| Internet-Draft | EVPN Port-Active Redundancy Mode | March 2024 |
| Brissette, et al. | Expires 5 September 2024 | [Page] |
The Multi-Chassis Link Aggregation Group (MC-LAG) technology enables establishing a logical link-aggregation connection with a redundant group of independent nodes. The purpose of multi-chassis LAG is to provide a solution to achieve higher network availability while providing different modes of sharing/balancing of traffic. RFC7432 defines EVPN-based MC-LAG with Single-active and All-active multi‑homing redundancy modes. This document expands on existing redundancy mechanisms supported by EVPN and introduces a new Port-Active redundancy mode.¶
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EVPN [RFC7432] defines the All-Active and Single-Active redundancy modes. All-Active redundancy provides per-flow load‑balancing for multi‑homing, and Single-active redundancy provides service carving where only one of the PEs in a redundancy relationship is active per service.¶
While these two multi‑homing scenarios are most widely utilized in data center and service provider access networks, there are scenarios where an active/standby multi‑homing at the interface level is useful and required. The main consideration for this new mode of load‑balancing is the determinism of traffic forwarding through a specific interface rather than statistical per-flow load‑balancing across multiple PEs providing multi‑homing. The determinism provided by active/standby multi‑homing at the interface level is also required for certain QoS features to work. While using this mode, customers also expect fast convergence during failure and recovery.¶
This document defines the Port-Active redundancy mode as a new type of multi-homing in EVPN and describes how this new mode operates and is to be supported via EVPN.¶
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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
When a CE is multi‑homed to a set of PE nodes using the [IEEE.802.1AX_2014] Link Aggregation Control Protocol (LACP), the PEs must act as if they were a single LACP speaker for the Ethernet links to form and operate as a Link Aggregation Group (LAG). To achieve this, the PEs connected to the same multi‑homed CE must synchronize LACP configuration and operational data between them. Interchassis Communication Protocol (ICCP) [RFC7275] has historically been used to achieve this. EVPN in [RFC7432] describes the case where a CE is multihomed to multiple PE nodes, using a LAG as a means to greatly simplify the procedure. The simplification, however, comes with a few assumptions:¶
This document relies on proper LAG operation as in [RFC7432]. Discrepancies from the list above are out of the scope of this document, as are LAG misconfiguration and miswiring detection across peering PEs.¶
+-----+
| PE3 |
+-----+
+-----------+
| MPLS/IP |
| CORE |
+-----------+
+-----+ +-----+
| PE1 | | PE2 |
+-----+ +-----+
| |
I1 I2
\ /
\ /
+---+
|CE1|
+---+
Figure 1 shows a MC-LAG multi‑homing topology where PE1 and PE2 are part of the same redundancy group providing multi‑homing to CE1 via interfaces I1 and I2. Interfaces I1 and I2 are members of a LAG running LACP. The core, shown as IP or MPLS enabled, provides a wide range of L2 and L3 services. MC-LAG multi‑homing functionality is decoupled from those services in the core and it focuses on providing multi‑homing to the CE. In Port-Active redundancy mode, only one of the two interfaces I1 or I2 would be in forwarding and the other interface will be in standby. This also implies that all services on the active interface are in active mode and all services on the standby interface operate in standby mode.¶
The use of Port-Active redundancy brings the following benefits to EVPN networks:¶
Replaces legacy MC-LAG ICCP-based solution, and offers the following additional benefits:¶
The following steps describe the proposed procedure with EVPN LAG to support Port-Active redundancy mode:¶
Non-DF routers SHOULD implement a bidirectional blocking scheme for all traffic comparable to [RFC7432] Single-Active blocking scheme, albeit across all VLANs.¶
The ES routes, running in Port-Active redundancy mode, are advertised with the new Port Mode Load-Balancing capability bit in the DF Election Extended Community defined in [RFC8584]. Moreover, the ES associated with the port leverages the existing procedure of Single-Active, and signals Single-Active Multihomed site redundancy mode along with Ethernet-AD per-ES route (Section 7.5 of [RFC7432]). Finally the ESI label-based split-horizon procedures in Section 8.3 of [RFC7432] should be used to avoid transient echo'ed packets when Layer‑2 circuits are involved.¶
The various algorithms for DF Election are discussed in Sections 4.2 to 4.5 for completeness even though the choice of algorithm in this solution doesn't affect complexity or performance as in other redundancy modes.¶
[RFC8584] defines a DF Election extended community, and a Bitmap (2 octets) field to encode "capabilities" to use with the DF election algorithm in the DF algorithm field:¶
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|D|A| |P| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This document defines the following value and extends the Bitmap field:¶
The default DF Election algorithm, or modulus-based algorithm as in
[RFC7432] and updated by [RFC8584], is used here, at the granularity
of ES only. Given that ES-Import Route Target extended community may be auto-derived and
directly inherits its auto-derived value from ESI bytes 1-6, many operators differentiate ESI
primarily within these bytes.
