Internet-Draft EVPN-lite August 2021
Wang & Chen Expires 3 March 2022 [Page]
Intended Status:
Standards Track
Y. Wang
ZTE Corporation
R. Chen
ZTE Corporation

Light Weighted EVPN


SRv6 EVPN [I-D.ietf-bess-srv6-services] is not sufficient for some light-weighted use cases. When PBB EVPN [RFC7623] over SRv6 is used to support these light-weighted EVPN services, it is complicated to make use of the SID list to carry a function that is aiming for C-MACs.

In [RFC8986], End.DX2 function is defined, this function can be used in EVPN VPLS. When it is used in EVPN VPLS, the data-plane learning defined in End.DT2U function can also be transplanted into End.DX2 function. On the basis of such extended End.DX2 function, SRv6 EVPN can be improved to meet all the requirements per [RFC7623] and bring us some other benefits. Such SRv6 EVPN is called light-weighted SRv6 EVPN, and it will be more simpler than PBB EVPN over SRv6.

It is easy for the light-weighted SRv6 EVPN to carry a SID that is aiming for customer ethernet packets, because there will be no other ethernet header between the SID list and the customer ethernet header. These SIDs may be user-defined functions for the customer ethernet headers.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

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This Internet-Draft will expire on 3 March 2022.

Table of Contents

1. Introduction

1.1. Background

When there are too many customer-MACs (C-MACs), the RRs and/or ASBRs will be overloaded by the RT-2 routes for these MACs according to [RFC7432]. This issue can be solved by light-weighted EVPNs. PBB EVPN [RFC7623] is a MPLS-based light-weighted EVPN solution. But in SRv6 network, PBB EVPN over SRv6 is not a good choice for light-weighted EVPN solution.

This document proposes some new extensions to [I-D.ietf-bess-srv6-services] to achieve all-active mode ES redundancy on TPEs and reduce the C-MAC loads for RRs and ASBRs at the same time. The new solution will work even more better than PBB EVPN under the help of these extensions, especially when there is no deployment of MPLS dataplane.

Furthermore, it naturally brings the benefits of high scalability, faster network convergence, and reduced operational complexity, and we call it light-weighted EVPNs because of these advantages.

1.2. Overview

In [RFC7432], the C-MACs is advertised via RT-2 route. This behavior is inheritted by [I-D.ietf-bess-srv6-services]. but in order to solve the C-MAC overload problem for RRs and ASBRs, we have to return to a PBB-like dataplane C-MAC learning procedures.

We discuss all the requirements for a light-weighted EVPN solution which pushes no C-MAC entries into the backbone network in Section 2. Note that some of these requirements is not supported well by PBB EVPN.

In this document, the light-weighted EVPN solutions are also called as EVPN-lite for short.

Note that the EVI here corresponds to the I-Component of [RFC7623], not the B-Component. In fact, there will be no typical B-components in EVPN-lite SRv6 solutions.

1.3. Terminology

Most of the terminology used in this documents comes from [RFC7432] and [I-D.ietf-bess-srv6-services] except for the following:

  • Light-weighted EVPN: The EVPN solution with high scalability and reduced operational complexity.
  • EVPN-lite: The Light-weighted EVPN is also called EVPN-lite for short.
  • C-MAC: Customer MAC, it is the same as the C-MAC of PBB EVPN.
  • ISID: a broadcast domain identifier in PBB I-Component.
  • LDV: Local Discreminating Value. It is similar to the Local Discreminating Value of type 3 ESI.
  • GDV: Global Discreminating Value. An identifier with global uniqueness.
  • EGD: EVI-GDV, an EVI's Global Discreminator, it is a GDV for an EVPN MAC-VRF. A EGD is used to idenfify an EVPN MAC-VRF in data plane. The EGD is a Global Discreminating Value (GDV) of that EVPN MAC-VRF, so it is also the abbreviation of EVPN-GDV. e.g. The EGD of [RFC7348] is a global VNI. In this draft, the EGD of an EVI is that MAC-VRF's VPN ID of global uniqueness.
  • AC SID: The End.DX2 SID of a specified AC, different ACs on the same ES have different AC SIDs.
  • ESI SID: An SRv6 SID whose function type is End.DX2AGG. Note that when the ESI is all-active mode, the ESI SID is the same on all PEs of that ES, according to Section 3.2. In such case, the ESI SID can be called as ES anycast SID too. Different ACs on the same ES have the same ESI SID with different Arg.ACI.
  • ESI IP: The End.DX2AGG SID of a specified ESI, but with an empty Arg.ACI.
  • ESI Prefix: The IPv6 Prefix that covers all AC-SIDs of the specified ESI.
  • Ingress AC SID: The End.DX2 SID of the ingress-AC on ingress-PE. Note that the Ingress AC SID are typically encapsulated as SRv6 Source IP in data plane.
  • EAD/ES: Ethernet A-D route per EVI, or RT-1 per ES route.
  • EAD/EVI: Ethernet A-D route per EVI, or RT-1 per ES route.
  • Arg.ACI: The argument part of a SID of the End.DX2AGG function is called as Arg.ACI, because the value of that argument will be an AC-ID.
  • RT-2: MAC/IP Advertise Route.
  • MAC Entry: An entry in the EVPN MAC table in data-plane.
  • GRT: Global Routing Table.

2. Requirements

Light-weighted SRv6 EVPNs should be provided together with the following requirements:

2.1. No C-MAC Awareness in the Backbone

In typical operation, an EVPN PE sends a BGP MAC Advertisement route per C-MAC address. In certain applications, this poses scalability challenges, as is the case in data center interconnect (DCI) scenarios where the number of virtual machines (VMs), and hence the number of C-MAC addresses, can be in the millions. This is called as C-MAC overload of DC Backbone. In such scenarios, it is required to reduce the number of BGP MAC Advertisement routes by relying on a 'EVPN-lite' scheme, as is provided by ESI and its equivalents (e.g. Pseudo B-MAC, ESI IP).

2.2. Flexible Multi-homing Remains Supported

Flexible multi-homing means that different ES instances can have different adjacent-PEs. We call all the adjacent-PEs of the same ES instances as that ES's location-set in this document. Flexible multi-homing means that different ES can have different location-set.

