Internet Engineering Task Force (IETF) R. Singh
Request for Comments: 8614 K. Kompella
Updates: 4761 Juniper Networks
Category: Standards Track S. Palislamovic
ISSN: 2070-1721 Nokia
June 2019
Internet Engineering Task Force (IETF) R. Singh
Request for Comments: 8614 K. Kompella
Updates: 4761 Juniper Networks
Category: Standards Track S. Palislamovic
ISSN: 2070-1721 Nokia
June 2019
Updated Processing of Control Flags for BGP Virtual Private LAN Service (VPLS)
BGP虚拟专用LAN服务（VPLS）控制标志的更新处理
Abstract
摘要
This document updates the meaning of the Control Flags field in the "Layer2 Info Extended Community" used for BGP Virtual Private LAN Service (VPLS) Network Layer Reachability Information (NLRI) as defined in RFC 4761. This document updates RFC 4761.
本文档更新了RFC 4761中定义的BGP虚拟专用LAN服务（VPLS）网络层可达性信息（NLRI）“Layer2信息扩展社区”中控制标志字段的含义。本文件更新了RFC 4761。
Status of This Memo
关于下段备忘
This is an Internet Standards Track document.
这是一份互联网标准跟踪文件。
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.
本文件是互联网工程任务组（IETF）的产品。它代表了IETF社区的共识。它已经接受了公众审查，并已被互联网工程指导小组（IESG）批准出版。有关互联网标准的更多信息，请参见RFC 7841第2节。
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8614.
有关本文件当前状态、任何勘误表以及如何提供反馈的信息，请访问https://www.rfc-editor.org/info/rfc8614.
Copyright Notice
版权公告
Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved.
版权（c）2019 IETF信托基金和被确定为文件作者的人员。版权所有。
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
本文件受BCP 78和IETF信托有关IETF文件的法律规定的约束(https://trustee.ietf.org/license-info)自本文件出版之日起生效。请仔细阅读这些文件，因为它们描述了您对本文件的权利和限制。从本文件中提取的代码组件必须包括信托法律条款第4.e节中所述的简化BSD许可证文本，并提供简化BSD许可证中所述的无担保。
Table of Contents
目录
1. Introduction ....................................................2
1.1. Terminology ................................................3
2. Problem Description .............................................3
3. Updated Meaning of Control Flags in the Layer2 Info Extended
Community .......................................................3
3.1. Control Word (C-Bit) .......................................4
3.2. Sequence Flag (S-Bit) ......................................4
4. Using Point-to-Multipoint (P2MP) LSPs as Transport for
BGP VPLS ........................................................5
5. Illustrative Diagram ............................................6
6. Treatment of C-Bits and S-Bits in Multihoming Scenarios .........7
6.1. Control Word (C-Bit) .......................................7
6.2. Sequence Flag (S-Bit) ......................................7
7. Security Considerations .........................................8
8. IANA Considerations .............................................8
9. References ......................................................8
9.1. Normative References .......................................8
9.2. Informative References .....................................9
Authors' Addresses .................................................9
1. Introduction ....................................................2
1.1. Terminology ................................................3
2. Problem Description .............................................3
3. Updated Meaning of Control Flags in the Layer2 Info Extended
Community .......................................................3
3.1. Control Word (C-Bit) .......................................4
3.2. Sequence Flag (S-Bit) ......................................4
4. Using Point-to-Multipoint (P2MP) LSPs as Transport for
BGP VPLS ........................................................5
5. Illustrative Diagram ............................................6
6. Treatment of C-Bits and S-Bits in Multihoming Scenarios .........7
6.1. Control Word (C-Bit) .......................................7
6.2. Sequence Flag (S-Bit) ......................................7
7. Security Considerations .........................................8
8. IANA Considerations .............................................8
9. References ......................................................8
9.1. Normative References .......................................8
9.2. Informative References .....................................9
Authors' Addresses .................................................9
"Virtual Private LAN Service (VPLS) Using BGP for Auto-Discovery and Signaling" [RFC4761] describes the concepts and signaling for using the Border Gateway Protocol (BGP) to set up a VPLS. It specifies the BGP VPLS Network Layer Reachability Information (NLRI) by which a Provider Edge (PE) router may require other PEs in the same VPLS to include (or not) the Control Word (CW) and sequencing information in VPLS frames sent to this PE.
