MPLS-based VPNs typically use a combination of Layer 2 and Layer 3 technologies to provide traffic separation and routing sequencing. The Layer 2 portion of the VPN provides functions such as path selection, tunneling, link aggregation, and Quality of Service (QoS). The Layer 3 portion handles customer route advertisement, address resolution, and other control plane duties.
MPLS-based VPNs offer many advantages over traditional IP-based VPNs, including:
Improved Performance: MPLS greatly reduces CPU overhead and packet processing time by eliminating the need to perform per-packet routing lookups. This can result in improved latency and jitter characteristics for real-time applications such as VoIP.
Addressing Scalability: In an MPLS-based VPN environment, each customer is only assigned a single unique label per VRF rather than a unique address per site like in an IP-based VPN. This drastically reduces the amount of address space required which makes it much easier to scale an MPLS-based VPN as compared to an IP-basedVPN1。
Ease of Provisioning: With an MPLS-basedVPN , new sites can be brought online simply by adding themto an existing VRFrather than provisioning entirely new addressingand routing infrastructure as would be required withan . also significantlyreduces configuration complexity since all that is requiredis advertise the new site’s routes into the appropriateVRF ；no static or default routes needbe configured。
省去配置新的地址和路由基础设施所需的时间：使用MPLS – basedVPN，只需将新站点的路由广播到适当的VRF中即可将其添加到现有VRF中；不需要配置静态或者特定的路由。这样大大减少了配置复杂度。
QoS Functionality: Since MPLS supports both intrinsic available withLayer2 technologies suchas Ethernetand , itcan be usedto provide end – to – end Qualityof Service(QoS) guarantees for voiceand video traffic traversingthe . 能够支持两个不同协议（也就是说一个是传统的IP方式，一个是通过label实现快速传输数据包并存储在frame里以UDP方式表征）。因此就具备了对整条vpn连接上行、下行数据流量都能进行监测、分析而不必依赖于TCP/IP 协议栈sniffer工具了。
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