For example, ES101's location-set is {PE1}, ES102's location-set is {PE2, PE3}, ES103's location-set is {PE1, PE3}, and ES104's location-set is {PE2,PE4}.

2.3. C-MAC Address Learning and Confinement

In EVPN, all the PE nodes participating in the same EVPN instance are exposed to all the C-MAC addresses learnt by any one of these PE nodes because a C-MAC learnt by one of the PE nodes is advertised in BGP to other PE nodes in that EVPN instance. This is the case even if some of the PE nodes for that EVPN instance are not involved in forwarding traffic to, or from, these C-MAC addresses. Even if an implementation does not install hardware forwarding entries for C-MAC addresses that are not part of active traffic flows on that PE, the device memory is still consumed by keeping record of the C-MAC addresses in the routing information base (RIB) table. In network applications with millions of C-MAC addresses, this introduces a non- trivial waste of PE resources. As such, it is required to confine the scope of visibility of C-MAC addresses to only those PE nodes that are actively involved in forwarding traffic to, or from, these addresses.

2.4. No C-MAC Flushing for All-Active ESes

Just as in [RFC7623], it is required to avoid C-MAC address flushing upon link, port, or node failure for remote All-Active multihomed segments.

Note that when an ES fails on one PE, it may still works well on another PE, so the C-MACs should not be flushed.

2.5. Independent C-MAC Flushing for Single-Active ESes

Just as in [RFC7623], upon single-active ESI's link or port failure, the C-MACs of other single-active ESes from the same PE will not be flushed.

2.6. Independent Convergency per <ESI, EVI>

When the physical port of an All-Active ES works well, but a single Ethernet Tag ID (ETI) of that ES fails (illustrated as the 'X' flag in Figure 1) on PE1, The traffic to that ETI of that ES will be re-routed to other adjacent PE of the same ES, but the traffic to other ETIs of the same ES will not be affected.

If PE1 is the last active link for that <ESI, ETI> before that failure, C-MAC flush should be triggered on the remote PEs. If that ESI is single-active, C-MAC flush should be triggered on the remote PEs too.

Note that when AC (ES link) fails but PE node still works well, there should not be steady bypassing traffic either. The steady bypassing problem is discussed in [].

2.7. ESI Route Aggregation in Backbone

In SRv6 EVPN, different sub-interfaces of the same ESI can have different AC-SIDs in order to achieve Independent Convergency per <ESI, EVI>. But only the common prefix (say ESI-Prefix) of them should be advertised in underlay network.

Note that only the common prefix need to be advertised in the overlay network before any of these sub-interfaces failed.

Note that different ESIs may use the same SRv6 locator. In such case, these ESI SIDs are aggregated into that anycast SRv6 locator while they are advertised in the underlay network.

2.8. Unequal load-balance

The light-weighted EVPNs should support the unequal load-balance defined in [I-D.ietf-bess-evpn-unequal-lb].

2.9. AC-aware Service Interface

In AC-aware bundling service interface [I-D.sajassi-bess-evpn-ac-aware-bundling], the ESes may make its two VLANs to be attached to the same broadcast domain. These two VLANs may be assigned to the same sub-interface, or to different sub-interfaces.

2.10. Ingress Filtering for Unicast Flows of E-Tree Services

The filtering needed by an E-Tree service for known unicast traffic should be performed at the ingress PE, thus providing very efficient filtering and avoiding sending known unicast traffic over the PSN to be filtered at the egress PE, as is done in traditional E-Tree solutions (i.e., E-Tree for VPLS [RFC7796]).

2.11. AC-Influenced DF Election

When the EAD/EVI route is not advertised before the corresponding ESI sub-interface fails, The AC-influenced DF Election procedures should elect the right DF before and after that failure.

Note that according to [RFC8584], the AC-influenced DF Election will be incorrect when no EAD/EVI route is advertised, even if no ESI sub-interface has failed at all.

The AC-influenced DF Election should support "service carving" like what [RFC7432] section 8.5 have done.

2.12. Synchronous MAC Entries in All-Active Mode

When a C-MAC Mx is learnt on an attachment circuit AC1 of an all-active Ethernet Segment ES21 on PE1, Mx should not be in unknown unicast state on PE2, which is also adjacent with ES21. And the outgoing interface of PE2's MAC entry of Mx should be AC2, which is an AC of ES21 and has the same VLAN as AC1.

3. Light-Weighted SRv6 EVPN Overview

3.1. Use Case

The physical links of these use cases are described in Appendix A. Here we discribe the ACs and broadcast domains. Note that the VN-10/VN-20/VUN1 in Figure 1 is the VPNx/VPNy/NIz in Figure 5.

The ethernet segment ES21's ESI is ESI21, the ES21 is attached to MAC-VRF VN-10 via attachment circuit AC1 on PE1, the ES21 is attached to MAC-VRF VN-10 via attachment circuit AC2 on PE2, We assign an End.DX2 SID DX2_AC1 to AC1, and we assign an End.DX2 SID DX2_AC2 to AC2. The ethernet segment ES3's ESI is ESI3, the ES3 is attached to MAC-VRF VN-10 via attachment circuit AC3 on PE3, We assign an End.DX2 SID DX2_AC3 to AC3.

Note that network instance VUN1 is the (virtual) underlay network of VN-10 and VN-20. Because that VN-10 and VN-20 are SRv6 EVPN MAC-VRFs, their underlay network will be the SRv6 network of the GRT.