“使用BGP进行自动发现和信令的虚拟专用LAN服务（VPLS）”[RFC4761]描述了使用边界网关协议（BGP）建立VPLS的概念和信令。它指定BGP VPLS网络层可达性信息（NLRI），通过该信息，提供商边缘（PE）路由器可能要求同一VPLS中的其他PE在发送到此PE的VPLS帧中包括（或不包括）控制字（CW）和序列信息。
The use of the CW helps prevent the misordering of IPv4 or IPv6 Pseudowire (PW) traffic over Equal-Cost Multipath (ECMP) paths or Link Aggregation Group (LAG) bundles. [RFC4385] describes the format for the CW that may be used over point-to-point PWs and over a VPLS. Along with [RFC3985], [RFC4385] also describes sequence number usage for VPLS frames.
CW的使用有助于防止IPv4或IPv6伪线（PW）流量在等成本多路径（ECMP）路径或链路聚合组（LAG）捆绑包上的顺序错误。[RFC4385]描述了可在点到点PWs和VPLS上使用的CW格式。除了[RFC3985]，[RFC4385]还描述了VPLS帧的序列号用法。
However, [RFC4761] does not specify the behavior of PEs in a mixed environment where some PEs support CW/sequencing and others do not.
但是，[RFC4761]未指定PEs在混合环境中的行为，其中一些PEs支持CW/测序，而其他PEs不支持。
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.
本文件中的关键词“必须”、“不得”、“必需”、“应”、“不应”、“建议”、“不建议”、“可”和“可选”在所有大写字母出现时（如图所示）应按照BCP 14[RFC2119][RFC8174]所述进行解释。
[RFC4761] specifies the VPLS BGP NLRI by which a given PE advertises the behavior expected by the multiple PEs participating in the same VPLS. The NLRI indicates the VPLS label that the various PE routers, which are referred to in the NLRI, should use when forwarding VPLS traffic to this PE. Additionally, by using the Control Flags, this PE specifies whether the other PEs (in the same VPLS) should use the CW or sequenced delivery for frames forwarded to this PE. These are indicated by the C-bits and the S-bits, respectively, in the Control Flags, as specified in Section 3.2.4 in [RFC4761].
[RFC4761]指定VPLS BGP NLRI，给定PE通过该NLRI宣传参与同一VPLS的多个PE预期的行为。NLRI表示NLRI中提及的各种PE路由器在将VPLS流量转发到此PE时应使用的VPLS标签。此外，通过使用控制标志，此PE指定其他PE（在同一VPL中）是否应使用CW或顺序传送转发到此PE的帧。根据[RFC4761]第3.2.4节的规定，这些由控制标志中的C位和S位分别表示。
[RFC4761] requires that if the advertising PE sets the C-bits and S-bits, the receiving PE MUST, respectively, insert a CW and include sequence numbers when forwarding VPLS traffic to the advertising PE.
[RFC4761]要求，如果广告PE设置了C位和S位，则接收PE在向广告PE转发VPLS流量时必须分别插入CW和序列号。
However, in a BGP VPLS deployment, there would often be cases where a PE receiving the VPLS BGP NLRI may not have the ability to insert a CW or include sequencing information inside PW frames. Thus, the behavior of CW processing and sequencing needs to be further specified.
然而，在BGP VPLS部署中，通常会出现这样的情况：接收VPLS BGP NLRI的PE可能无法在PW帧内插入CW或包含序列信息。因此，需要进一步规定CW处理和排序的行为。
This document updates the meaning of the Control Flags in the Layer2 Info Extended Community in the BGP VPLS NLRI. It also specifies the forwarding behavior for a mixed-mode environment where not every PE in a VPLS has the ability or the configuration to honor the Control Flags received from the PE advertising the BGP NLRI.
