First of all, the MPLS VPN network is divided into two areas: the client's IP network and the provider's internal (backbone) MPLS network, which is required to interconnect the clients' networks.

In general, each client can have several geographically separate IP networks, each of which in turn can include several subnets connected by routers. Such geographically isolated network "islands" of the corporate network are usually called sites. Sites belonging to the same client exchange IP packets through the provider's network and form this client's virtual private network. For example, a corporate network that links the central office network to three remote branch offices can be said to have four sites. To exchange routing information within a site, nodes use one of the Interior Gateway Protocol (IGP), the scope of which is limited to an autonomous system: RIP, OSPF, or IS-IS.

The router that connects the customer site to the provider backbone is called the Customer Edge router (CE). As a component of the client's network, CE is unaware of the existence of the VPN from vpn firefox. It can be connected to the provider's backbone by several channels.

The provider's backbone network is an MPLS network, where IP packets are forwarded based not on IP addresses, but on local labels (for more details about technologies of this type, see N. Olifer's article "Paths and Roads Through the Network" in this issue). An MPLS network consists of Label Switch Routers (LSRs) that route traffic along pre-mapped Label Switching Paths (LSPs) according to the values ??of the labels. An LSR is a hybrid of an IP router and a switch, with the IP router taking the ability to determine the topology of the network using routing protocols and choosing rational traffic paths, and from the switch the technique of forwarding packets using labels and local switching tables. LSRs are often referred to simply as routers for short, and for good reason - they can just as well forward packets based on an IP address when MPLS is disabled.

In the provider's network, among LSRs, there are Provider Edge routers (PE), to which client sites and internal routers of the provider's backbone network (Provider router, P) are connected via CE routers. CE and PE routers are usually connected directly by a physical link that runs some kind of link-layer protocol, such as PPP, FR, ATM, or Ethernet. Communication between CE and PE is based on standard protocols of the TCP / IP stack, MPLS support is needed only for internal PE interfaces (and all P interfaces). It is sometimes useful to distinguish between the ingress PE and the egress (remote) PE regarding the direction of traffic.

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