                       PE1   |                        |
                     +-------+-------+ ------------>  |
               V1P1  |  ___(VN-10)   | VUN1: DX2AGG:: |
    EVC1(V1)   V2P1  | /DX2_AC1  \   | through IGP or |
SN1--O=------=========<       (VUN1) | RT-1 per ES    |  PE3
       \    /  ESI21 | \___      /   |           +----+-----+
        |  |     +   | X   (VN-20)   |           |          |
        X  X     |   +----------+----+           |          |
        |P3|     |              |                |   (VN-10)--+H3
        |  |     |              | | RT2(VN:C-MAC)|    /     |
     V1 |  | V2  | DX2AGG::/96  | | EVI-RT       | (VUN1)   |
        |  |     |              | V ES-Import RT |    \     |
        |  |     |              |                |   (VN-20)--+H4
        |  |     |   +----------+----+           |          |
         \/      +   | X___(VN-10)   |           |          |
         /\    ESI21 | /DX2_AC2  \   |           +----+-----+
SN2--O===--===========<       (VUN1) |                |
    EVC2(V2)   V1P2  | \___      /   |                |
               V2P2  |     (VN-20)   |                |
                     +-------+-------+                |
                       PE2   |                        |
Figure 1: EVPN-lite SRv6 Usecase

We use IMET routes to build a broadcast-list. The broadcast-list is used to forward BUM traffics. The data-plane MAC learning for BUM traffics produces the first batch of C-MAC entries. The subsequent C-MAC entries can be learnt from Unicast traffics and/or BUM traffics. It is clear that we don't use MAC/IP routes to advertise C-MAC entries as usual, that is for fear that the RRs and/or SPEs are overloaded by these C-MACs.

3.2. Solution Overview

3.2.1. Aggregatable End.DX2 SID

When an Ethernet Segment ES21 is attached to an EVI, the attachment-circuit AC1 for that <ESI,EVI> is assigned with an End.DX2 SID. Different ACs of the same ESI are assigned with different End.DX2 SIDs, we call them AC SIDs in this document. But these different End.DX2 SIDs must be able to be aggregated into the same prefix, and this prefix are called as ESI prefix in light-weighted SRv6 EVPNs. The format of aggregatable End.DX2 SIDs is illustrated in the following figure:

    |<---   ESI-Prefix(128-N bits)   ---->|<----     N bits     --->|
    |    Block   |   Node     | ESI.LDV   |          AC-ID          |
    |<------ Locator -------->|<------------- Function ------------>|

Figure 2: End.DX2 SID Formart for Aggregation

Note that the ESI.LDV field is the Local Discreminator Value (LDV) of the ESI (especially the type 3/4/5 ESI). The AC-ID field is the identifier of the AC's EVI. The ESI.LDV field and the AC-ID field are integrated into the End.DX2 SID's Function part.

Note that in "AC-aware bundling service interface" the AC-ID field MUST be the same as the Attachment Circuit ID of [I-D.sajassi-bess-evpn-ac-aware-bundling]. But in other service interfaces the AC-ID field can also be the EGD of that AC's MAC-VRF. Note that the EGD has a global meaning like a global VNI or a PBB I-SID, while the ordinary AC-ID part for an aggregatable End.DX2 SID typically is only a VLAN-ID on that ES.

Note that the ESI IP of an AC is that AC's End.DX2 SID but with a zero AC-ID. The AC SIDs have non-zero AC-IDs, but the ESI-IPs always have zero AC-IDs. Becuase an ESI-IP identifies an ESI, not an AC.

Note that if ESI21 is single-active mode, DX2_AC1 is different from DX2_AC2, but if ESI21 is all-active mode, DX2_AC1 is the same as DX2_AC2, we can call them DX2_SID21 in such case.

3.2.2. The Advertisement of ESI-Prefixes

The ESI-prefixes of DX2_AC1 and DX2_AC2 are defined in Figure 2, and they are called ESI_Prefix1 and ESI_Prefix2 respectively. We can use IGP protocols to advertise these ESI-Prefixes to PE3 respectively in SRv6 underlay. So we don't have to use EAD/ES route or EAD/EVI route in SRv6 EVPN in this section.

Note that the SRv6 SID in IMET route is an End.DT2M SID but with a zero argument length.

Note that if ESI21 is single-active mode, ESI_Prefix1 is different from ESI_Prefix2, but if ESI21 is all-active mode, ESI_Prefix1 is the same as ESI_Prefix2.

Note that when PE1 node fails and the ESI is all active, the PLR node will do underlay anycast FRR switching for DX2_SID21(=DX2_AC2=DX2_AC1). This will bring out fast network convergency.

Note that when the PE-CE link of ESI21 fails, the IGP route of ESI_Prefix1 will be withdrawn, So there will be no steady bypassing for that ES, but a temporary bypassing can be performed to further improve the convergency.

When two ESes are attached to the same redundancy group of PEs, they can share the same anycast SRv6 Locator. In such case, only the common SRv6 Locator is advertised by the underlay network. But they should have different ESI-Prefix. Because that the ESI-SID Aggregation is not recommanded to be activated in order to avoid the steady bypass problems described in Section 5.1.

The detailed comparisons between light-weighted SRv6 EVPN and PBB EVPN over SRv6 is described in Section 6.

3.3. Packet Walkthrough

3.3.1. PE1 forward ARP Request to PE2/PE3

  • When H1 (of SN1) requests H3's ARP, PE1 will receive the ARP Request BUM1 from AC1 of ESI21. PE1 will forward the ARP Request following the broadcast-list of AC1's MAC-VRF VN-10. The broadcast-list is constructed by the IMET routes from PE2 and PE3. The End.DX2 SID of AC1 is named as DX2_AC1.

    PE1 will forward the ARP Request to PE2 and PE3. The inner SMAC of the ARP request is M1 which is H1's MAC address.

  • In this step, PE1 will forward the ARP Request BUM1 to PE2/PE3 with the following SRv6 encapsulation: It's underlay Source IP is the End.DX2 SID (DX2_AC1) on PE1 for the ingress AC; It's underlay Destination IP is the End.DT2M SID (whose argument length is zero) on PE2/PE3.

    Note that the underlay SIP will be the End.DT2U SID (because they don't need any dedicated End.DX2 SIDs) for the single-homed ingress ACs. The multi-homed ingress ACs with single-active behavior may not be assigned with a dedicated ESI-Prefix either. In such situations, the underlay SIP can be the End.DT2U SID too. Note that in such situations, the AC SIDs of all single-active ESIs for the same EVI are aggregated into the same End.DT2U SID.

3.3.2. PE2/PE3's Dataplane MAC Learning

  • When PE2/PE3 receives the ARP Request packet BUM1, they do dataplane MAC learning independently. They will learn that M1 is behind DX2_AC1.

    Note that when PE2 learns that M1 is behind DX2_AC1, it will assume that M1 is behind the local AC (AC2) whose End.DX2 SID (DX2_AC2) is the same as DX2_AC1 too. The local AC may have more higher priority than the remote one.