本文档更新了BGP VPLS NLRI中Layer2信息扩展社区中控制标志的含义。它还指定了混合模式环境中的转发行为，在这种环境中，VPLS中的每个PE都不能或配置为接受从播发BGP NLRI的PE接收的控制标志。
3. Updated Meaning of Control Flags in the Layer2 Info Extended Community
3. 更新了Layer2信息扩展社区中控制标志的含义
[RFC4761] does not allow for the CW setting to be negotiated. In a typical implementation, if a PE sets the C-bit, it expects to receive VPLS frames with a CW and will send frames the same way. If the PEs at the two ends of a PW do not agree on the setting of the C-bit, the PW does not come up. The behavior is similar for the S-bit.
[RFC4761]不允许协商CW设置。在一个典型的实现中，如果PE设置了C位，它期望接收带有CW的VPLS帧，并以相同的方式发送帧。如果PW两端的PEs不同意C位的设置，则PW不会出现。S位的行为类似。
This memo updates the meaning of the C-bit and the S-bit in the Control Flags.
此备忘录更新控制标志中C位和S位的含义。
If a PE sets the C-bit in its NLRI, it means that the PE has the ability to send and receive frames with a CW.
如果PE在其NLRI中设置了C位，则意味着PE能够使用CW发送和接收帧。
- If the PEs at both ends of a PW set the C-bit, CWs MUST be used in both directions of the PW.
- 如果PW两端的PEs设置了C位，则必须在PW的两个方向上使用CWs。
- If both PEs send a C-bit of 0, CWs MUST NOT be used on the PW.
- 如果两个PEs都发送C位0，则不得在PW上使用CWs。
These two cases behave as before.
这两种情况与以前一样。
However, if the PEs at both ends of the PW do not agree on the setting of the C-bit, CWs MUST NOT be used in either direction on that PW, but the PW MUST NOT be prevented from coming up due to this mismatch. So, the PW will still come up but will not use the CW in either direction. This behavior is changed from the behavior described in [RFC4761] where the PW does not come up.
但是，如果PW两端的PEs对C位的设置不一致，则不得在该PW的任何方向上使用CWs，但不得因该不匹配而阻止PW出现。因此，PW仍然会出现，但不会在任何方向使用CW。此行为与[RFC4761]中描述的行为不同，其中PW未出现。
If a PE sets the S-bit in its NLRI, it means that the PE has the ability to set sequence numbers as described in Section 4.1 in [RFC4385] and process sequence numbers as described in Section 4.2 in [RFC4385].
如果PE在其NLRI中设置了S位，则表示PE能够按照[RFC4385]第4.1节中的说明设置序列号，并按照[RFC4385]第4.2节中的说明处理序列号。
- If the PEs at both ends of a PW set the S-bit, non-zero sequence numbers MUST be used in both directions of the PW.
- 如果PW两端的PEs设置了S位，则必须在PW的两个方向上使用非零序编号。
- If both PEs send an S-bit of 0, sequence numbers MUST NOT be used on the PW.
- 如果两个PEs发送的S位均为0，则不得在PW上使用序列号。
These two cases behave as before.
这两种情况与以前一样。
[RFC4761] does not allow for the S-bit setting to be negotiated either. In a typical implementation, if the PE sets the S-bit in the advertised NLRI, it expects to receive VPLS frames with non-zero sequence numbers and will send outgoing frames over the PW with non-zero sequence numbers.
[RFC4761]也不允许协商S位设置。在典型的实现中，如果PE在公布的NLRI中设置S位，则它期望接收具有非零序编号的VPLS帧，并将通过具有非零序编号的PW发送传出帧。
This memo further specifies the expected behavior when the PEs at the ends of the PW advertise differing S-bit values. If the PEs at both ends of the PW do not agree on the setting of the S-bit, then the PW SHOULD NOT come up. This is to avoid running into out-of-sequence ordering scenarios when the multiple PEs that are enabling multihoming for a site have differing S-bit advertisements as described in Section 4.2 in [RFC4385]. However, if a deployment is known to not utilize multihoming, a user-configurable way to override
该备忘录进一步规定了PW末端的PEs公布不同S位值时的预期行为。如果PW两端的PEs不同意S位的设置，则PW不应出现。如[RFC4385]第4.2节所述，当为一个站点启用多主的多个PE具有不同的S位播发时，这是为了避免出现无序排序的情况。但是，如果已知部署未使用多宿主，则是一种用户可配置的覆盖方式
this recommendation MAY be provided by an implementation whereby the PW is allowed to come up. In that case, the PE advertising the S-bit as 0 should set sequence numbers in the frames as 0, and the PW receiving the frames should not expect to receive non-zero sequence numbers.
