| Internet-Draft | EVPN MH for L2-GW Protocols | March 2022 |
| Brissette, et al. | Expires 8 September 2022 | [Page] |
The existing EVPN multi-homing load-balancing modes defined are Single-Active and All-Active. Neither of these multi-homing mechanisms adequately represent ethernet-segments facing access networks with Layer-2 Gateway protocols such as G.8032, (M)STP, REP, MPLS-TP, etc. These loop-preventing Layer-2 protocols require a new multi-homing mechanism defined in this draft.¶
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Existing EVPN Single-Active and All-Active multi-homing mechanisms do not address the additional requirements of loop-preventing Layer-2 gateway protocols such as G.8032, (M)STP, REP, MPLS-TP, etc.¶
These Layer-2 Gateway protocols require that a given L2 flow of a VLAN be only active on one of the PEs in the multi-homing group, while another L2 flow may be active on the other PE. This is in contrast with Single-Active redundancy mode where all flows of a VLAN are active on a single multi-homing PEs and it is also in contrast with All-Active redundancy mode where all flows of a VLAN are active on all PEs in the redundancy group.¶
This draft defines a new multi-homing mechanism "Single-Flow-Active" specifying that a VLAN can be active on all PEs in the redundancy group but each unique L2 flow of that VLAN can be active on only one of the PEs in the redundancy group at a time. In fact, the Designated Forwarder election algorithm for these L2 Gateway protocols, is not per VLAN but rather for a given L2 flow. A selected PE in the redundancy group must be the only Designated Forwarder for a specific L2 flow, but the decision is not taken by the PE. The loop-prevention blocking scheme occurs in the access network, by the Layer-2 protocol.¶
EVPN multi-homing procedures need to be enhanced to support Designated Forwarder election for all traffic (both known unicast and BUM) on a per L2 flow basis. The Single-Flow-Active multi-homing mechanism also requires new EVPN considerations for aliasing, mass-withdraw, fast-switchover and [RFC9135] as described in the solution section.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].¶
The EVPN L2GW framework for L2GW protocols in Access-Gateway mode, consists of the following rules:¶
Upon receiving ESI label BGP Extended Community with the single-flow-active load-balancing mode, remote PE MUST:¶
For fast-convergence, remote PE3 SHOULD set up two distinct backup paths on a per-flow basis:¶
The backup paths so created, operate as in Section 8.4 of [RFC7432] where the backup PE of the redundancy group MAY immediately be selected for forwarding upon detection of a specific subset of failures: Ethernet A-D per ES route withdraw, Active PE loss of reachability (via IGP detection). An Ethernet A-D per EVI withdraw MUST NOT result in automatic switching to the backup PE as only a subset of the hosts may be changing reachability to the Backup PE, and the remote cannot determine which.¶
+---+
|CE3|
+---+
|
|
+-----+
+-----| PE3 |-----+
| +-----+ |
| |
| MPLS/IP |
| CORE |
| |
+-----+ +-----+
| PE1 |-----------| PE2 |
+-----+ +-----+
AC1| |AC2
| |
+---+ +---+
|CE1| |CE2|
+---+ +---+
| |
| +---+ |
+----|CE4|---/ /--+
+---+
Figure 1 shows a typical EVPN network with an access network running a L2GW protocol, typically one of the following: G.8032, (M)STP, REP, MPLS‑TP ([RFC6378]), etc. The L2GW protocol usually starts from AC1 (on PE1) up to AC2 (on PE2) in an open "ring" manner. AC1 and AC2 interfaces of PE1 and PE2 are participants in the access protocol.¶
The L2GW protocol is used for loop avoidance. In above example, the loop is broken on the right side of CE4.¶
PE1 and PE2 are peering PEs in a redundancy group, and sharing a same ESI. In the proposed Single-Flow-Active mode, load-balancing at PE1 and PE2 shares similarities with singular aspects of both Single-Active and All-Active. Designated Forwarder election must not compete with the L2GW protocol and must not result in blocked ports or portions of the access may become isolated. Additionally, the reachability between CE1/CE4 and CE2 is achieved with the forwarding path through the EVPN MPLS/IP core side. Thus, the ESI-Label filtering of [RFC7432] is disabled for Single-Flow-Active Ethernet segments.¶
Finally, PE3 behaves according to EVPN [RFC7432] rules for traffic to/from PE1/PE2. Peering PE, selected per L2 flow, is chosen by the L2GW protocol in the access, and is out of EVPN control.¶
From PE3 point of view, the L2 flows from PE3 destined to CE1/CE4 transit via edge node PE1 and the L2 flows destined to CE2 transit via edge node PE2. A specific unicast L2 flow never goes to both peering PEs. Therefore, aliasing of [RFC7432] Section 8.4 cannot be performed by PE3. That node operates in a single-active fashion for each of the unicast L2 flows.¶
The backup path of [RFC7432] Section 8.4 which is also setup for single-active rapid convergence on a per-VLAN basis, is not applicable here. For example, in Figure 1, if a failure happens between CE1 and CE4 the loop-prevention at the right of CE4 is released and:¶
On PE3, there is no way to know which L2 flow specifically is affected. During the transition time, PE3 may flood until unicast traffic recovers properly.¶
In order to address rapid Layer-2 convergence requirement, topology change notification received from the L2GW protocols must be sent across the EVPN network to perform the equivalent of legacy L2VPN remote MAC flush.¶
The generation of TCN is done differently based on the access protocol. In the case of REP and G.8032, TCN gets generated in both directions and thus both of the dual-homing PEs receive it. However, with (M)STP, TCN gets generated only in one direction and thus only a single PE can receive it. That TCN is propagated to the other peering PE for local MAC flushing, and relaying back into the access.¶