    After the dataplane MAC learning, the ARP request packet BUM1 is broadcasted to the local ACs, behind one of which is H3.

3.3.3. PE2 Discard ARP Request to H1

  • On receiving BUM1 from PE1, PE2 use the ingress ESI information (DX2_AC1) in BUM1 to determine its ingress ESI-Prefix, When ESI21 is all-active mode and PE2 is about to forward the ARP request to H1, PE2 will find that the AC SID (DX2_AC2) for the outgoing AC (AC2) is of the same ESI-Prefix, so PE2 discards it for ESI loop-free considerations.

    Note that before that ARP Request packet is discarded, its source-MAC can be learnt, especially in "AC-aware bundling service interface". The MAC entry is learnt against DX2_AC1, but it will consider the local sub-interface (of the same AC SID) on that ES as its outgoing interface, in order to avoid unknown-unicast flooding.

    When ESI21 is single-active mode, the outgoing AC may be in blocking state, otherwise its corresponding sub-interface on H1 will take charge of packet-drop behavior instead. So although the AC-SID (DX2_AC2) for the outgoing AC is not the same as DX2_AC1, no loop will arise in the Ethernet Segment.

  • In this step, PE2 can compare the ingress AC-SID of BUM1 and the AC-SID of outgoing AC directly, no SID-to-ESI lookup needed.

3.3.4. PE3 Forward ARP Replay to PE1/PE2

  • When H3 replies to H1 for the ARP request BUM1, PE3 will forward the ARP reply U1 according to the MAC entry M1 learnt previously as above.

    PE3 will forward the ARP reply U1 to PE1 or PE2 according to DX2_AC1's SRv6 locator's IGP route.

    When ESI21 is all-active mode, DX2_AC1 will be the same as DX2_AC2, in such case, we call both of them DX2_SID21 instead. The traffics to M1 will be load-balanced between PE1 and PE2. Because that DX2_SID21's locator is advertised by both PE1 and PE2 in the underlay IGP protocol.

  • In this step, PE3 will forward the ARP reply U1 to PE1 with the following SRv6 encapsulation: It's underlay Source IP is the End.DX2 SID on PE3 for AC3; It's underlay Destination IP is the End.DX2 SID (DX2_AC1) on PE1 for AC1 according to the MAC entry M1.

    Note that if the DIP is just the anycast node SID of PE1 and PE2, when the PE-CE link of ESI21 fails, the traffic will be steadily bypassed untill that link recovers again. That's why MAC-entries should be learnt against AC-SIDs.

3.3.5. PE1 Forward ARP Replay to H1

  • When PE1 receives the ARP reply packet U1 from PE3, PE1 first match the packet to its MAC-VRF VN-10 by U1's destination End.DX2 SID. And PE1 will not discard it because the egress AC's AC-SID is not the same as the ingress AC-SID (which is represented by U1's source IP).

  • In this step, When PE1 receives the SRv6 encapsulated ARP reply packet U1 from PE3, PE1 first match the packet to the End.DX2 SID of AC1 by DIP, then match the packet to AC1's MAC-VRF VN-10.

4. Decapsulation Optimizations

4.1. Decapsulation Aggregation

We want to decapsulation the packets destining to different ESIs for the same EVI using the same forwarding entry. In order to achieve this benefit, we can use an AC's EVI's EGD as that AC's AC SID's AC-ID.

These AC SIDs are aggregatable End.DX2 SIDs, so we can consider the ESI prefix aggregated from these End.DX2 SIDs as a new SRv6 function called End.DX2AGG SID, The format of the End.DX2AGG SID is illustrated in the following figure:

    |<------ Locator -------->|<- FUNC -->|<------ Arg.ACI -------->|
    |    Block   |   Node     | ESI.LDV   |        EGD            |L|

Figure 3: End.DX2AGG SID Format

Note that whether these SIDs are considered as lots of End.DX2 SIDs or are considered as a single End.DX2AGG SID with different arguments, it is just a local matter of their PE node's independent choice, other PEs of the same EVI won't be aware of the difference of these two implementations.

A SID with the End.DX2AGG function is called as an "ESI SID" in this document. The ESI's ESI-Prefix is the locator and fuction part of its corresponding ESI SID. The argument part of the ESI SID is the AC-ID for the corresponding AC's End.DX2 SID. The AC-ID plus the ESI.LDV works like the function part of an End.DX2 SID. The argument part of an ESI SID is called as Arg.ACI in this document.

Note that the Arg.ACI comprises EGD (EVPN Global Discreminator) and L bit. The EGD identifies the EVI of that AC. When that AC is a leaf AC, the L bit is 1, otherwise the L bit is 0.

Note that when AC-ID is the EGD, PE2 can still decapsulate the packet following the End.DX2 function or following the End.DX2AGG function. It is just a local matter, while the End.DX2AGG function can reduce the decapsulation forwarding entries. But when AC-ID is that AC's VLAN-IDs, PE2 have to decapsulate the packet following the End.DX2 function.

4.2. End.DX2AGG Function and Arg.ACI

The "Endpoint with decapsulation and Aggregated L2 table forwarding" behavior (End.DX2AGG for short) is a variant of the End.DX2 behavior.

Two of the applications of the End.DX2AGG behavior are the EVPN VPLS [RFC7432] and the EVPN ETREE [RFC8317] use-cases.

Any SID instance of this behavior is associated with an ESI E. The behavior also takes an argument: "Arg.ACI". This argument provides a local mapping to an EVI V. The outgoing interface corresponds to <ESI E, EVI V> is OIF, and the EVI V's bridge table is L2 Table T .

The End.DX2AGG SID MUST be the last segment in a SR Policy.

When N receives a packet whose IPv6 DA is S and S is a local End.DX2AGG SID, the processing is identical to the End.DT2U behavior except for the Upper-layer header processing which is as follows:

   S01. If (Upper-Layer Header type == 143(Ethernet) ) {
   S02.    Remove the outer IPv6 Header with all its extension headers.
   S03.    Determine the L2 Table T using Arg.ACI.
   S04.    Learn the exposed MAC Source Address in L2 Table T.
   S05.    Find out the OIF, Forward the Ethernet frame to the OIF.
   S06. } Else {
   S07.    Process as per Section 4.1.1 of [RFC8986].
   S08. }

Note that the OIF can be found out using the MAC-entries in L2 Table T, when the EVI V is an E-LAN service.

