PBB-EVPN ISID-based CMAC-Flush

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Table of Contents

1. Introduction

[RFC7623] defines how Provider Backbone Bridging (PBB) can be combined with Ethernet VPN (EVPN) to deploy ELAN services in very large MPLS networks. [RFC7623] also describes how Single-Active Multi-homing and per-I-SID Load-Balancing can be provided to access devices and aggregation networks. When Access Ethernet/MPLS Networks exists, [I-D.ietf-bess-evpn-virtual-eth-segment] describes how virtual Ethernet Segments can be associated to a group of Ethernet Virtual Circuits (EVCs) or even Pseudowires (PWs). In order to speed up the network convergence in case of failures on Single-Active Multi-Homed Ethernet Segments, [RFC7623] defines a CMAC-flush mechanism that works for different Ethernet Segment BMAC address allocation models.¶

In some cases, the administrative entities that manage the access devices or aggregation networks do not demand Multi-Homing Ethernet Segments (ES) from the PBB-EVPN provider, but simply multiple single-homed ES. If that is the case, the PBB-EVPN network is no longer aware of the redundancy offered by the access administrative entity. Figure 1 shows an example where the PBB-EVPN network provides four different Attachment Circuits (ACs) for I-SID1, with those ACs not being part of any ES or vES (therefore they are referred to as null vES).¶

     <----G.8032--><--PBB-EVPN Network---><----MPLS---->
          Access          MPLS                Access
           Ring                               Network
     I-SID1    ESI +-----+         +-----+
     +----+    null| PE1 |---------| PE3 |
     |CE1 |--------|BMAC1|         |BMAC3| ESI null
     +----+  active|     |         |     |----+ PW
       |           +-----+         +-----+     \active
       |             |                 |        \  +----+
       |             |                 |         ==|CE3 |I-SID1
       |             |                 |        /  +----+
       |stb    ESI +-----+         +-----+     / PW
     +----+    null| PE2 |         | PE4 |----+ standby
     |CE2 |--------|BMAC2|         |BMAC4| ESI null
     +----+  active|     |---------|     |
     I-SID1        +-----+         +-----+

In the example in Figure 1, CE1, CE2 and CE3 are attached to the same service, identified by I-SID1, in the PBB-EVPN PEs. CE1 and CE2 are connected to the PEs via G.8032 Ethernet Ring Protection Switching, and their ACs to PE1 and PE2 are represented by a port and VLAN identifier. CE3 is dual-homed to PE3 and PE4 through an active-standby PW, and its AC to the PEs is represented by a PW. Each of the four PEs uses a dedicated BMAC address as source MAC address (BMAC1, BMAC2, BMAC3 and BMAC4, respectively) when encapsulating customer frames in PBB packets and forwarding those PBB packets to the remote PEs as per [RFC7623]. There are no multi-homed Ethernet Segments defined in the PBB-EVPN network of the example, that is why the four ACs in Figure 1 show the text "ES null", which means the Ethernet Segment Identifier on those ACs is zero. Since there are no multi-homed ES defined, the PEs keep their ACs active as long as the physical connectivity is established and the CEs are responsible for managing the redundancy, avoiding loops and providing per-I-SID load balancing to the PBB-EVPN network.¶

For instance, CE2 will block its link to CE1 and CE3 will block its forwarding path to PE4. In this situation, a failure in one of the redundant ACs will trigger the CEs to start using their redundant paths, however those failures will not trigger any CMAC-flush procedures in the PEs that implement [RFC7623], since the PEs are not using the PBB-EVPN multi-homing procedures. For example, if the active PW from CE3 (to PE3) fails, PE3 will not issue any CMAC-flush message and therefore the remote PEs will continue pointing at PE3's BMAC to reach CE3's CMACs, until the CMACs age out in the I-SID1 forwarding tables.¶

[RFC7623] provides a CMAC-flush solution based on a shared BMAC update along with the MAC Mobility extended community where the sequence number is incremented. However, the procedure is only used along with multi-homed Ethernet Segments. Even if that procedure could be used for null Ethernet Segments, as in the example of Figure 1, the [RFC7623] CMAC-flush procedure would result in unnecessary flushing of unaffected I-SIDs on the remote PEs, and subsequent flooding of unknown unicast traffic in the network.¶

This document describes an extension of the [RFC7623] CMAC-flush procedures, so that in the above failure example, PE3 can trigger a CMAC-flush notification that makes PE1, PE2 and PE4 flush all the CMACs associated to PE3's BMAC3 and (only) I-SID1. This new CMAC-flush procedure explained in this document will be referred to as "PBB-EVPN I-SID-based CMAC-flush" and can be used in PBB-EVPN networks with null or non-null (virtual) Ethernet Segments.¶

2. Solution requirements

The following requirements are followed by the CMAC-flush solution described in this document:¶

1. The solution solves black-hole scenarios in case of failures on null ES ACs (Attachment Circuits not associated to ES, that is, ESI=0) when the access device/network is responsible for the redundancy.¶
2. This extension works with Single-Active non-null ES and virtual ES, irrespective of the PE BMAC address assignment (dedicated per-ES BMAC or shared BMAC, as in [RFC7623]).¶
3. In case of failure on the egress PE, the solution provides a CMAC-flush notification at BMAC and I-SID granularity level.¶
4. The solution provides a reliable CMAC-flush notification in PBB-EVPN networks that use Route-Reflectors (RRs), without causing "double flushing" or no flushing for certain I-SIDs due to the notification messages being aggregated at the RR.¶
5. The solution coexists in [RFC7623] networks where there are PEs that do not support this specification.¶
6. The solution SHOULD be enabled/disabled by an administrative option on a per-PE and per-I-SID basis.¶

