H3C Technologies H3C Intelligent Management Center - Manual (Page 89)

2. After receiving the packet, the core network encapsulates the packet with the new public network
label and the MAC address of the new next hop device, and then forwards the packet to the
specified PE accordingly.
3. The PE sends the packet to the right CE through the AC according to the private network label.
VLL overview
VLL (Virtual Leased Line): VLL provides point-to-point L2VPN services over the public network. With VLL,
two sites can communicate as if they were directly connected. VLL is a form of MPLS L2VPN. It uses inner
labels to identify virtual lines (Layer 2 tunnels), hereinafter referred to as virtual circuits (VCs), and uses
outer labels to identify public tunnels. The network devices of the service provider do not need to
maintain any Layer 2 information but perform MPLS forwarding on the public networks based on MPLS
labels. In VLL, a Forwarding Equivalence Class (FEC) is about the VC identifier and some Layer 2
information.
Basic concepts of VLL
With the existing technologies, VLL can be implemented mainly in two methods, which use different
signaling protocols to exchange VC information. One method is called Martini (the L2VPN Management
only support this method). The Martini method uses a standard two-layer label stack. The inner labels
(VC labels) are exchanged through the extended LDP. The Martini draft extends the standard LDP, adding
a new FEC type, the VC FEC, for exchanging VC labels. A VC FEC identifies a VC by VC Type and VC
ID. VC Type indicates the link layer encapsulation type and VC ID uniquely indentifies a VC of the same
type on a PE. The PEs that connect two CEs switch VC labels through LDP and bind the corresponding CEs
by the VC ID. A VC between the two CEs are then established and the two CEs can transport Layer 2
data over the VC. The Martini method does not provide the local switching function. Outer labels is used
to transport VC data over the service provider network. Because PEs can identify data of different VCs by
inner label, multiple VCs can share the public tunnel.
PBB overview
PBB meets the Metro Ethernet Network (MEN) requirements. In the MEN service hierarchy model, PBB is
at the carrier level, used to establish MAC tunnels to separate services of different customers on the
backbone. A network employing 802.1ah is called a Provider Backbone Bridge Network (PBBN).
Basic concepts of PBB
In a typical PBBN, the two types of devices are:
• BEB (Backbone Edge Bridge)—Like a PE in an MPLS network. It performs MAC-in-MAC
encapsulation for the incoming packets and then delivers the encapsulated packets to subsequent
devices, which forwards the packets according to the B-MAC and B-VID. It also de-encapsulates the
received MAC-in-MAC packets to standard Ethernet packets and then looks up the forwarding table
and sends the packets from the corresponding outgoing interfaces.
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