5. Advanced Considerations

5.1. ESI SID Aggregation

There are obvious difference between "Route Aggregation" and "SID Aggregation" for an ESI. The "ESI Route Aggregation" is that different End.DX2AGG SIDs are advertised by underlay protocols in a common SRv6 locator, but different ESIs still have different End.DX2AGG SIDs. The "ESI SID Aggregation" is that different ESIs use the same SRv6 SID.

Note that the "ESI Route Aggregation" is recommanded as long as it is possible, but the "ESI SID Aggregation" can only be used under certain restraints.

When two ESes are attached to the same redundancy group of PEs, they can share the same SRv6 SID. But this will bring out some issues too. One of these issues is that they may be attached to different groups of PEs in the future. Another issue is that when only one of the ESes fails, that common SRv6 SID can't be withdrawn by that PE, so the steady bypass of that ES arises immediately after its failture on that PE. If these issues are not so important in some scenarios, The ESI-SID Aggregation may be activated. This is an option.

Note that when ESI SID Aggregation is activated, the local-bias ES split-horizon procedures or its variations should be used.

Note that ESI SID Aggregation works well with single-active ESIs (see Section 3.3), its steadby bypassing problem will arise with all-active ESIs only.

Note that the sub-interfaces of the same ESI may be assigned with different End.DX2 SIDs, and these End.DX2 SIDs can be aggregated into a common prefix, this common prefix is assigned with that ESI. In such case, only the common prefix should be advertised before any of the sub-interfaces fails. But this is not considered as "ESI SID Aggregation", this is "ESI Route Aggregation".

5.2. ESI/AC SID Advertisement Optimization

5.2.1. Advertise ESI-Locators in Underlay Network

The End.DX2AGG SIDs can be advertised as an IP prefix in underlay IGP protocols. Although it is the aggregation of many AC SIDs, the ESI SIDs may still be too many for the underlay network. And the core routers who are service-agnostic have to install these ESI prefixes.

In order to solve these problems, only the anycast SRv6 locators (say ESI-Locators) of such ESI prefixes should be advertised in the underlay network.

Note that in such case the ESI/AC SID typically don't have to be advertised by EVPN routes in overlay network, unless some special features (i.e. unequal load-balance) should be providered together.

5.2.2. Using EAD/EVI Route to Advertise AC SIDs

When the EAD/EVI routes here are used to advertise AC SIDs, the End.DX2 SIDs are advertised in their SRv6 L2 Service TLVs, not in their next hops. Their next hops will be the node SID of the advertising PE.

In such case, the EAD/EVI routes will be installed as overlay routes, and the AC SIDs learnt in the MAC entries is treated as the overlay indexes for recursion.

In all-active mode, when an AC of a <ESI, EVI> fails on one PE, all other PEs of that <ESI, EVI> should use EAD/EVI route to advertise its AC SID.

5.2.3. Using EAD/ES Route to Advertise ESI SIDs

In section 6.1.1 of [I-D.ietf-bess-srv6-services], the SRv6 L2 Service TLVs of EAD ES routes just carry the Arg.FE2 infomations. Here the SRv6 L2 Service TLVs of EAD ES routes carry the ESI SIDs.

EAD/ES routes will be advertised/imported for EVIs but they should be installed into Global Routing Table (GRT). Because there isn't a dedicated B-component in EVPN-lite SRv6 like that in PBB VPLS and PBB EVPN. The GRT plays a B-Component role in EVPN-lite SRv6.

Note that the EAD/ES routes won't be installed as overlay routes like the EAD/EVI routes, because that we want to reduce the forwarding table consumption.

Although ESI SIDs are installed into GRT, they are awared only on PE nodes, the transit nodes in underlay network won't be aware of ESI SIDs (they may aware the locators of these SIDs) in order to reduce the FIB consumption.

Note that when the EAD/ES route here is used to advertise ESI SID, the End.DX2AGG SID is advertised in its SRv6 L2 Service TLV, not in its nexthop. Its nexthop will be the node SID of the advertising PE.

Note that in such case, the SRv6 source IP in the dataplane should be set to the entire AC SID of the ingress AC, not just the ESI IP whose AC-ID part is zero.

5.2.4. The Reduction of EAD/EVI Routes

In order to solve the problem described in Section 2.6, we may have to advertise AC SIDs in the overlay network. But the amount of AC SIDs may be hundreds of times larger than ESI SIDs. It is necessary for the light-weighted SRv6 EVPNs to reduce the advertisement of AC SIDs.

The AC SID of a specified <ESI,EVI> will not be advertised by its PEs, until these PEs know that the <ESI,EVI> fails on at least one of them.

Note that the entire AC SID for that <ESI,EVI> can be used as the source IP of the SRv6 encapsulation before that AC SID is advertised via EVPN routes. Because that when a MAC is learnt over that AC SID, the packet for that MAC can also be forwarded according to the ESI-Prefix or ESI-Locator of the corresponding ESI SID due to the longest match procedures of IP lookup. AC-DF per EVI Mode for Light-Weighted EVPNs

When the EAD/EVI routes are not advertised, the AC-influenced DF-Election per [RFC8584] can't work. So the AC-DF per EVI procedures are required. The AC-DF per EVI procedures includes two steps. The first step is the AC-DF per EVI capability negotiation procedure, and the second step is the AC-DF per EVI DF-election procedure.

The Capability negotiation procedures and the DF-Election procedures follow []. On Receiving Reverse EAD/EVI Routes

In all-active mode, when a PE X receives a reverse EAD/EVI route ([]), that PE x can use nomal EAD/EVI route to advertise its local AC SID of that <ESI,EVI>.

Note that no EAD/EVI route have to be advertised before receiving the corresponding reverse EAD/EVI routes. This can greatly reduce the amount of EAD/EVI routes.

5.3. Unequal LB Advertisement

When the ESI SIDs are advertised by EVPN routes for the overlay network according to Section 5.2.2, we can advertise the EVPN Link Bandwidth extended community (see [I-D.ietf-bess-evpn-unequal-lb]) along with the ESI SIDs using EAD/ES routes.

