Re: [bess] shepherd review of draft-ietf-bess-evpn-etree

["Ali Sajassi (sajassi)" <[e-mail]>]

Hi Thomas,

Your suggestion regarding multiple MAC-VRFs per EVI for E-TREE, impacts lot more sections than just section 2.2 for which you suggested some texts. It drastically  impacts section 3.1 (known unicast traffic), and it also impacts section 3.2 (BUM traffic) and section 5.1. Furthermore, it creates a new paradigm for EVPN that was never intended for because of creating two MAC-VRFs (and two bridge tables) for the same VLAN. The WG LC was completed on 3/29/16 and I am sure it is not your intention to have major changes to the doc at this stage where multiple vendors have already implemented the draft.

This draft talks about two kinds of traffic filtering: a) ingress filtering for known unicast and b) egress filtering for BUM traffic. What you are suggesting is an alternate mechanism for ingress filtering. Although having multiple VRFs (and forwarding tables) are fine for IP-VPNs because the unknown traffic is always dropped, multiple VRFs for the same VLAN is not OK for L2 traffic because of flooding of unknown traffic. That’s why in section 6 of RFC 7432, for all service interface types, the draft talks about a single MAC-VRF per EVI per PE and in case of VLAN-aware mode,  multiple VLANs per MAC-VRF but only a single bridge table per VLAN. In other words, the bottom line is that there can only be a single bridge table per VLAN in order to avoid unnecessary flooding. When you have two MAC-VRFs per VLAN (one for root ACs and another for Leaf ACs), then you either need to duplicate lots of MAC addresses between these two VRFs, or do lookup on both of these VRFs. Either ways this is not a good option relative to keeping a single VRF table for both root and leaf sites and just have a single-bit indication on whether a MAC is associated with root or leaf (as currently described approach in the draft).  I

Regards,
Ali

P.S., I included a bit more comment inline …


From: Thomas Morin <[e-mail]<mailto:[e-mail]>>
Organization: Orange
Date: Tuesday, December 6, 2016 at 1:55 AM
To: Cisco Employee <[e-mail]<mailto:[e-mail]>>, "[e-mail]<mailto:[e-mail]>" <[e-mail]<mailto:[e-mail]>>, Loa Andersson <[e-mail]<mailto:[e-mail]>>, "George Swallow -T (swallow - MBO PARTNERS INC at Cisco)" <[e-mail]<mailto:[e-mail]>>, Eric Rosen <[e-mail]<mailto:[e-mail]>>, BESS <[e-mail]<mailto:[e-mail]>>
Cc: Martin Vigoureux <[e-mail]<mailto:[e-mail]>>
Subject: Re: shepherd review of draft-ietf-bess-evpn-etree

Hi Ali,

Ali Sajassi (sajassi):
Thanks again for your additional comments. Below, please find my comment resolutions. Let me know please if there are any further comments.

Answers below...


   EVI. The purpose of this topology constraint is to avoid having PEs
   with only  Leaf sites importing and processing BGP MAC routes from
   each other. To support such topology constrain in EVPN, two BGP
   Route-Targets (RTs) are used for every EVPN Instance (EVI): one RT is
   associated with the Root sites and the other is associated with the
   Leaf sites. On a per EVI basis, every PE exports the single RT
   associated with its type of site(s). Furthermore, a PE with Root
   site(s) imports both Root and Leaf RTs, whereas a PE with Leaf
   site(s) only imports the Root RT.

The text seems to imply that the above is sufficient to deliver the service, but I fail to see what would prevent Leaf-to-Leaf traffic between Leaves bound to the same MAC-VRF (ES2 and ES3 in firgure1).  Shouldn't the text mention the use of a split-horizon in Leaf MAC-VRFs ?

Agree, nice catch!. I changed the first sentence from:
"In such scenario, an EVPN PE implementation MAY provide E-TREE service using topology constraint among the PEs belonging to the same EVI."
TO
"In such scenario, topology constraint, provided by BGP Route Target (RT) import/export policies among the PEs belonging to the same EVI, can be used to restrict the communications among Leaf PEs."

The sentence above does not address my question in fact, which was about communication between Leaf ACs (rather than about communication between Leaf PEs)
Let me restate here, more clearly:  I fail to see what would prevent Leaf-to-Leaf traffic between **ACs** bound to the same MAC-VRF (ES2 and ES3 in firgure1).  Shouldn't the text mention the use of a split-horizon in Leaf MAC-VRFs ?