该建议可通过允许出现PW的实施来提供。在这种情况下，将S位广告为0的PE应将帧中的序列号设置为0，并且接收帧的PW不应期望接收非零序号。
BGP VPLS can be used over point-to-point Label Switched Paths (LSPs) acting as transport between the VPLS PEs. Alternately, BGP VPLS may also be used over Point-to-Multipoint (P2MP) LSPs with the source of the P2MP LSP rooted at the PE advertising the VPLS BGP NLRI.
BGP VPL可在点对点标签交换路径（LSP）上使用，作为VPL PE之间的传输。或者，BGP vpl也可以在点对多点（P2MP）LSP上使用，P2MP LSP的源扎根于发送VPLS BGP NLRI广告的PE。
In a network that uses P2MP LSPs as transport for a VPLS, there may be some PEs that support the CW while others may not. The behavior is similar for the sequencing of VPLS frames.
在使用P2MP LSP作为VPLS传输的网络中，可能有一些PE支持CW，而其他PE可能不支持CW。VPLS帧的排序行为类似。
In such a setup, a source PE that supports CW should set up two different P2MP LSPs such that:
在这种设置中，支持CW的源PE应设置两个不同的P2MP LSP，以便：
- One P2MP LSP will transport CW-marked frames to those PEs that advertised the C-bit as 1.
- 一个P2MP LSP将把CW标记的帧传输到那些将C位播发为1的PE。
- The other P2MP LSP will transport frames without the CW to those PEs that advertised the C-bit as 0.
- 另一个P2MP LSP将不带CW的帧传输到那些将C位播发为0的PE。
Using two different P2MP LSPs to deliver frames with and without the CW to different PEs ensures that a P2MP root PE honors the C-bit advertised by the other P2MP PEs.
使用两个不同的P2MP LSP向不同的PE传送带CW和不带CW的帧，确保P2MP根PE尊重其他P2MP PE播发的C位。
However, the set of leaves on the two P2MP LSPs (rooted at the given PE) MUST NOT contain any PEs that advertised a value for the S-bit different from what the root PE itself is advertising. PEs that advertised their S-bit values differently (from what the P2MP root PE advertised) will not be on either of the P2MP LSPs. This ensures that the P2MP root PE is sending VPLS frames only to those PEs that agree on the setting of the S-bit.
但是，两个P2MP LSP（以给定PE为根）上的叶子集不得包含为S位播发与根PE本身播发的值不同的任何PE。以不同方式公布其S位值（与P2MP根PE公布的S位值不同）的PE将不会出现在任何P2MP LSP上。这确保P2MP根PE仅向同意S位设置的PE发送VPLS帧。
The ingress router for the P2MP LSP should send separate NLRIs for the cases of using the CW and for not using the CW.
