PBB-EVPN ISID-based C-MAC-Flush

Introduction defines how Provider Backbone Bridging (PBB) can be combined with Ethernet Virtual Private Networks (EVPN) to deploy ELAN services in very large MPLS networks. 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, describes how virtual Ethernet Segments (ES) 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, defines a Customer MAC flush mechanism that works for different Ethernet Segment B-MAC 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. Single-homed ES use null ESIs, whereas multi-homed ES use non-null ESIs. If case of using single-homed ES, the PBB-EVPN network is no longer aware of the redundancy offered by the access administrative entity. shows an example where the PBB-EVPN network provides four different Attachment Circuits for I-SID1, with those Attachment Circuits not being part of any Ethernet Segment or virtual Ethernet Segment (therefore they are referred to as null virtual Ethernet Segment).

PBB-EVPN and non-ES based redundancy <--PBB-EVPN Network---><----MPLS----> Access MPLS Access Ring Network I-SID1 ESI +------+ +------+ +----+ null| PE1 |---------| PE3 | |CE1 |--------|B-MAC1| |B-MAC3| ESI null +----+ active| | | |----+ PW | +------+ +------+ \active | | | \ +----+ | | | ==|CE3 |I-SID1 | | | / +----+ |stb ESI +------+ +------+ / PW +----+ null| PE2 | | PE4 |----+ standby |CE2 |--------|B-MAC2| |B-MAC4| ESI null +----+ active| |---------| | I-SID1 +------+ +------+ ]]> In the example in , 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 Attachment Circuits 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 Attachment Circuit to the PEs is represented by a PW. Each of the four PEs uses a dedicated Backbone MAC address as source MAC address (B-MAC1, B-MAC2, B-MAC3 and B-MAC4, respectively) when encapsulating customer frames in PBB packets and forwarding those PBB packets to the remote PEs as per . There are no multi-homed Ethernet Segments defined in the PBB-EVPN network of the example, that is why the four Attachment Circuits in show the text "ESI null", which means the Ethernet Segment Identifier on those Attachment Circuits is zero. Since there are no multi-homed ES defined, the PEs keep their Attachment Circuits 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. Examples of CEs managing their own redundancy are described in , or for active/standby Pseudowires. For instance, in normal conditions, 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 Attachment Circuits will trigger the CEs to start using their redundant paths, however those failures will not trigger any Customer MAC flush procedures in the PEs that implement , since the PEs are not using the PBB-EVPN multi-homing procedures. For example:

if the active PW from CE3 (to PE3) fails and the failure is due to the entire PE3 node failing, then the procedures in guarantee that all the remote PEs flush all the Customer MACs associated with B-MAC3 (the B-MAC of PE3). In this case, CE3 detects the fault due to the PW going operationally down.
however, if the active PW from CE3 (to PE3) fails (but PE3 is still operationally up), following the procedures in , neither PE3 nor PE4 issue a Customer MAC flush message and therefore the remote PEs will continue pointing at PE3's Backbone MAC to reach CE3's Customer MACs, until the Customer MACs age out in the I-SID1 forwarding tables. In this case, PE3 may use any of the existing PW notifications so that CE3 switches the active PW to PE4.
the same issue is exposed when the active PW from CE3 switches over from PE3 to PE4 due to a manual operation on CE3; that is, neither PE3 nor PE4 trigger any Customer MAC flush notification and the remote PEs continue sending the traffic to PE3 until the Customer MACs age out.

provides a Customer MAC flush solution based on a shared Backbone MAC 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 , the Customer MAC 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. For instance, in case CE3 switches its active PW over to PE4, a potential solution reusing the existing C-MAC Flush notifications in could be for PE3 to issue an update for the MAC/IP Advertisement route of B-MAC3 with a higher sequence number. However, this update would have caused unnecessary Customer MAC flushing for all the I-SIDs attached to PE3 (supposing multiple I-SIDs in PE3), and not only I-SID1. This document describes an extension of the Customer MAC flush procedures, so that in the above failure example, PE3 can trigger a Customer MAC flush notification that makes PE1, PE2 and PE4 flush all the Customer MACs associated to PE3's B-MAC3 and (only) I-SID1. In order to do so, this specification introduces the encoding of the I-SID in the MAC/IP Advertisement routes advertised for the B-MACs. The Customer MAC flush procedure explained in this document is referred to as "PBB-EVPN I-SID-based C-MAC-flush" and can be used in PBB-EVPN networks with null or non-null (virtual) Ethernet Segments. This specification assumes that the reader is familiar with the procedures in .

