学习rfc 7432 多归场景

8. Multihoming Functions

This section discusses the functions, procedures, and associated BGP

routes used to support multihoming in EVPN. This covers both

multihomed device (MHD) and multihomed network (MHN) scenarios.

本节讨论功能，章程，用于在EVPN中支持多归的BGP route。覆盖多归设备（MHD）和多归网络（MHN）场景

8.1. Multihomed Ethernet Segment Auto-discovery

PEs connected to the same Ethernet segment can automatically discover

each other with minimal to no configuration through the exchange of

the Ethernet Segment route.

链接同一ES的PE可以使用最小的配置通过交换ES路由互相发现

8.1.1. Constructing the Ethernet Segment Route

The Route Distinguisher (RD) MUST be a Type 1 RD [RFC4364]. The

value field comprises an IP address of the PE (typically, the

loopback address) followed by a number unique to the PE.

The Ethernet Segment Identifier (ESI) MUST be set to the 10-octet

value described in Section 5.

The BGP advertisement that advertises the Ethernet Segment route MUST

also carry an ES-Import Route Target, as defined in Section 7.6.

The Ethernet Segment route filtering MUST be done such that the

Ethernet Segment route is imported only by the PEs that are

multihomed to the same Ethernet segment. To that end, each PE that

is connected to a particular Ethernet segment constructs an import

filtering rule to import a route that carries the ES-Import Route

Target, constructed from the ESI.

构造ES路由

RD必须是类型1RD（RFC4364）。值字段由PE的IP地址（通常是环回地址）及紧随的对PE唯一的数字。ESI必须置为第5节描述的10字节值。通告ES路由的BGP通告必须携带ES-Import Route Target扩展团体属性。ES路由仅被多归到同一ES的PE Import这一ES路由过滤必须完成。最后，链接到某一ES的PE构造从其ESI一个过滤规则来import携带ES-Import Route Target的路由。

8.2. Fast Convergence

In EVPN, MAC address reachability is learned via the BGP control

plane over the MPLS network. As such, in the absence of any fast

protection mechanism, the network convergence time is a function of

the number of MAC/IP Advertisement routes that must be withdrawn by

the PE encountering a failure. For highly scaled environments, this

scheme yields slow convergence.

在EVPN，MAC地址可达性是通过BGP控制面在MPLS网络学习的。同样得，缺乏快速保护机制，网络收敛时间是指MAC/IP通告路由在PE故障时被撤掉的数目的功能。大数据环境下，这个模式收敛很慢。

To alleviate this, EVPN defines a mechanism to efficiently and

quickly signal, to remote PE nodes, the need to update their

forwarding tables upon the occurrence of a failure in connectivity to

an Ethernet segment. This is done by having each PE advertise a set

of one or more Ethernet A-D per ES routes for each locally attached

Ethernet segment (refer to Section 8.2.1 below for details on how

these routes are constructed). A PE may need to advertise more than

one Ethernet A-D per ES route for a given ES because the ES may be in

a multiplicity of EVIs and the RTs for all of these EVIs may not fit

into a single route. Advertising a set of Ethernet A-D per ES routes

for the ES allows each route to contain a subset of the complete set

of RTs. Each Ethernet A-D per ES route is differentiated from the

other routes in the set by a different Route Distinguisher (RD).

Upon a failure in connectivity to the attached segment, the PE

withdraws the corresponding set of Ethernet A-D per ES routes. This

triggers all PEs that receive the withdrawal to update their next-hop

adjacencies for all MAC addresses associated with the Ethernet

segment in question. If no other PE had advertised an Ethernet A-D

route for the same segment, then the PE that received the withdrawal

simply invalidates the MAC entries for that segment. Otherwise, the

PE updates its next-hop adjacencies accordingly.

为了减缓此影响，EVPN定义了对于端对PE在到一个ES的链接故障发生时更新他们的转发表的需求的有效，快速的信号的机制。这是通过每台PE通告一组一个或者多个ES A-D路由给每个链接此ES的PE（路由构造见8.2.1）。一台PE对于一个ES可能需要通告多条ES A-D路由，因为该ES可能在多个EVI里且所有EVI的RT可能不适合一条单一路由。为该PE通告一组ES ad路由允许每条路由包含所有RT的一个子组。ES A-D路由通过RD来区分彼此。一旦和接入ES的链路发生故障，PE发相应的ES A-D撤销路由。这会触发所有接收到该撤销路由的PE去更新他们所有和发生问题ES关联的MAC地址的下一跳。如果没有其它PE通告为同样的ES通告ES A-D路由，收到ES A-D撤销路由的PE仅无效该ES的MAC入口。否则，该PE照着更新下一跳信息。

》有ack机制保证PE知道是否有其它PE收到路由？

8.2.1. Constructing Ethernet A-D per Ethernet Segment Route

This section describes the procedure