Note that these extra information (which are advertised along with the EVPN routes) are awared by the PEs only. The underlay network don't have to be aware of it.

Note that when the EVPN Link Bandwidth extended community is advertised along with the ESI SID, The nexthop of the EAD/ES route should not be set to the anycast ECMP Node SID of the advertising PE (egress-PE). On receiving such EAD/ES route, the ingress PE may push this EAD/ES route's nexthop onto the End.DX2AGG/End.DX2 SID when constructing the SID stack, if unequal-LB is required.

Note that the association between an ESI SID and its corresponding Node SID is also advertised by EAD/ES routes. In such case, when the ESI SIDs are used as destination IP addresses, they should be hiden in the SRH behind the node SID of the corresponding egress PE router. This need to be encapsulated under the help of EAD/ES routes of overlay network. So the ESI SIDs must be advertised in overlay network in such case.

Although these ESI SIDs (that are used as destination IP addresses to PE X) won't be exposed untill data packets reached the egress PE X, the ESI-Locator of them should also be advertised in underlay network because that their corresponding AC SIDs will be encapsulated as source IPs for some other data packets whose ingress PE is PE X. and these source IPs may be checked by underlay URPF (Unicast Reverse Path Forwarding) procedures.

5.4. AC-aware Bundling Service Interface

In AC-aware bundling service interface, Attachment Circuit ID extended community ([I-D.sajassi-bess-evpn-ac-aware-bundling]) or ACI-specific SOI extended community ([]) should be used in ARP/ND synchronization.

Note that each VLAN of the same AC of the same MAC-VRF will have the same End.DX2 SID,

Note that in "AC-aware bundling service interface", the AC-ID inside that DX2_AC1 can help the MAC entry to be installed for the correct outgoing interface. Such MAC entry is called as the synced MAC entry.

Note that the MAC enties which are learnt against a DX2-SID should have low preference than which are received over a RT-2 route, when they are installed to the MAC table.

5.5. C-MAC Flush Notification Procedure

The withdraw of an ESI/AC SID Advertisement (as an overlay route) can (if it is the only advertisement of that ESI/AC SID at that time) be used as C-MAC (which was learnt against that ESI/AC SID) flush notification.

Note that in single active mode, the ESI-Prefixes of DX2_AC1 and DX2_AC2 are different, so each withdraw of DX2_AC1 or DX2_AC2 will be for the single advertisement of that SID.

When "AC-DF per EVI" (Section is used, the reverse EAD/EVI routes can be used to trigger C-MAC flush for specified AC SIDs. In such case, these reverse EAD/EVI routes should not use EVI-RT format to carry their EVI's route-target. Because that EVI-RT format is not visible to RT constraints mechanism.

5.6. E-Tree Support Considerations

E-tree Supprot extensions is similar to [RFC8317] section 5 except for the following notable differences: The leaf B-MACs are replaced by leaf ESI-SIDs, the root B-MACs are replaced by root ESI-SIDs. The PBB encapsulation is replaced by SRv6 encapsulation, the B-component is replaced by the underlay GRT. The B-MAC Advertisement Route is replaced by EAD/EVI route or EAD/ES Route.

As illustrated in Figure 3, the root AC-SID and leaf AC-SID of the same AC can be considered as the same ESI-SID with different Arg.ACI. Even the EGD part of their Arg.ACIs are the same EGD, only the L bit of their Arg.ACIs are different. The L bit of the leaf AC-SID is set to 1. The L bit of the root AC-SID is set to 0.

On the ingress PE, when the L bit of the destination SID for the DMAC of a data packet is 1, and that data packet's ingress AC is a leaf AC, that data packet should be dropped.

5.7. MAC-Synchronization in All-Active Mode

When a host H1 of subnet SN1 sends an ARP Request REQ_P1, then REQ_P1 will be forwarded by EVC1 to either PE1 or PE2, not to the both. But when H3 send an ARP Reply REP_P2 to H1, then PE3 may load-balance REP_P2 to either PE1 or PE2, not to the both.

When REQ_P1 is load-balanced (see Appendix A.2.1.1) by EVC1 to PE1, not to PE2, but PE3 load-balance REP_P2 to PE2, The MAC entry of H1 would not have been prepared on PE2 for REP_P2. So the fowarding of REP_P2 will follow the unknown-unicast procedures.

PE1 MUST use RT-2 route RT2S (RT-2 for Synchronization only) to advertise the MAC/IP entry of H1 to other PEs (e.g. PE2) on ES21. These RT-2 routes should be advertised along with an EVI-RT ([I-D.ietf-bess-evpn-igmp-mld-proxy]) and an ES-Import RT.

When PE2 receives RT2S, the MAC entry Mx should be installed with AC2 as its actual outgoing-interface. When PE3 receives RT2S, RT2S MUST not be imported into VN-10 because that the ES-Import RT of RT2S can be resolved to a local ES of PE3.

As a result of that, the synchronized MAC entries will not be imported by their external remote PEs, they are imported just by their internal remote PEs.

The IP address field of NLRI of RT2S can be set to H1's IP address, which is obtained through ARP snooping. This IP address can be used to trigger ARP probing when PE1 fails.

When C-MAC Mx is aged out by PE1, the RT2S MUST be withdrawn, thus PE2's MAC entry of Mx will be deleted. In such case, ARP probing for Mx should not be triggered in order not to hold a MAC entry for Mx when Mx will not connect to others for a long time.

Note that in other light-weighted EVPNs, the VUN1 may be a backbone-VPLS (B-VPLS), in such case, the IP address field of NLRI can be used to distinguish the RT-2 routes of C-MACs from the RT-2 routes of B-MACs.

5.8. EVPN IRB Support Considerations

The dataplane in this draft is no more complex than typical SRv6 EVPN. So it will work as efficient as we should expect in SRv6 EVPN IRB usecase.

5.8.1. EVPN IRB Extended Mobility

In EVPN IRB usecase, [I-D.ietf-bess-evpn-irb-extended-mobility] defines some optional extensions to support some specific IRB usecases. In these specific IRB usecases, the <MAC,IP> bindings will change across VM-moves. These extensions can't be applied to light-weighted EVPNs, just like they can't be applied to PBB EVPNs either.

