>, Michael Gorokhovsky <>, Shell Nakash <>, Ron Sdayoor <>, Rotem Cohen <>
Thread-Topic: A question regarding Single-Active ES redundancy mode and DF election in RFC 7432
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Date: Tue, 04 Sep 2018 05:00:20 +0000
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Subject: Re: [bess] A question regarding Single-Active ES redundancy mode and DF election in RFC 7432
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Hi Sasha,

I don’t see any contradiction between the two statements from RFC 7432 that you mentioned below. For All-Active, DF election is for BUM traffic of a given VLAN (or group of VLANs in case of VLAN bundling) in the egress direction toward an ES. For Single-Active, DF election is for all traffic of a given VLAN (or group of VLANs …) in both directions of an ES. Now with respect to notification of active VLANs to a CE device: MVRP mechanism that is mentioned in the RFC is an IEEE standard way of doing such thing. However, if the CE support E-LMI, then that protocol can be used as well. Regarding LAG, it can be used to connect a CE in an active/standby mode where one link is active and another link in standby mode (assuming two-link bundle). You cannot use LAG to do active/standby on a per VLAN basis (aka EVPN single-active).

I will be travelling over next few days with limited email access, so please expect some delay for my responses.


From: Alexander Vainshtein <>
Date: Sunday, September 2, 2018 at 6:09 AM
To: Cisco Employee <>
Cc: "" <>, Michael Gorokhovsky <>, Shell Nakash <>, Ron Sdayoor <>, Rotem Cohen <>
Subject: A question regarding Single-Active ES redundancy mode and DF election in RFC 7432

Ali and all,
I have a question regarding one of the aspects of RFC 7432, namely operation of the default Designated Forwarder (DF) election process on an Ethernet Segment (ES) that operates in the Single-Active Redundancy Mode.

RFC 7432 defines the Single-Active Redundancy Mode in Section 3 as following:
“Only a single PE, among all the PEs attached to an Ethernet segment, is allowed to forward traffic to/from that Ethernet segment for a given VLAN”.

The same RFC in Section 8.5 also specifies that the DF for a specific VLAN on a multi-homed Ethernet segment (ES) is the only PE attached to this segment that is responsible for sending BUM traffic for this VLAN to the CE. It also defined the default DF election procedure that elects a single “live” PE on the specific ES as the DF for each specific EVI that is represented on this ES.

These two definitions look contradictory to me, because:

  1.  The default DF election procedure only involves the PEs attached to the specific ES
  2.  In the Single-Active Redundancy mode the elected DF for a specific VLAN must also be the only PE that is allowed to forward traffic received with this VLAN from the CEs to the peer PEs. It is not clear to me, how this can be achieved.
     *   The RFC mentions MVRP as a possible method to notify the attached CEs that a specific PE is NOT a DF for a specific VLAN in the case of an ES that operates in the Single-Active Redundancy Mode. Does this mean that CEs that are attached to a multi-homed ES operating in Single-Active Redundancy Mode SHOULD support MVRP?
     *   Are there any alternatives to MVRP that can be used for this purpose. In particular, is it possible to use Ethernet Local Management Interface (E-LMI) as defined in MEF-16<> for this purpose?
     *   The RFC mentions LAG as the method to connect the CE to a multi-homed ES operating in the All-Active Redundancy Mode. Is it possible to connect a CE that uses LAG to a multi-homed ES operating in the Single-Active Redundancy Mode?

Your feedback would be highly appreciated.

Regards, and lots of thanks in advance,

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