As a result, bytes 3‑6 are used to determine the designated forwarder using Modulo-based DF
assignment, achieving good entropy during Modulo calculation across ESIs:
Assuming a redundancy group of N PE nodes, the PE with ordinal i is the DF for an <ES>
when (Es mod N) = i, where Es represents bytes 3‑6 of that ESI.¶
Highest Random Weight (HRW) algorithm defined in [RFC8584] MAY also be used and signaled, and modified to operate at the granularity of <ES> rather than per <ES, VLAN>.¶
Section 3.2 of [RFC8584] describes computing a 32-bit CRC over the concatenation of Ethernet Tag and ESI. For Port-Active redundancy mode, the Ethernet Tag is simply omitted from the CRC computation and all references to (V, Es) are replaced by (Es), as repeated and summarised below.¶
DF(Es) denotes the DF and BDF(Es) denote the BDF for the Ethernet Segment Es; Si is the IP address of PE i; and Weight is a function of Si, and Es.¶
Where:¶
When the new capability 'Port Mode' is signaled, the preference-based DF Election algirithm in [I-D.ietf-bess-evpn-pref-df] is modified to consider the port only and not any associated Ethernet Tags. Furthermore, the Port Mode capability MUST be compatible with the 'Don't Preempt' bit. When an interface recovers, a peering PE signaling D bit will enable non-revertive behavior at the port level.¶
The AC-DF bit defined in [RFC8584] MUST be set to 0 when advertising Port Mode Designated Forwarder Election capability (P=1). When an AC (sub-interface) goes down, it does not influence the DF Election. The peer's Ethernet A-D per EVI is ignored in all Port Mode DF Election algorithms.¶
Upon receiving the AC-DF bit set (A=1) from a remote PE, it MUST be ignored when performing Port Mode DF Election.¶
To improve the convergence, upon failure and recovery, when the Port-Active redundancy mode is used, some advanced synchronization between peering PEs may be required. Port-Active is challenging in the sense that the "standby" port may be in a down state. It takes some time to bring a "standby" port to an up state and settle the network. For IRB and L3 services, ARP / ND cache may be synchronized. Moreover, associated VRF tables may also be synchronized. For L2 services, MAC table synchronization may be considered.¶
Finally, for members of a LAG running LACP the ability to set the "standby" port in "out-of-sync" state a.k.a "warm‑standby" can be leveraged.¶
The EVPN Layer 2 Attributes Extended Community ("L2-Attr") defined in [RFC8214] SHOULD be advertised in the Ethernet A-D per ES route for fast convergence.¶
Only the P and B bits of the Control Flags field in the L2-Attr Extended Community are relevant to this document, and only in the context of Ethernet A-D per ES routes:¶
For L2-Attr Extended Community sent and received in Ethernet A-D per EVI routes used in [RFC8214], [RFC7432] and [I-D.ietf-bess-evpn-vpws-fxc]:¶
Implementations that comply with [RFC7432] or [RFC8214] only (i.e., implementations that predate this specification) will not advertise the EVPN Layer 2 Attributes Extended Community in Ethernet A-D per ES routes. That means that all remote PEs in the ES will not receive P and B bit per ES and will continue to receive and honour the P and B bits received in Ethernet A-D per EVI route(s). Similarly, an implementation that complies with [RFC7432] or [RFC8214] only and that receives an L2-Attr Extended Community in Ethernet A-D per ES routes will ignore it and continue to use the default path resolution algorithm:¶
A common deployment is to provide L2 or L3 service on the PEs providing multi‑homing. The services could be any L2 EVPN such as EVPN VPWS, EVPN [RFC7432], etc. L3 service could be in a VPN context [RFC4364] or in a global routing context. When a PE provides first hop routing, EVPN IRB could also be deployed on the PEs. The mechanism defined in this document is used between the PEs providing L2 and/or L3 services, when active/standby redundancy at the interface level is desired.¶
A possible alternate solution to the one described in this document is MC-LAG with ICCP [RFC7275] active-standby redundancy. However, ICCP requires LDP to be enabled as a transport of ICCP messages. There are many scenarios where LDP is not required e.g. deployments with VXLAN or SRv6. The solution defined in this document with EVPN does not mandate the need to use LDP or ICCP and is independent of the underlay encapsulation.¶
This document solicits the allocation of the following values from the "BGP Extended Communities" registry group :¶
The same Security Considerations described in [RFC7432] and [RFC8584] are valid for this document.¶
By introducing a new capability, a new requirement for unanimity (or lack thereof) between PEs is added. Without consensus on the new DF Election procedures and Port Mode, the DF Election algorithm falls back to the default DF Election as provided in [RFC8584] and [RFC7432]. This behavior could be exploited by an attacker that manages to modify the configuration of one PE in the ES so that the DF Election algorithm and capabilities in all the PEs in the ES fall back to the default DF Election. If that is the case, the PEs will be exposed to the same unfair load balancing, service disruption, and possibly black-holing or duplicate traffic mentioned in those documents and their security sections.¶
The authors thank Anoop Ghanwani for his comments and suggestions and Stephane Litkowski for his careful review.¶
In addition to the authors listed on the front page, the following coauthors have also contributed to this document:¶