5.8.2. Anycast IRB interfaces

When an EVPN IRB interface (on PE1) ping a host H1, the corresponding ICMP Echo Request will be delivered to host H1, whether host H1 is PE1's local host or not . but if that IRB interface is an anycast IRB interface, and host H1 is a local host of PE2 (not of PE1), naturally the Echo Reply for that Echo Request will be delivered to the nearest anycast IRB interface on PE2 (not on PE1) only. Constructing GW-list

The MAC/IP of an anycast IRB interface should be advertised along with a Default Gateway Extended Community.

The PEs in which resides the anycast IRB interface of a subnet forms the "GW-list" of that subnet. The "GW-list" of a BD can be constructed from such MAC/IP routes (with Default Gateway extended community of corresponding subnet). Flooding over GW-list

Echo Replies received by any of the anycast IRB interfaces MUST be flooded over the GW-list of that BD. So that the PE which originated the previous Echo Request can receive the synced Echo Replies.

Note that the Echo Replies between two hosts of that BD will not be flooded, because that they will not be received by any of the anycast IRB interfaces.

6. Comparison with Other Solutions

6.1. Detailed Comparisons with PBB EVPN over SRv6

The "PBB EVPN over SRv6 underlay" solution will be complex, if we address too much things to it. I have some examples in the following:

  • The upper-layer header for SRv6 is the PBB-header for B-MACs, not the ethernet header for C-MACs, so the SID list (SR-Path or network programming Instructions) in the SRH can't be constructed for the sake of the I-Component. For example, when a SRv6 SID for MAC-guarding (or something else, just an example) present in the SRH for PBB EVPN SRv6, I think it means BMAC-guarding, no C-MAC guarding.

  • The B-MACs for the all-active ESIs can't be aggregated, but the SRv6 SIDs for ESIs can be aggregated. The underlay can advertise the ESI-Locators only, so the burden of the underlay network may not be increased too much. When the underlay routes is aggregated, the C-MACs can also be learnt against /128 source-IP, it is the advantage of a light-weighted SRv6 EVPN, which can't be gained from a PBB header.

  • The B-MACs are for overlay protection (the real overlay is the I-VPLS, but the B-VPLS is also an overlay network from the viewpoint of the SRv6 network). But the SRv6 SIDs for ESIs will be for underlay protection, it works like the egress protection. They are two different types of protecting solutions.

  • Light-weighted SRv6 EVPN can support AC-influenced DF Election, but PBB EVPN over SRv6 can't.

  • Although PBB EVPN can be transplanted into SRv6 networks along with the PBB header (say PBB EVPN over SRv6), It seems to be more complicated to me. Take the EVPN IRB usecases for example, that requires seven sequences of header processing, like (SRv6/B-MAC/C-MAC)(Inner-IP)(C-MAC/B-MAC/SRv6), during the overlay L3 forwarding. I think it will be horrible enough for some ASICs to implement it. When the processing is simplified as (SRv6/C-MAC)(Inner-IP)(C-MAC/SRv6), it sounds like a step forward, not backward, IMHO. We can achieve this goal easily inside the EVPN framework, only if the data-plane learning can still be considered as an option after PBB EVPN.

Fortunately, SRv6 is just too young to have a transplantation of PBB EVPN. So it will waste nothing for the SRv6 nodes to give up the PBB header that is never used by these SRv6 nodes. Note that the SRv6 functions (End.DT2U and End.DT2M) for L2VPNs have source-IP-based data-plane learning for a long time already.

Although the extensions in [I-D.ietf-bess-evpn-irb-extended-mobility] can't be applied to PBB EVPNs or light-weighted EVPNs. This will not prevent PBB EVPNs and light-weighted EVPNs from supporting typical IRB use-cases. Note that these extensions are optional.

6.2. Detailed Comparisons with Anycast Node SID

Note that SRv6 Anycast Node SID is the ultimate aggregation of ESI SIDs. Such ESI SID aggregation will have some problems as described in Section 5.1.

7. Security Considerations

Security considerations will be added in future versions.

8. IANA Considerations

8.1. End.DX2AGG SID

IANA is requested to allocate a new code points for the new SRv6 Endpoint Behaviors defined in this document.

| Type | Description | Reference     |
| TBD1 | End.DX2AGG  | This Document |

Figure 4: End.DX2AGG

9. Acknowledgements

The authors would like to thank the following for their comments and review of this document:

Ye Shu.

10. Normative References

Sajassi, A., Thoria, S., Mishra, M. P., Drake, J., and W. Lin, "IGMP and MLD Proxy for EVPN", Work in Progress, Internet-Draft, draft-ietf-bess-evpn-igmp-mld-proxy-12, , <>.
Malhotra, N., Sajassi, A., Rabadan, J., Drake, J., Lingala, A., and S. Thoria, "Weighted Multi-Path Procedures for EVPN Multi-Homing", Work in Progress, Internet-Draft, draft-ietf-bess-evpn-unequal-lb-14, , <>.
Dawra, G., Filsfils, C., Talaulikar, K., Raszuk, R., Decraene, B., Zhuang, S., and J. Rabadan, "SRv6 BGP based Overlay Services", Work in Progress, Internet-Draft, draft-ietf-bess-srv6-services-07, , <>.
Sajassi, A., Brissette, P., Mishra, M., Thoria, S., Rabadan, J., and J. Drake, "AC-Aware Bundling Service Interface in EVPN", Work in Progress, Internet-Draft, draft-sajassi-bess-evpn-ac-aware-bundling-04, , <>.
Wang, Y., "AC-Influenced DF Election per EVI", Work in Progress, Internet-Draft, draft-wang-bess-evpn-ac-df-per-evi-00, , <>.
Wang, Y., "Ethernet Tag ID Usage Update for Ethernet A-D per EVI Route", Work in Progress, Internet-Draft, draft-wang-bess-evpn-ether-tag-id-usage-03, , <>.
Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A., Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, , <>.
Sajassi, A., Ed., Salam, S., Bitar, N., Isaac, A., and W. Henderickx, "Provider Backbone Bridging Combined with Ethernet VPN (PBB-EVPN)", RFC 7623, DOI 10.17487/RFC7623, , <>.
Sajassi, A., Ed., Salam, S., Drake, J., Uttaro, J., Boutros, S., and J. Rabadan, "Ethernet-Tree (E-Tree) Support in Ethernet VPN (EVPN) and Provider Backbone Bridging EVPN (PBB-EVPN)", RFC 8317, DOI 10.17487/RFC8317, , <>.
Rabadan, J., Ed., Mohanty, S., Ed., Sajassi, A., Drake, J., Nagaraj, K., and S. Sathappan, "Framework for Ethernet VPN Designated Forwarder Election Extensibility", RFC 8584, DOI 10.17487/RFC8584, , <>.
Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer, D., Matsushima, S., and Z. Li, "Segment Routing over IPv6 (SRv6) Network Programming", RFC 8986, DOI 10.17487/RFC8986, , <>.

