1.1. Conventions and Terminology

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.¶

AC:

Attachment Circuit¶

A-D per ES route:

Auto-Discovery per Ethernet Segment route. Refers to the EVPN Ethernet Auto-Discovery per ES route defined in [RFC7432].¶

Arg.FE2:

Refers to the ESI filtering argument used for Split Horizon as specified in [RFC9252].¶

BD:

Broadcast Domain. Refers to an emulated Ethernet, such that two systems on the same BD will receive each other's BUM traffic. In this document, BD also refers to the instantiation of a BD on an EVPN PE. An EVPN PE can be attached to one or multiple BDs of the same tenant.¶

BUM:

Broadcast, Unknown Unicast, and Multicast¶

DF:

Designated Forwarder. As defined in [RFC7432], an ES may be multihomed (attached to more than one PE). An ES may also contain multiple BDs of one or more EVIs. For each such EVI, one of the PEs attached to the segment becomes that EVI's DF for that segment. Since a BD may belong to only one EVI, we can speak unambiguously of the BD's DF for a given segment.¶

ES:

Ethernet Segment¶

ESI:

Ethernet Segment Identifier¶

EVI:

EVPN Instance¶

EVI-RT:

EVI Route Target. Refers to a group of NVEs attached to the same EVI that will share the same EVI-RT.¶

GENEVE:

Generic Network Virtualization Encapsulation [RFC8926] (see tunnel type 19 in [TUNNEL-ENCAP]).¶

MPLS tunnels and non-MPLS NVO tunnels:

Refers to Multiprotocol Label Switching (or the absence of it) Network Virtualization Overlay tunnels. NVO tunnels use an IP encapsulation for overlay frames, where the source IP address identifies the ingress NVE and the destination IP address identifies the egress NVE.¶

MPLSoUDP:

Multiprotocol Label Switching over User Datagram Protocol [RFC7510] (see tunnel type 13 in [TUNNEL-ENCAP]).¶

MPLSoGRE:

Multiprotocol Label Switching over Generic Network Encapsulation [RFC4023] (see tunnel type 11 in [TUNNEL-ENCAP]).¶

MPLSoX:

Refers to MPLS over any IP encapsulation, for example, MPLSoUDP or MPLSoGRE.¶

NVE:

Network Virtualization Edge¶

NVGRE:

Network Virtualization Using Generic Routing Encapsulation [RFC7637] (see tunnel type 9 in [TUNNEL-ENCAP]).¶

VXLAN:

Virtual eXtensible Local Area Network [RFC7348] (see tunnel type 8 in [TUNNEL-ENCAP]).¶

VXLAN-GPE:

VXLAN Generic Protocol Extension [VXLAN-GPE] (see tunnel type 12 in [TUNNEL-ENCAP]).¶

SHT:

Split Horizon Type. Refers to the Split Horizon method that a PE intends to use and advertises in an A-D per ES route.¶

SRv6:

Segment Routing over IPv6 (see [RFC8402] and [RFC8754]).¶

This document also assumes familiarity with the terminology of [RFC7432] and [RFC8365].¶

2.1. The Split Horizon Type

[RFC8365] does not include any explicit indication regarding the Split Horizon method in the A-D per ES route. In this document, the Split Horizon procedure defined in Section 8.3.1 of [RFC8365] is considered the default behavior, presuming that Local Bias is employed exclusively for IP tunnels, while ESI Label-based Split Horizon is used for IP-based MPLS tunnels. This document specifies that the two high-order bits in the Flags octet (bits 6 and 7) constitute the "Split Horizon Type" or "SHT" field, where:¶

 7 6 5 4 3 2 1 0
+-+-+-+-+-+-+-+-+
|SHT|       |RED|
+-+-+-+-+-+-+-+-+
RED = "Multihoming Redundancy Mode" field (section 5)

SHT bit 7 6
-----------
        0 0  --> Default SHT
                 Backwards compatible with [RFC8365] and [RFC7432]
        0 1  --> Local Bias
        1 0  --> ESI Label-based filtering
        1 1  --> reserved for future use

• SHT = 00 is backwards compatible with [RFC8365] and [RFC7432], and indicates that the advertising NVE intends to use the default or built-in SHT. The default SHT is shown in Table 1 for each encapsulation. An egress NVE that follows the [RFC8365] behavior and does not support this specification will ignore the SHT bits (which is equivalent to processing them as a value of 00).¶

• SHT = 01 indicates that the advertising NVE intends to use Local Bias procedures in the ES for which the AD per-ES route is advertised.¶

• SHT = 10 indicates that the advertising NVE intends to use the ESI Label-based Split Horizon method procedures in the ES for which the AD per-ES route is advertised.¶

• SHT = 11 is a reserved value, for future use.¶

2.2. Use of the Split Horizon Type in A-D per ES Routes

The following behavior is observed:¶

• An SHT value of 01 or 10 MUST NOT be used with encapsulations that support only one SHT in Table 1, and MAY be used by encapsulations that support the two SHTs in Table 1.¶

• An SHT value different than 00 expresses the intent to use a specific Split Horizon method, but does not reflect the actual operational SHT used by the advertising NVE, unless all the NVEs attached to the ES advertise the same SHT.¶

• In case of an inconsistency in the SHT value advertised by the NVEs attached to the same ES for a given EVI, all the NVEs MUST revert to the behavior in [RFC8365] and use the default SHT in Table 1, irrespective of the advertised SHT.¶