In fact, PEs have no direct visibility on failures happening in the access network nor on the impact of those failures over the connectivity between CE devices. Hence, both peering PEs require to perform a local MAC flush on corresponding interfaces.¶
There are two options to relay the access protocol's TCN to the peering PE: in-band or out-of-band messaging. The first method is better for rapid convergence, and requires a dedicated channel between peering PEs. An EVPN-VPWS connection MAY be dedicated for that purpose, connecting the Untagged ACs of both PEs. The latter choice relies on the MAC Mobility BGP Extended Community applied to the Ethernet A-D per EVI route, detailed below. It is a slower method but has the advantage of avoiding a dedicated channel between peering PEs.¶
Peering PE in Single Flow Active mode, upon receiving notification of a protocol convergence-event from access (such as TCN), MUST:¶
The MAC-Flush procedure described in [RFC7623] is borrowed, and the MAC mobility BGP Extended community is signaled along with the Ethernet A-D per EVI route from a PE in Single-Flow-Active mode.¶
When MAC Mobility BGP Extended Community is received on the Ethernet A-D per EVI route, it indicates to all remote PEs that all MAC addresses associated with that EVI/ESI are "flushed" i.e. must be unresolved.¶
Remote PEs, having previously received Ethernet A-D per ES with Single Flow Active indication from an originating PE, treat the MAC Mobility indication to simply invalidate the MAC entries for that originating PE on an EVI/ESI basis, similar to [RFC7432]'s mass-withdraw mechanism.¶
They remain unresolved until the remote PE receives a route update (or withdraw) for those MAC addresses. Note: the MAC may be re-advertised by the same PE, but also some are expected to have moved to a multi-homing peer, within the same ESI, due to the L2 protocol's action.¶
The sequence number of the MAC Mobility extended community is of local significance from the originating PE, and is not used for comparison between peering PEs. Rather, it is used to signal via BGP successive MAC Flush requests from a given PE per EVI/ESI.¶
As a reference, an alternative solution which achieves some, but not all, of the requirements exists:¶
On the PE1 and PE2,¶
While this solution is feasible, it is considered to fall short of the requirements listed in Section 2, namely for all aspects meant to achieve fast-convergence.¶
A PE which receives an Ethernet A-D per ES route with the Single-Flow-Active bit set in the ESI-flags, and which does not support/understand this bit, SHALL discard the bit and continue operating per [RFC7432] (All-Active). The operator should understand the usage of single-flow-active load-balancing mode else it is highly recommended to use the two-ESI approach as described in Section 3.3.1¶
The remote PE3 which does not support Single-Flow-Active redundancy mode as described, will ECMP traffic to peering PE1 and PE2 in the example topology above (Figure 1), per [RFC7432], Section 8.4 aliasing and load-balancing rules. PE1 and PE2, which support the Single-Flow-Active redundancy mode MUST setup redirections towards the PE at which the flow is currently active (sub-optimal Layer-2 forwarding and sub-optimal Layer-3 routing).¶
Thus, while PE3 will ECMP (on average) 50% of the traffic to the incorrect PE using [RFC7432] operation, PE1 and PE2 will handle this gracefully in Single-Flow-Active mode and redirect across peering pair of PEs appropriately.¶
No extra route or information is required for this. The [RFC7432] and [RFC9135] route advertisements are sufficient.¶
In order to signal the new EVPN load-balancing mode (single-flow-active), this document claims the following value from the "EVPN ESI Multihoming Attributes" registry's RED field setup by Section 7.5 of [I-D.ietf-bess-rfc7432bis].¶
Multihomed site redundancy mode:¶
EVPN Inter-subnet forwarding procedures in [RFC9135] works with the current proposal and does not require any extension. Host routes continue to be installed at PE3 with a single remote nexthop, no aliasing.¶
However, leveraging the same-ESI on both L2GW PEs enables ARP/ND synchronization procedures which are defined for All-Active redundancy in [RFC9135]. In steady-state, on PE2 where a host is not locally-reachable the routing table will reflect PE1 as the destination. However, with ARP/ND synchronization based on a common ESI, the ARP/ND cache may be pre-populated with the local AC as destination for the host, should an AC failure occur on PE1. This achieves fast-convergence.¶
When a host moves to PE2 from the PE1 L2GW peer, the MAC mobility sequence number is incremented to signal to remote peers that a 'move' has occurred and the routing tables must be updated to PE2. This is required when an Access Protocol is running where the loop is broken between two CEs in the access and the L2GWs, and the host is no longer reachable from the PE1-side but now from the PE2-side of the access network.¶
EVPN Multi-Homing Mechanism for Layer-2 Gateway Protocols solves a true problem due to the wide legacy deployment of these access L2GW protocols in Service Provider networks. The current draft has the main advantage to be fully compliant with [RFC7432] and [RFC9135].¶
The same Security Considerations described in [RFC7432] and [RFC9135] remain valid for this document.¶
Authors would like to thank Thierry Couture for valuable review and inputs with respect to access protocol deployments related to procedures proposed in this document.¶
This document solicits the allocation of the following values from the "EVPN ESI Multihoming Attributes" registry setup in [I-D.ietf-bess-rfc7432bis], and updates the listing of redundancy modes values:¶
RED Multihomed site redundancy mode
00 = All-Active
01 = Single-Active
10 = Single-Flow-Active
¶