Terminology and Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

AC:: Attachment Circuit.
B-MAC:: Backbone MAC address.
B-MAC/0 route:: an EVPN MAC/IP Advertisement route that uses a B-MAC in the MAC address field and a zero Ethernet Tag ID.
B-MAC/I-SID route:: an EVPN MAC/IP Advertisement route that uses a B-MAC in the MAC address field and an I-SID in the Ethernet Tag field, and it is used to notify remote PEs about the required C-MAC-flush procedure for the C-MACs associated with the advertised B-MAC and I-SID.
CE:: Customer Edge router.
C-MAC:: Customer MAC address.
ES, and ESI:: Ethernet Segment and Ethernet Segment Identifier.
EVI:: EVPN Instance.
EVPN:: Ethernet Virtual Private Networks, as in .
G.8032:: Ethernet Ring Protection .
I-SID:: Service Instance Identifier.
MAC-VRF:: A Virtual Routing and Forwarding table for MAC addresses.
PBB-EVPN:: Provider-Backbone-Bridging and EVPN, as in .
PE:: Provider Edge router.

Familiarity with the terminology in is expected.

Solution requirements The following requirements are followed by the C-MAC-flush solution described in this document:

The solution MUST prevent black-hole scenarios in case of failures on null ES ACs (Attachment Circuits not associated to ES, that is, their corresponding ESI is zero) when the access device/network is responsible for the redundancy.
This extension described in this document MUST work with Single-Active non-null ES and virtual ES, irrespective of the PE B-MAC address assignment (dedicated per-ES B-MAC or shared B-MAC, as in ).
In case of failure on the egress PE, the solution MUST provide a C-MAC-flush notification at B-MAC and I-SID granularity level.
The solution MUST provide a reliable C-MAC-flush notification in PBB-EVPN networks that use Route-Reflectors (RRs). MAC flushing needs to be provided to all affected I-SIDs in spite of the BGP flush notification messages being aggregated at the RR.
The solution MUST coexist in networks where there are PEs that do not support this specification.
The solution SHOULD be enabled/disabled by an administrative option on a per-PE and per-I-SID basis (as opposed to be always enabled for all the I-SIDs).

EVPN BGP Encoding for ISID-based C-MAC-flush The solution does not use any new BGP attributes but reuses the MAC Mobility extended community as an indication of C-MAC-flush (as in ) and encodes the I-SID in the Ethernet Tag field of the EVPN MAC/IP advertisement route. As a reference, shows the MAC Mobility extended community and the EVPN MAC/IP advertisement route that are used specified in and used in this document as a C-MAC-flush notification message.

CMAC-Flush notification encoding: BMAC/ISID route Where:

The route's route distinguisher and route targets 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. 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 C-MACs upon reception of the route.
The MAC address field encodes the B-MAC Address for which the PE that receives the route must flush the C-MACs upon reception of the route.
The MAC Mobility extended community is used as in , where an increment in the sequence number between two updates for the same B-MAC/I-SID will be interpreted as a C-MAC-flush notification for the corresponding B-MAC and I-SID.

All the other fields are set and used as defined in . This document will refer to this route as the B-MAC/I-SID route, as opposed to the EVPN MAC/IP Advertisement route used in that contains a specific B-MAC, and the Ethernet Tag ID set to zero. This document uses the term B-MAC/0 route to represent a B-MAC route advertised with Ethernet Tag ID = 0. Note that this B-MAC/I-SID route will be accepted and reflected by any RR, since no new attributes or values are used. A PE receiving the route will process the received B-MAC/I-SID update only in case of supporting the procedures described in this document.