对于使用CW和不使用CW的情况，P2MP LSP的入口路由器应发送单独的NLRI。
-----
/ A1 \
---- ____CE1 |
/ \ -------- -------- / | |
| A2 CE2- / \ / PE1 \ /
\ / \ / \___/ | \ -----
---- ---PE2 | \
| | \ -----
| Service Provider Network | \ / \
| | CE5 A5
| ___ | / \ /
\ / \ PE4_/ -----
PE3 / \ /
|------/ \------- -------
---- / | ----
/ \/ \ / \ CE = Customer Edge Device
| A3 CE3 --CE4 A4 | PE = Provider Edge Router
\ / \ /
---- ---- A<n> = Customer site n
-----
/ A1 \
---- ____CE1 |
/ \ -------- -------- / | |
| A2 CE2- / \ / PE1 \ /
\ / \ / \___/ | \ -----
---- ---PE2 | \
| | \ -----
| Service Provider Network | \ / \
| | CE5 A5
| ___ | / \ /
\ / \ PE4_/ -----
PE3 / \ /
|------/ \------- -------
---- / | ----
/ \/ \ / \ CE = Customer Edge Device
| A3 CE3 --CE4 A4 | PE = Provider Edge Router
\ / \ /
---- ---- A<n> = Customer site n
Figure 1: Example of a VPLS
图1：VPLS的示例
In the above topology, let there be a VPLS configured with the PEs as displayed. Let PE1 be the PE under consideration that is CW enabled and sequencing enabled. Let PE2 and PE3 also be CW enabled and sequencing enabled. Let PE4 not be CW enabled or have the ability to include sequence numbers. PE1 will advertise a VPLS BGP NLRI, containing the C/S-bits marked as 1. PE2 and PE3, on learning of the NLRI from PE1, will include the CW and non-zero sequence numbers in the VPLS frames being forwarded to PE1 as described in Section 4 in [RFC4385]. However, PE4, which does not have the ability to include a CW or include non-zero sequence numbers, will not.
在上述拓扑中，假设有一个VPLS配置了PEs，如图所示。假设PE1是考虑中的启用CW和启用排序的PE。让PE2和PE3同时启用CW和排序。让PE4不启用CW或具有包含序列号的能力。PE1将公布一个VPLS BGP NLRI，其中包含标记为1的C/S位。在从PE1学习NLRI时，PE2和PE3将包括转发给PE1的VPLS帧中的CW和非零序编号，如[RFC4385]第4节所述。然而，不具备包含CW或非零序编号能力的PE4将不会。
As per [RFC4761], PE1 would expect all other PEs to forward CW-containing frames that have non-zero sequence numbers. That expectation cannot be met by PE4 in this example. Thus, as per [RFC4761], the PW between PE1 and PE4 does not come up.
根据[RFC4761]，PE1期望所有其他PE转发包含非零序编号的CW帧。在本例中，PE4无法满足该期望。因此，根据[RFC4761]，PE1和PE4之间的PW不会出现。
However, this document addresses how an implementation should support BGP VPLS in a network where a subset of the BGP VPLS PEs support the CW and/or frame sequencing. PE1 will not bring up the PW with PE4 due to the S-bit mismatch, unless overridden by local configuration on PE1 and PE4 as specified in Section 3.2. If PE4 instead was to advertise a C-bit of 0 and an S-bit of 1, then the PW between PE1 and PE4 would come up despite the CW mismatch. Additionally, PE1 would set up its data plane such that it will strip the CW only for those
但是，本文档说明了在BGP VPLS PE的子集支持CW和/或帧排序的网络中，实现应如何支持BGP VPLS。由于S位不匹配，PE1不会启动带有PE4的PW，除非按照第3.2节的规定，由PE1和PE4上的本地配置覆盖。如果PE4改为公布C位0和S位1，那么尽管CW不匹配，PE1和PE4之间的PW仍会出现。此外，PE1将设置其数据平面，这样它将仅为这些对象剥离CW
VPLS frames that are received from PEs that have indicated their desire to receive CW-marked frames. So, PE1 will set up its data plane to strip the CW only for VPLS frames received from PE2 and PE3, and it will expect to process PW frames containing non-zero sequence numbers as described in Section 4.2 in [RFC4385]. PE1 will set up its data plane to not strip the CW from frames received from PE4, and it would expect PE4 to send frames with non-zero sequence numbers. All frames sent by PE4 to PE1 over the PW would have a non-zero sequence number.
从表示希望接收CW标记帧的PEs接收的VPLS帧。因此，PE1将设置其数据平面，以便仅对从PE2和PE3接收的VPLS帧剥离CW，并期望处理包含非零序编号的PW帧，如[RFC4385]第4.2节所述。PE1将设置其数据平面，以不从从PE4接收的帧中剥离CW，并期望PE4发送具有非零序编号的帧。通过PW由PE4发送到PE1的所有帧将具有非零序编号。
In a multihomed environment, different PEs may effectively represent the same service destination endpoint. It could be assumed that the end-to-end PW establishment process should follow the same rules when it comes to CW requirements, meaning that setting the C-bit would be enforced equally toward both primary and backup designated forwarders.