OK. I mentioned the use of split-horizon filtering explicitly for blocking inter-Leaf communication within the same PE.

"In such scenario, using tailored BGP Route Target (RT) import/export policies among the PEs belonging to the same EVI, can be used to restrict the communications among Leaf PEs. To restrict the communications among leaf sites connected to the same PE  and belonging to the same EVI, split-horizon filtering is used - i.e., the interfaces associated with Leaf sites are placed in the same split-horizon group. "


Mentioning this here is an improvement.
Perhaps a reference is needed to explain what a split-horizon _group_ is though, or simply rephrase to avoid the notion ?

Proposal: "split-horizon is used to block traffic from one Leaf interface to another Leaf interface of a given E-TREE EVI".

Ali> OK. Done.


(assuming the previous point is resolved:)

With this mechanism above, isn't it possible to have on a given PE, for a single E-TREE EVI, both Leaves and Roots, as long as distinct MAC-VRFs are used (one for Leaves and one for Roots) ?   (it seems to me that the assymetric import/export RT would do what is needed to build an E-TREE, we would just have a particular case where a Leaf MAC-VRF and a Root MAC-VRF for a given E-TREE end up on a single PE)

That’s not possible because per definition of an EVI, there is only a single MAC-VRF per EVI for a PE.

Where can I read such a definition ? (the Terminology section in RFC7432 does not say that, unless I'm missing something).
And that seems a completely arbitrary restriction.
(just thinking that a given PE device can be split in two logical devices show that it can work)

Section 6 of RFC7432 where it gives definitions for different service interface types, it specifies the relationship between MAC-VRF and VLAN (bridge table) and how many MAC-VRF (and bridge tables) can be per EVI.

This section of RFC7434 discusses many different things for the different variants.
Can you provide a specific pointer about "how many MAC-VRFs can be per EVI" ?

Ali> Section 6 of RFC7432 spells out the relationship between EVI, MAC-VRF, and bridge tables for all service interfaces very clearly. In all service interfaces, the RFC says there is one MAC-VRF per EVI on a given PE. Now, if the service interface is “vlan-aware”, then there are several bridge tables for that single MAC-VRF – ie, one bridge table per VLAN. In all service interfaces, you can ONLY have one bridge table per VLAN.


In bridging world, there can only be a single bridge table per VLAN in a device.

I still don't find here anything that would preclude having, on a given PE, for a given E-TREE EVI, one Leaves MAC-VRF and one Roots MAC-VRF: can't these two MAC-VRFs use different internal VLANs (with translation if the external VLANs are constrained).

Ali>  Lets assume we are using vlan-based service and thus there is only a single bridge table per MAC-VRF, then what you are suggesting is two use two MAC-VRFs (two bridge tables) for the same EVI (same VLAN). This results in some duplications of MAC addresses and would only work if flooding is disabled (more on this later).

Besides, I don’t understand what good does it do to have two MAC-VRFs on the same PE (one for Leafs and another for Roots)

Well, the "what is good for" is pretty simple: it means you can have, just by tailoring the import/export policies like in 2.1, something as useful as the scenario in 2.2.

There can only be a single bridge table per VLAN. Now even if you add some kind of logic to form two logical PEs in single physical PE, you end up replicating all the MAC addresses associated with the root sites in two bridge tables.

Your point above certainly does not sound to me as "it can't be done": some may think that the above is an acceptable cost, some others may find ways to make this "replication" with a low overhead, on some platforms the cost may be negligible, etc.




because Leafs and Roots need to talk to each other and thus we want them to be in the same MAC-VRF.

The fact that Leafs and Roots need to talk to each other does not mean that they *have* to be in the same MAC-VRF, you can rely on the local MPLS dataplane inside the PE to carry the traffic between Roots and Leaves can be passed between a Leaf MAC-VRF and a Root MAC-VRF (and you can possibly implement a shortcut not involving MPLS encap/decap).

Anything is possible but at what cost.

You know, for cost it is not always obvious to reach conclusions that are true for all implementations and all targets.

The current proposal is very efficient in terms of forwarding path as well as control plane.

Sure, but what I question is not the new solution but the lack of discussion on why using the existing specs was not considered good enough.


I think that my concern of clearly explaining the scenarios and motivations for this new spec could be addressed by splitting section 2.2 into a 2.2.1 describing the approach from 2.1 and its possible drawbacks, and a 2.2.2 having essentially the content of current section 2.2.

