| Internet-Draft | EVPN MH Port-Active | November 2021 |
| Brissette, et al. | Expires 16 May 2022 | [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 load‑balancing mode. The current draft expands on existing redundancy mechanisms supported by EVPN and introduces support for a new port-active load‑balancing mode.¶
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EVPN, as per [RFC7432], provides all-active per flow load‑balancing for multi‑homing. It also defines single-active with service carving mode, where one of the PEs, in 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 active-standby per interface multi‑homing load‑balancing is useful and required. The main consideration for this 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 per interface is also required for certain QOS features to work. While using this mode, customers also expect minimized convergence during failures.¶
A new type of load‑balancing mode, port-active load‑balancing, is defined. This draft describes how the new load‑balancing mode can be supported via EVPN. The new mode may also be referred to as per interface active/standby.¶
+-----+
| 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 protocol. The core, shown as IP or MPLS enabled, provides 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. With per-port active/standby load‑balancing, only one of the two interface I1 or I2 would be in forwarding, 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 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].¶
When a CE is multi‑homed to a set of PE nodes using the [802.1AX] 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 among them. Interchassis Communication Protocol (ICCP) [RFC7275] has been used for that purpose. EVPN LAG simplifies greatly that solution. Along with the simplification comes few assumptions:¶
Any discrepancies from this list are left for future study. Furthermore, mis-configuration and mis-wiring detection across peering PEs are also left for further study.¶
Following steps describe the proposed procedure with EVPN LAG to support port-active load‑balancing mode:¶
The ES routes, running in port-active load‑balancing mode, are advertised with a new capability in the DF Election Extended Community as defined in [RFC8584]. Moreover, the ES associated to the port leverages existing procedure of single-active, and signals single-active bit along with Ethernet-AD per-ES route. Finally, as in [RFC7432], the ESI-label based split-horizon procedures 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 4.4 for completeness, although the choice of algorithm in this solution doesn't affect complexity or performance as in other load-balancing modes.¶
[RFC8584] defines a DF Election extended community, and a Bitmap field to encode "capabilities" to use with the DF election algorithm in the DF algorithm field. Bitmap (2 octets) is extended by the following value:¶
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| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 the fact, ES-Import RT community inherits from ESI only byte 1-6, many deployments differentiate ESI within these bytes only. For Modulo calculation, bytes 3‑6 are used to determine the designated forwarder using Modulo-based DF assignment.¶
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].¶
[RFC8584] describes computing a 32 bit CRC over the concatenation of Ethernet Tag and ESI. For port-active load‑balancing mode, the Ethernet Tag is simply removed from the CRC computation.¶
When the new capability 'Port-Mode' is signaled, the algorithm is modified to consider the port only and not any associated Ethernet Tags. Furthermore, the "port-based" capability MUST be compatible with the "Don't Preempt" bit. When an interface recovers, a peering PE signaling D-bit will enable non-revertive behaviour at the port level. The AC-DF bit MUST be set to zero. When an AC (sub-interface) goes down, it does not influence the DF election.¶
To improve the convergence, upon failure and recovery, when port‑active load‑balancing mode is used, some advanced synchronization between peering PEs may be required. Port-active is challenging in a sense that the "standby" port is in down state. It takes some time to bring a "standby" port in 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 L2 Info Extended Community MAY be advertised in Ethernet A-D per ES route for fast convergence. Only the P and B bits are relevant to this specification. When advertised, the L2 Info Extended Community SHALL have only P or B bits set and all other bits must be zero. MTU must also be zero. Remote PE receiving optional L2 Info Extended Community on Ethernet A-D per ES routes SHALL consider only P and B bits. P and B bits received on Ethernet A-D per EVI routes per [RFC8214] are overridden.¶
Implementations that comply with [RFC7432] or [RFC8214] only (i.e., implementations that predate this specification) will not advertise the L2 Info 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 a L2 Info Extended Community will ignore it and will 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 VPN context [RFC4364] or in global routing context. When a PE provides first hop routing, EVPN IRB could also be deployed on the PEs. The mechanism defined in this draft is used between the PEs providing the L2 and/or L3 service, when the requirement is to use per port active.¶
A possible alternate solution is the one described in this draft 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 draft with EVPN does not mandate the need to use LDP or ICCP and is independent of the underlay encapsulation.¶
The use of port-active multi‑homing brings the following benefits to EVPN networks:¶
Replaces legacy MC-LAG ICCP-based solution, and offers following additional benefits:¶
This document solicits the allocation of the following values:¶
The same Security Considerations described in [RFC7432] are valid for this document.¶
The authors thank Anoop Ghanwani for his comments and suggestions.¶