在多宿环境中，不同的PE可以有效地表示相同的服务目标端点。可以假设，当涉及CW要求时，端到端PW建立过程应遵循相同的规则，这意味着设置C位将对主指定转发器和备用指定转发器进行同等强制。
However, in the multihoming case, each PW SHOULD be evaluated independently. Assuming the network topology specified in Section 5, there could be the case where the PW between PE2 and PE1 could have the CW signaled via the extended community and would be used in the VPLS frame, while the PE2-to-PE4 PW would not insert the CW in the VPLS frame due to a C-bit mismatch. The multihoming behavior of the rest of the PEs should simply follow the rules specified in [VPLS-MULTIHOMING].
然而，在多归宿情况下，每个PW都应该独立评估。假设第5节中规定的网络拓扑，可能存在这样的情况：PE2和PE1之间的PW可以通过扩展社区发送CW信号，并将在VPLS帧中使用，而PE2到PE4 PW由于C位不匹配而不会在VPLS帧中插入CW。其余PEs的多归宿行为应遵循[VPLS-multihoming]中规定的规则。
In a multihomed environment, different PEs may effectively represent the same service destination endpoint. In this case, the rules for end-to-end PW establishment SHOULD follow the same behavior as that described in Section 3.2 when it comes to S-bit requirements. Consider the case described in Section 5 with CE5 having a connection to multiple PEs (multihomed) to PE4 and PE1. The PW's behavior is similar to that for the CW scenario such that the S-bit evaluation SHOULD be independent per PW. So, in the case where PE4 does not set the S-bit in its advertised NLRI, there is an S-bit mismatch between PE1 and PE4. This mismatch prevents the PW establishment between PE1 and PE4. So, only one PW -- between PE1 and PE2 -- would be established for the multihomed site shown. Thus, even though CE5 is physically multihomed, due to PE4's lack of support for sending frames with non-zero sequence numbers, there would be no PW between PE2 and PE4. CE5 would effectively not be multihomed.
在多宿环境中，不同的PE可以有效地表示相同的服务目标端点。在这种情况下，当涉及S位要求时，端到端PW建立的规则应遵循第3.2节中描述的相同行为。考虑第5节中描述的情况，CE5与PE4和PE1具有多个PES（多宿主）的连接。PW的行为类似于CW场景，因此s位评估应独立于每个PW。因此，在PE4未在其公布的NLRI中设置S位的情况下，PE1和PE4之间存在S位不匹配。这种不匹配防止了PE1和PE4之间的PW建立。因此，对于所示的多宿主站点，只会建立一个PW——在PE1和PE2之间。因此，即使CE5在物理上是多址的，由于PE4不支持使用非零序号发送帧，PE2和PE4之间也不会有PW。CE5实际上不会是多宿主的。
This document updates the behavior specified in [RFC4761]. The security considerations discussed in [RFC4761] apply. This document essentially addresses BGP VPLS behavior for PEs when the C-bit value, the S-bit value, or both values advertised by a given PE are different from what another PE in the VPLS is advertising. Any bit-flipping media errors leading to causing this mismatch of C/S-bits between PEs do not adversely affect the availability of the PWs. Rather, they cause CWs to not be used or cause the NLRI-advertising PE to not expect non-zero sequenced frames, for the C-bit and the S-bit, respectively, being mismatched across PEs. This is no worse than the previous behavior where any bit-flipping media errors leading to a mismatch of the C/S-bits between PEs would cause the PW to not come up.