3. EVPN BGP Encoding for ISID-based CMAC-flush

The solution does not use any new BGP attributes but reuses the MAC Mobility extended community as an indication of CMAC-flush (as in [RFC7623]) and encodes the I-SID in the Ethernet Tag field of the EVPN MAC/IP advertisement route. As a reference, Figure 2 shows the MAC Mobility extended community and the EVPN MAC/IP advertisement route that are used specified in [RFC7432] and used in this document as a CMAC-flush notification message.¶

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=0x06     | Sub-Type=0x03 |   Flags       |   Reserved=0  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       Sequence Number                         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            +---------------------------------------+
            |  RD                                   |
            +---------------------------------------+
            |  ESI = 0                              |
            +---------------------------------------+
            |  Ethernet Tag ID = I-SID              |
            +---------------------------------------+
            |  MAC Address Length = 48              |
            +---------------------------------------+
            |  BMAC Address                         |
            +---------------------------------------+
            |  IP Address Length = 0                |
            +---------------------------------------+
            |  MPLS Label1                          |
            +---------------------------------------+

Where:¶

• The route's RD and RT are the ones corresponding to its EVI. Alternatively to the EVI's RT, the route MAY be tagged with an RT auto-derived from the Ethernet Tag (I-SID) instead. [RFC7623] describes how the EVPN MAC/IP Advertisement routes can be advertised along with the EVI RT or an RT that is derived from the I-SID.¶
• The Ethernet Tag encodes the I-SID for which the PE that receives the route must flush the CMACs upon reception of the route.¶
• The MAC address field encodes the BMAC Address for which the PE that receives the route must flush the CMACs upon reception of the route.¶
• The MAC Mobility extended community is used as in [RFC7623], where a delta in the sequence number between two updates for the same BMAC/I-SID will be interpreted as a CMAC-flush notification for the corresponding BMAC and I-SID.¶

All the other fields are set and used as defined in [RFC7623]. This document will refer to this route as the BMAC/I-SID route, as opposed to the [RFC7623] BMAC/0 route (BMAC route sent with Ethernet Tag ID = 0).¶

Note that this BMAC/I-SID route will be accepted and reflected by any [RFC7432] RR, since no new attributes or values are used. A PE receiving the route will process the received BMAC/I-SID update only in case of supporting the procedures described in this document.¶

4. Solution description

Figure 1 will be used in the description of the solution. CE1, CE2 and CE3 are connected to ACs associated to I-SID1, where no (Multi-Homed) Ethernet Segments have been enabled, and the ACs and PWs are in active or standby state as per Figure 1.¶

Enabling or disabling I-SID-based CMAC-flush SHOULD be an administrative choice on the system that MAY be configured per I-SID (I-Component). When enabled on a PE:¶

1. The PE will be able to generate BMAC/I-SID routes as CMAC-Flush notifications for the remote PEs.¶
2. The PE will be able to process BMAC/I-SID routes received from remote PEs.¶

When I-SID-based CMAC-flush is disabled, the PE will follow the [RFC7623] procedures for CMAC-flush.¶

This CMAC-flush specification is described in three sets of procedures:¶

• I-SID-based CMAC-flush activation¶
• CMAC-flush notification generation upon AC failures¶
• CMAC-flush process upon receiving a CMAC-flush notification¶

5. Conclusions

The I-SID-based CMAC-flush solution described in this document has the following benefits:¶

1. The solution solves black-hole scenarios in case of failures on null ES ACs, since the CMAC-flush procedures are independent of the Ethernet Segment definition.¶
2. This extension can also be used with Single-Active non-null ES and virtual ES, irrespective of the PE BMAC address assignment (dedicated per-ES BMAC or shared BMAC).¶
3. It provides a CMAC-flush notification at BMAC and I-SID granularity level, therefore flushing a minimum number of CMACs and reducing the amount of unknown unicast flooding in the network.¶
4. It provides a reliable CMAC-flush notification in PBB-EVPN networks that use RRs. RRs will propagate the CMAC-flush notifications for all the affected I-SIDs and irrespective of the order in which the notifications make it to the RR.¶
5. The solution can coexist in a network with systems supporting or not supporting this specification.¶

6. Security Considerations

Security considerations described in [RFC7623] apply to this document.¶

In addition, this document suggests additional procedures, that can be activated on a per I-SID basis, and generate additional EVPN MAC/IP Advertisement routes in the network. The format of these additional EVPN MAC/IP Advertisement routes is backwards compatible with [RFC7623] procedures and should not create any issues on receiving PEs not following this specification, however, the additional routes may consume extra memory and processing resources on the receiving PEs. Because of that, it is RECOMMENDED to activate this feature only when necessary (when multi-homed networks or devices are attached to the PBB-EVPN PEs), and not by default in any PBB-EVPN PE.¶

7. IANA Considerations

8. Acknowledgments

The authors want to thank Vinod Prabhu, Sriram Venkateswaran, Laxmi Padakanti, Ranganathan Boovaraghavan for their review and contributions.¶

9. Contributors

Authors' Addresses