11. Informative References

Malhotra, N., Sajassi, A., Pattekar, A., Rabadan, J., Lingala, A., and J. Drake, "Extended Mobility Procedures for EVPN-IRB", Work in Progress, Internet-Draft, draft-ietf-bess-evpn-irb-extended-mobility-05, , <>.
Wang, Y. and R. Chen, "EVPN Egress Protection", Work in Progress, Internet-Draft, draft-wang-bess-evpn-egress-protection-04, , <>.
Balus, F., Ed., Sajassi, A., Ed., and N. Bitar, Ed., "Extensions to the Virtual Private LAN Service (VPLS) Provider Edge (PE) Model for Provider Backbone Bridging", RFC 7041, DOI 10.17487/RFC7041, , <>.
Mahalingam, M., Dutt, D., Duda, K., Agarwal, P., Kreeger, L., Sridhar, T., Bursell, M., and C. Wright, "Virtual eXtensible Local Area Network (VXLAN): A Framework for Overlaying Virtualized Layer 2 Networks over Layer 3 Networks", RFC 7348, DOI 10.17487/RFC7348, , <>.
Sajassi, A., Ed., Drake, J., Ed., Bitar, N., Shekhar, R., Uttaro, J., and W. Henderickx, "A Network Virtualization Overlay Solution Using Ethernet VPN (EVPN)", RFC 8365, DOI 10.17487/RFC8365, , <>.

Appendix A. Explanation for Physical Links of the Use-cases

There are three PEs, two L2NEs (Layer 2 Network Elements) and five L3NEs (Layer 3 Network Elements) in abobe network. The PEs are PE1, PE2 and PE3. The L2NEs are L2NE1 and L2NE2. The L3NEs are N1/N2/N3/N4/N5. They are all illustrated in Figure 5.

There are 9 physical links among these 10 physical devices as illustrated in Figure 5. These physical links are called as PLi (i=1,2...8). The two physical ports of the same physical link PLi are both called as Pi (i=1,2...8).

As illustrated in Figure 5, some of these physical ports may have subinterfaces. When a subinterface's VLAN ID is j and it is physical port Pi's subinterface, that subinterface is called as Pi.j. For example, P1.2 is a subinterface of physical port P1 and its VLAN ID is 2.

There are three NIs (Network Instances) among PE1, PE2 and PE3. They are VPNx, VPNy and NIz. Two subinterfaces are attached to VPNx, they are P1.1 and P2.1. Other two subinterfaces are attached to VPNy, they are P1.2 and P2.2. N3 is also attched to VPNx, while N5 is also attached to VPNy.

There are two EVCs (Ethernet Virtual Connections) between L2NE1 and L2NE2, they are EVC1 and EVC2. The L2NE1's EVC1 instance (which is illustrated as the "O" on L2NE1) have three member interfaces, they are P4, P1.1 and P3.1, where P3.1 and P1.1 are of the same protection-group. The L2NE2's EVC1 instance have two member interfaces, they are P3.1 and P2.1. The L2NE2's EVC2 instance (which is illustrated as the "O" on L2NE2) have three member interfaces, they are P5, P2.2 and P3.2, where P3.1 and P1.1 are of the same protection-group. The L2NE1's EVC2 instance have two member interfaces, they are P3.2 and P1.2. The L2NE2's EVC1 instance and L2NE1's EVC2 instance are both CCC (Circuit Cross Connection) local connections.

VPNx and VPNy are associated to NIz on each PE.

A.1. Failure Detections for P1.2 (or P2.1)

There is a CFM session CFM1 between P1.2 of PE1 and L2NE2's P3.2, when physical port P3 fails, the CFM session CFM1 will go down. There is a CFM session CFM2 between P2.1 of PE2 and L2NE1's P3.1, when physical port P3 fails, the CFM session CFM2 will go down.

A.2. Protection Approaches for N1 (or N2)

A.2.1. CCC-Approaches

The L2NE1's EVC1 instance and L2NE2's EVC2 instance are both CCC local connections too. In L2NE1's EVC1 instance, P1.1 and P3.1 are of the same protection-group PG1. In L2NE2's EVC2 instance, P2.2 and P3.2 are of the same protection-group PG2. In PG1, both P1.1 and P3.1 will receive data packets. In PG2, both P2.2 and P3.2 will receive data packets.

A.2.1.1. CCC Active-Active Protection

L2NE1 (or L2NE2) will load-balance N1's (N2's) data packets between P1.1 and P3.1 (or P2.2 and P3.2).

A.2.1.2. CCC Active-Standby Protection

In PG1, P1.1 is the active path, P3.1 is the backup path. In PG2, P2.2 is the active path, P3.2 is the backup path.

That's saying that L2NE1 (or L2NE2) will not send N1's (or N2's) data packets over P3.1 (or P3.2), unless P1.1 (or P2.2) or P1 (or P2) has been in failure before that data forwarding.

A.2.2. VSI-Approaches

L2NE1's EVC2 instance and L2NE2's EVC1 instance are both VSI instances in this case. P1.1, P3.1, P2.2 and P3.2 are all individual ACs in these VSIs.

Note that L2NE2's EVC1 instance and L2NE1's EVC2 instance are still both CCC local connections in this case, and there is no PG1 or PG2 in this case, and there are no PWs in this case.

Authors' Addresses

Yubao Wang
ZTE Corporation
No.68 of Zijinghua Road, Yuhuatai Distinct
Ran Chen
ZTE Corporation
No. 50 Software Ave, Yuhuatai Distinct