Here is a proposal:

2.2 Scenario 2: Leaf of Root site(s) per AC

   In these scenarii, a PE receives traffic from either Root OR Leaf
   sites (but not both) on a given Attachment Circuit (AC) of an EVI. In
   other words, an AC (ES or ES/VLAN) is either associated with Root(s)
   or Leaf(s) (but not both).

2.2.1 Scenario 2a: Leaf OR Root site(s) per AC, separate Leaf/Root MAC-VRFs

                     +---------+            +---------+
                     |   PE1   |            |   PE2   |
    +---+            |  +---+  |  +------+  |  +---+  |            +---+
    |CE1+-----ES1----+--+   |  |  |      |  |  |MAC+--+---ES2/AC1--+CE2|
    +---+    (Leaf)  |  |MAC|  |  | MPLS |  |  |VRF|  |   (Leaf)   +---+
                     |  |VRF|  |  |  /IP |  |  '---'  |
                     |  |   |  |  |      |  |  .---.  |
                     |  |   |  |  |      |  |  |MAC|  |            +---+
                     |  |   |  |  |      |  |  |VRF+--+---ES2/AC2--+CE3|
                     |  +---+  |  +------+  |  +---+  |   (Root)   +---+
                     +---------+            +---------+

   Figure 2: Scenario 2a

   In this scenario, the RT constraint procedures described in section 2.1 could
   also be used. The feasibility and efficiency of this approach depends on
   platforms specifics.

   This approach will lead to duplication of a large proportion of MAC addresses on
   PEs having both Leaf and Root sites, and is hence considered less suitable for
   deployment contexts where the vast majority of PEs are likely to ultimately
   have both Leaf and Root sites attached to them.

2.2.2 Scenario 2b: Leaf OR Root site(s) per AC, single MAC-VRF

                     +---------+            +---------+
                     |   PE1   |            |   PE2   |
    +---+            |  +---+  |  +------+  |  +---+  |            +---+
    |CE1+-----ES1----+--+   |  |  |      |  |  |   +--+---ES2/AC1--+CE2|
    +---+    (Leaf)  |  |MAC|  |  | MPLS |  |  |MAC|  |   (Leaf)   +---+
                     |  |VRF|  |  |  /IP |  |  |VRF|  |
                     |  |   |  |  |      |  |  |   |  |            +---+
                     |  |   |  |  |      |  |  |   +--+---ES2/AC2--+CE3|
                     |  +---+  |  +------+  |  +---+  |   (Root)   +---+
                     +---------+            +---------+

   Figure 2: Scenario 2b

   This scenario will alleviate keys drawbacks from Scenario 2a, in particular
   by avoiding duplication of MAC addresses on Leaf/Root PEs and avoiding the
   operational overhead of managing more than one RT.

   This approach comes at the expense of having routes for unneeded MAC addresses
   on Leaf-only PEs, and is hence considered less suitable for deployment contexts
   where the vast majority of PEs would remain Leaf-only.   Unlike Scenario 1 and Scenario 2a, this scenario requires additional procedures
   provided in this document.




(And this last sentence should be added to section 2.3 as well)


For this scenario, if for a given
   EVI, the majority of PEs will eventually have both Leaf and Root
   sites attached, even though they may start as Root-only or Leaf-only
   PEs, then it is recommended to use a single RT per EVI and avoid
   additional configuration and operational overhead.

Why this recommendation ?
Even with a majority of PEs having both Leaves and Roots, there can remain (up to 49% of) PEs having only Leaves, which will uselessly have all routes to other Leaves.

So "it is recommended" above, deserves to be explained more, I think.

OK, I changed “majority” to “vast majority” :-)

My point was not to nit pick on "majority", but was that you should explain why you recommend that.
As the text currently reads, the cost of the recommendation can be identified: having useless routes on the fraction of PEs having only Leaves.
But the gain brought by the recommendation is not even mentioned, not to say explained.
Hence: why ?
(Why is it a useful tradeoff to have useless routes on some, even if only one, PE ?)

Changed the last sentence from:
"then it is recommended to use a single RT per EVI and avoid additional configuration and operational overhead.”
To
"then it is recommended to use a single RT per EVI and avoid additional configuration and operational overhead
at the expense of having unwanted MAC addresses on the Leaf PEs."

Ok. I adapted and incorporated this addition into my proposed text splitting 2.2 into a 2.2.1 and a 2.2.2.

Best,

-Thomas