• An SHT different than 00 MUST NOT be set if the "Single-Active" bit is set. A received A-D per ES route where the "Single-Active" and SHT bits are different than zero MUST follow the treat-as-withdraw behavior in [RFC7606].¶

• The SHT MUST have the same value in each Ethernet A-D per ES route that an NVE advertises for a given ES and a given encapsulation (see Section 3 for NVEs supporting multiple encapsulations).¶

As an example, egress NVEs that support IP-based MPLS tunnels, such as MPLSoGRE or MPLSoUDP, will advertise A-D per ES routes for the ES along with the BGP Encapsulation Extended Community, as defined in [RFC9012]. This extended community indicates the encapsulation type (MPLSoGRE or MPLSoUDP) and may use the SHT value of 01 or 10 to signify the intent to use Local Bias or the ESI Label, respectively.¶

An egress NVE MUST NOT use an SHT value other than 00 when advertising an A-D per ES route with [RFC9012] Tunnel encapsulation types of VXLAN (type 8), NVGRE (type 9), MPLS (type 10), or no BGP Tunnel Encapsulation Extended Community at all. In all these cases, it is presumed that there is no choice for the Split Horizon method; therefore, the SHT value MUST be set to 00. If a route with any of the mentioned encapsulation options is received and has an SHT value different than 00, it SHOULD apply the treat-as-withdraw behavior, per [RFC7606].¶

An egress NVE advertising A-D per ES route(s) for an ES with GENEVE encapsulation ([RFC9012], Tunnel encapsulation type 19, [EVPN-GENEVE]) MAY use an SHT value of 01 or 10. A value of 01 indicates the intent to use Local Bias, regardless of the presence of an Ethernet option TLV with a non-zero Source-ID, as described in [EVPN-GENEVE]. A value of 10 indicates the intent to use ESI Label-based Split Horizon, and it is only valid if an Ethernet option TLV with a non-zero Source-ID is present. A value of 00 indicates the default behavior outlined in Table 1, which is to use Local Bias if:¶

1. no ESI Label is present in the Ethernet option TLV, or¶
2. there is no Ethernet option TLV.¶

Otherwise, the ESI Label Split Horizon method is applied.¶

These procedures assume a single encapsulation supported in the egress NVE. Section 3 describes additional procedures for NVEs supporting multiple encapsulations.¶

2.3. The ESI Label Value in A-D per ES Routes

This document also updates [RFC8365] regarding the value that is advertised in the ESI Label field of the ESI Label extended community, as follows:¶

• The A-D per ES route(s) for an ES MAY have an ESI Label value of zero if the SHT value is 01. Section 2.2 specifies the scenarios where the SHT can be 01. An ESI Label value of zero eliminates the need to allocate labels in cases where they are not utilized, such as in the Local Bias method.¶

• The A-D per ES route(s) for an ES MAY have an ESI Label value of zero for VXLAN or NVGRE encapsulations.¶

2.4. Backwards Compatibility with RFC 8365 NVEs

As discussed in Section 2.2, this specification is backwards compatible with the Split Horizon filtering behavior in [RFC8365] and a non-upgraded NVE can be attached to the same ES as other NVEs supporting this specification.¶

An NVE maintains an administrative SHT value for an ES, which is advertised alongside the A-D per ES route, and an operational SHT value, which is the one actually used regardless of what the NVE has advertised. The administrative SHT matches the operational SHT if all the NVEs attached to the ES have the same administrative SHT.¶

This document assumes that an implementation of [RFC7432] or [RFC8365] that does not support the specifications in this document will ignore the values of all the Flags in the ESI Label extended community, except for the "Single-Active" bit. Based on this assumption, a non-upgraded NVE will disregard any SHT value other than 00. If an upgraded NVE receives at least one A-D per ES route for the ES with an SHT value of 00, it MUST revert its operational SHT to the default Split Horizon method, as described in Table 1, irrespective of its administrative SHT.¶

For instance, consider an NVE attached to ES N that receives two A-D per ES routes for N from different NVEs, NVE1 and NVE2. If the route from NVE1 has an SHT value of 00 and the one from NVE2 has an SHT value of 01, the NVE MUST use the default Split Horizon method specified in Table 1 as its operational SHT, regardless of its administrative SHT.¶

All NVEs attached to an ES with an operational SHT value of 10 MUST advertise a valid, non-zero ESI Label. If the operational SHT value is 01, the ESI Label MAY be zero. If the operational SHT value is 00, the ESI Label may be zero only if the default encapsulation supports Local Bias exclusively, and the NVEs do not require the presence of a valid, non-zero ESI Label.¶

If an NVE changes its operational SHT value from 01 (Local Bias) to 00 (Default SHT) due to the presence of a new non-upgraded NVE in the ES, and it previously advertised a zero ESI Label, it MUST send an update with a valid, non-zero ESI Label, unless all the non-upgraded NVEs in the ES support only Local Bias. For example, consider NVE1 and NVE2 using MPLSoUDP as encapsulation, attached to the same Ethernet Segment ES1, and advertising an SHT value of 01 (Local Bias) with a zero ESI Label value. Suppose NVE3, which does not support this specification, joins ES1 and advertises an SHT value of 00 (default). Upon receiving NVE3's A-D per ES route, NVE1 and NVE2 MUST update their A-D per ES routes for ES1 to include a valid, non-zero ESI Label value. The assumption here is that NVE3 only supports the default ESI Label-based Split Horizon filtering.¶