In layer-3 networking, routing takes instance MAC and IP addresses out of the network core, reducing state churn. The only time there would be a routing state change is in the case of a Top of Rack (ToR) switch failure or a link failure in the backbone itself. Other advantages of using a layer-3 architecture include: Layer-3 networks provide the same level of resiliency and scalability as the Internet. Controlling traffic with routing metrics is straightforward. You can configure layer-3 to use Border Gateway Protocol (BGP) confederation for scalability. This way core routers have state proportional to the number of racks, not to the number of servers or instances. There are a variety of well tested tools, such as Internet Control Message Protocol (ICMP) to monitor and manage traffic. Layer-3 architectures enable the use of quality of service (QoS) to manage network performance.

在第三层网络中，路由将实例的MAC地址和IP地址从网络核心中移除，减少了状态变化。只有在Top of Rack (ToR)交换机故障或骨干链路故障的情况下，才会发生路由状态的变化。使用第三层架构的其他优势包括：

第三层网络提供与互联网相同级别的弹性和可扩展性。

使用路由度量控制流量非常直观。

可以配置第三层使用边界网关协议（BGP）联盟来实现可扩展性。这样，核心路由器的状态与机架数量成比例，而不是与服务器或实例数量成比例。

有各种经过充分测试的工具，例如Internet控制报文协议（ICMP），用于监视和管理流量。

第三层架构使得可以利用服务质量（QoS）来管理网络性能。

相关问题

The main limitation of layer-3 networking is that there is no built-in isolation mechanism comparable to the VLANs in layer-2 networks. Furthermore, the hierarchical nature of IP addresses means that an instance is on the same subnet as its physical host, making migration out of the subnet difficult. For these reasons, network virtualization needs to use IP encapsulation and software at the end hosts. This is for isolation and the separation of the addressing in the virtual layer from the addressing in the physical layer. Other potential disadvantages of layer-3 networking include the need to design an IP addressing scheme rather than relying on the switches to keep track of the MAC addresses automatically, and to configure the interior gateway routing protocol in the switches.

第三层网络的主要限制是没有内建的与第二层网络中的VLAN相媲美的隔离机制。此外，IP地址的分层结构意味着实例和其物理主机位于同一个子网中，使得迁移到子网之外变得困难。因此，网络虚拟化需要在终端主机上使用IP封装和软件来实现隔离，并将虚拟层中的寻址与物理层中的寻址分离。第三层网络的其他潜在缺点包括需要设计一个IP寻址方案，而不是依靠交换机自动跟踪MAC地址，并需要在交换机上配置内部网关路由协议。

翻译 There are several reasons a network designed on layer-2 protocols is selected over a network designed on layer-3 protocols. In spite of the difficulties of using a bridge to perform the network role of a router, many vendors, customers, and service providers choose to use Ethernet in as many parts of their networks as possible. The benefits of selecting a layer-2 design are: Ethernet frames contain all the essentials for networking. These include, but are not limited to, globally unique source addresses, globally unique destination addresses, and error control. Ethernet frames can carry any kind of packet. Networking at layer-2 is independent of the layer-3 protocol. Adding more layers to the Ethernet frame only slows the networking process down. This is known as nodal processing delay. You can add adjunct networking features, for example class of service (CoS) or multicasting, to Ethernet as readily as IP networks. VLANs are an easy mechanism for isolating networks. Most information starts and ends inside Ethernet frames. Today this applies to data, voice, and video. The concept is that the network will benefit more from the advantages of Ethernet if the transfer of information from a source to a destination is in the form of Ethernet frames. Although it is not a substitute for IP networking, networking at layer-2 can be a powerful adjunct to IP networking. Layer-2 Ethernet usage has additional benefits over layer-3 IP network usage: Speed Reduced overhead of the IP hierarchy. No need to keep track of address configuration as systems move around. Whereas the simplicity of layer-2 protocols might work well in a data center with hundreds of physical machines, cloud data centers have the additional burden of needing to keep track of all virtual machine addresses and networks. In these data centers, it is not uncommon for one physical node to support 30-40 instances.

选择基于第二层协议设计的网络而不是基于第三层协议设计的网络有几个原因。尽管使用桥接器来执行路由器的网络角色存在困难，但许多供应商、客户和服务提供商选择尽可能多地在其网络的各个部分使用以太网。选择第二层设计的好处包括：

以太网帧包含了进行网络通信所需的全部要素，包括但不限于全球唯一的源地址、全球唯一的目标地址和错误控制。

以太网帧可以承载任何类型的数据包。第二层网络与第三层协议无关。

增加以太网帧的层数只会减慢网络处理过程，这被称为节点处理延迟。

你可以像在IP网络中一样容易地为以太网添加附加的网络功能，例如服务类别（CoS）或组播。

虚拟局域网（VLAN）是隔离网络的一种简单机制。

大多数信息都以以太网帧的形式开始和结束。这适用于数据、语音和视频。这个概念是，如果将信息从源传输到目标的方式是以太网帧，网络将从以太网的优势中获益更多。

尽管第二层网络无法替代IP网络，但它可以作为IP网络的一个有力补充。

与第三层IP网络使用相比，第二层以太网使用具有额外的优势：

速度更快。

减少了IP层次结构的开销。

无需跟踪系统移动时的地址配置。

尽管第二层协议的简单性可能在拥有数百台物理机器的数据中心中运作良好，但云数据中心还需要跟踪所有虚拟机的地址和网络，这是一个额外的负担。在这些数据中心中，一个物理节点支持30-40个实例并不罕见。