本文档更新了[RFC4761]中指定的行为。[RFC4761]中讨论的安全注意事项适用。当给定PE发布的C位值、S位值或这两个值与VPLS中的另一个PE发布的值不同时，本文档主要解决PE的BGP VPLS行为。导致PEs之间C/S位不匹配的任何位翻转介质错误不会对PWs的可用性产生不利影响。相反，它们导致不使用CWs，或者导致NLRI广告PE不期望C位和S位分别在PE之间不匹配的非零序列帧。这并不比之前的行为更糟糕，因为任何位翻转介质错误都会导致PEs之间的C/S位不匹配，从而导致PW无法出现。
This document has no IANA actions.
本文档没有IANA操作。
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.
[RFC2119]Bradner，S.，“RFC中用于表示需求水平的关键词”，BCP 14，RFC 2119，DOI 10.17487/RFC2119，1997年3月<https://www.rfc-editor.org/info/rfc2119>.
[RFC4761] Kompella, K., Ed. and Y. Rekhter, Ed., "Virtual Private LAN Service (VPLS) Using BGP for Auto-Discovery and Signaling", RFC 4761, DOI 10.17487/RFC4761, January 2007, <https://www.rfc-editor.org/info/rfc4761>.
[RFC4761]Kompella，K.，Ed.和Y.Rekhter，Ed.，“使用BGP进行自动发现和信令的虚拟专用LAN服务（VPLS）”，RFC 4761，DOI 10.17487/RFC4761，2007年1月<https://www.rfc-editor.org/info/rfc4761>.
[RFC4385] Bryant, S., Swallow, G., Martini, L., and D. McPherson, "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for Use over an MPLS PSN", RFC 4385, DOI 10.17487/RFC4385, February 2006, <https://www.rfc-editor.org/info/rfc4385>.
[RFC4385]Bryant，S.，Swallow，G.，Martini，L.，和D.McPherson，“用于MPLS PSN的伪线仿真边到边（PWE3）控制字”，RFC 4385，DOI 10.17487/RFC4385，2006年2月<https://www.rfc-editor.org/info/rfc4385>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8174]Leiba，B.，“RFC 2119关键词中大写与小写的歧义”，BCP 14，RFC 8174，DOI 10.17487/RFC8174，2017年5月<https://www.rfc-editor.org/info/rfc8174>.
[RFC3985] Bryant, S., Ed. and P. Pate, Ed., "Pseudo Wire Emulation Edge-to-Edge (PWE3) Architecture", RFC 3985, DOI 10.17487/RFC3985, March 2005, <https://www.rfc-editor.org/info/rfc3985>.
[RFC3985]Bryant，S.，Ed.和P.Pate，Ed.，“伪线仿真边到边（PWE3）架构”，RFC 3985，DOI 10.17487/RFC3985，2005年3月<https://www.rfc-editor.org/info/rfc3985>.
[VPLS-MULTIHOMING] Kothari, B., Kompella, K., Henderickx, W., Balus, F., and J. Uttaro, "BGP based Multi-homing in Virtual Private LAN Service", Work in Progress, draft-ietf-bess-vpls-multihoming-03, March 2019.
[VPLS-MULTIHOMING]Kothari，B.，Kompella，K.，Henderickx，W.，Balus，F.，和J.Uttaro，“虚拟专用LAN服务中基于BGP的多主服务”，正在进行的工作，草案-ietf-bess-VPLS-MULTIHOMING-032019年3月。
Authors' Addresses
作者地址
Ravi Singh Juniper Networks 1133 Innovation Way Sunnyvale, CA 94089 United States of America
Ravi Singh Juniper Networks 1133 Innovation Way Sunnyvale，加利福尼亚州，美国94089
Email: ravis@juniper.net
Email: ravis@juniper.net
Kireeti Kompella Juniper Networks 1133 Innovation Way Sunnyvale, CA 94089 United States of America
Kireeti Kompella Juniper Networks 1133 Innovation Way Sunnyvale，加利福尼亚州，美国94089
Email: kireeti@juniper.net
Email: kireeti@juniper.net
Senad Palislamovic Nokia 600 Mountain Avenue Murray Hill, NJ 07974-0636 United States of America
Senad Palislamovic诺基亚600 Mountain Avenue Murray Hill，NJ 07974-0636美国
Email: Senad.palislamovic@nokia.com
Email: Senad.palislamovic@nokia.com