Solution description 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 . Enabling or disabling I-SID-based C-MAC-flush SHOULD be an administrative choice on the system that MAY be configured per I-SID (I-Component, Service Instance Component), as opposed to being configured for all I-SIDs. When enabled on a PE:

The PE will be able to generate B-MAC/I-SID routes as C-MAC-Flush notifications for the remote PEs.
The PE will be able to process B-MAC/I-SID routes received from remote PEs.

The PE MUST follow the procedures for C-MAC-flush. This specification brings some additional procedures when I-SID-based C-MAC-flush is enabled. This C-MAC-flush specification is described in three sets of procedures:

I-SID-based C-MAC-flush activation
C-MAC-flush notification generation upon AC failures
C-MAC-flush process upon receiving a C-MAC-flush notification

ISID-based C-MAC-Flush activation procedures The following behavior MUST be followed by the PBB-EVPN PEs following this specification. is used as a reference.

As in , each PE advertises a shared B-MAC in a B-MAC/0 route (with B-MAC1, B-MAC2, B-MAC3 and B-MAC4 in the MAC address field, respectively). This is the B-MAC that each PE will use as B-MAC SA (Source Address) when encapsulating the frames received on any local single-homed AC. Each PE will import the received B-MAC/0 routes from the remote PEs and will install the B-MACs in its B-component (Backbone Component) MAC-VRF. For instance, PE1 will advertise B-MAC1/0 and will install B-MAC2, B-MAC3 and B-MAC4 in its MAC-VRF.
Assuming I-SID-based C-MAC-flush is activated for I-SID 1, the PEs will advertise the shared B-MAC with I-SID 1 encoded in the Ethernet Tag. That is, PE1 will advertise B-MAC1/1 and will receive B-MAC2/1, B-MAC3/1 and B-MAC4/1. The receiving PEs MUST use these B-MAC/I-SID routes only for C-MAC-flush procedures and they MUST NOT be used them to add/withdraw any B-MAC entry in the MAC-VRFs. As per , only B-MAC/0 routes can be used to add/withdraw B-MACs in the MAC-VRFs.
The above procedure MAY also be used for dedicated B-MACs (B-MACs allocated per Ethernet Segment).

C-MAC-Flush generation If, for instance, there is a failure on PE1's AC, PE1 will generate an update including B-MAC1/1 along with the MAC Mobility extended community where the Sequence Number has been incremented. The reception of the B-MAC1/1 with an increment in the sequence number will trigger the C-MAC-flush procedures on the receiving PEs.

An AC going operationally down MUST generate a B-MAC/I-SID with a higher Sequence Number. If the AC going down makes the entire local I-SID go operationally down, the PE will withdraw the B-MAC/I-SID route for the I-SID.
An AC going operationally up SHOULD NOT generate any B-MAC/I-SID update, unless it activates its corresponding I-SID, in which case the PE will advertise the B-MAC/I-SID route.
An AC receiving a G.8032 flush notification or a flush message in any other protocol from the access network MAY propagate it to the remote PEs by generating a B-MAC/I-SID route update with higher Sequence Number.

C-MAC-flush process upon receiving a CMAC-flush notification A PE receiving a C-MAC-flush notification will follow these procedures:

A received B-MAC/I-SID route (with non-zero I-SID) MUST NOT add/remove any B-MAC to/from the MAC-VRF.
An update of a previously received B-MAC/I-SID route with an increment Sequence Number, MUST flush all the C-MACs associated to that I-SID and B-MAC. C-MACs associated to the same I-SID but different B-MAC MUST NOT be flushed.
A received B-MAC/I-SID withdraw (with non-zero I-SID) MUST flush all the C-MACs associated to that B-MAC and I-SID.

Note that the C-MAC-flush procedures described in for B-MAC/0 routes are still valid and a PE receiving C-MAC-flush notification messages MUST observe the behavior specified in .

Conclusions The I-SID-based C-MAC-flush solution described in this document has the following benefits:

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

Security Considerations Security considerations described in 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 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.

IANA Considerations This document requests no actions from IANA.

Acknowledgments The authors want to thank , , , and for their review and contributions.

Contributors