Difference between the RD and RT
RD与RT的区别
The Route-Distinguisher (RD) & Route-Target (RT) are two different concepts that are both used in an MPLS VPN. The RD is used to keep all prefixes in the BGP table unique, and the RT is used to transfer routes between VRF’s/VPNS. Let’s take a look at an example.
RD与RT尽管都用在MPLS VPN中（译者注：EVPN中也有用到），但它们是两种不同的概念。RD用于确保BGP表中路由前缀的唯一性，RT则用于在VRFs/VPNs之间传递路由条目。参考如下示例。
In the diagram below, the orange = customer A, and the red = customer B. The ISP is creating a VPN for Customer A’s sites between Cambridge and Birmingham, and also creating a VPN between Customer B’s sites in Cambridge and Birmingham.
如图所示，橙色代表用户A，红色代表用户B。ISP为用户A位于剑桥和伯明翰的两个站点间创建了VPN，同样也为用户B在这两地间创建了VPN。
RD
The problem here, is that in Birmingham, Customer A and Customer B are using the same prefixes (10.1.0.0/24 & 10.2.0.0/24). So how is PE2 going to keep the prefixes in the BGP table unique? The answer is, PE2 needs to create a separate RD for each customer. An RD is a 64 bit value that gets prepended in front of the 32 bit IP address in order to make a globally unique 96 bit address. On PE2 if we configure Customer A to use an RD of 1:1, and Customer B with an RD of 2:2, then in the BGP vpnv4 unicast table we will see the following entries:
那么问题来了：在伯明翰的站点，用户A和B使用了相同的IP前缀（10.1.0.0/24 & 10.2.0.0/24），那么PE2路由器如何确保BGP表中IP前缀的唯一性呢？答案是PE2为每位用户分配单独的RD。RD长度为64位，再通过添加32位的IP地址后缀，共同组成一个唯一的96位地址。若在PE2上配置用户A使用RD 1:1，用户B使用RD 2:2，那么在BGP VPNV4单播表中我们可以看到如下的路由条目：
1:1:10.1.0.0/24
1:1:10.2.0.0/24
2:2:10.1.0.0/24
2:2:10.2.0.0/24
RT
So by pre-pending an RD in front of the route, we have created a globally unique set of BGP prefixes in the vpnv4 BGP table that can be shared between peers. The next question is, how does PE1 know which of these routes belong to Customer A, and which of these routes belong to Customer B? All we’ve done so far is just ensure the routes are globally unique in the vpnv4 BGP table. The table currently contains 4 entries that are literally just considerd prefixes (although it’s very wierd to look at). So the answer is; by using a route-target. A route target is kind of like a little tag that is attached to a route. So PE2 may add, let’s say 100:100 to routes from Customer A in Birmingham, then when PE1 checks the vpnv4 BGP table he can choose to pick out (import) routes that have this 100:100 value and put them into a separate Virtual Routing & Forwarding (VRF) table for Customer A. The configuration regarding RD’s and RT’s is displayed below.
通过在路由条目前添加RD前缀，在BGP VPNV4表中创建了全局唯一的BGP前缀，以此实现BGP对等体之间交换路由条目。接踵而来的问题是： PE1如何知道哪些路由条目属于用户A，哪些路由条目属于用户B？通过设置RD，我们仅仅确保了路由条目在BGP VPNV4表中的唯一性。当前BGP表中存在的4条路由，看上去仅仅就是前缀信息（尽管看上去很奇怪）。问题的答案是：使用RT。RT类似于添加在路由条目上的标签。PE2 为用户A在伯明翰的路由条目添加RT 100:100；当 PE1检视BGP VPNV4表时，挑出拥有100:100标签的路由条目，写入用户A的VRF中。关于RD,RT的配置示例如下。
**PE2**
vrf definition CustomerA
rd 1:1
route-target import 100:100
route-target export 100:100
int fa0/0
description connection to Customer A Birmingham Site
ip vrf forwarding CustomerA
So from the output, you can see that Customer A routes are made unique by prepending an RD of 1:1 to each prefix. Each route is assigned a route-target of 100:100 because of the VRF association with the interface. PE2 then exports this route target so that each of the routes will have 100:100 attached in the RT field within the BGP extended community attribute as they are sent to PE1. PE1 can then import the route-target of 100:100 so that it can associate those routes tagged with this value with the Customer A VRF.
从上述输出中可见，用户A的路由都添加RD 1:1的前缀，每条路由都制定了100:100的RT值。PE2 通过BGP的扩展属性，在导出用户A的路由时都会在RT字段填充100:100，然后发送至PE1。 PE1 通过设置导入RT值100:100，确保了拥有相同RT值的路由条目能被导入用户A的VRF。
Summary
The RD is used purely for the purpose of ensuring routes are unique per VPN. So 10 different VPN’s could use 10.0.0.0/24, and each instance of this prefix would be globally unique. The route-target is used to identify a subset of routes within the BGP vpnv4 unicast table that should be used in a VRF for a particular customer.
RD值纯用于确保每个VPN下路由条目（在BGP表中）的唯一性。因此，10个不同的VPN都能拥有10.0.0.0/24网段的路由，并且（在BGP表中）每个10.0.0.0/24网段的路由条目都是全局唯一的。RT值则用于识别BGP VPNV4表中的每条路由条目具体属于哪个用户的哪个VRF。
