3Com Switch 8807, Configuration Manual

Page: 614 / 883

614 C
HAPTER
51: MPLS VLL
Figure 147 Structure of an MPLS L2VPN packet
The fields in an MPLS L2VPN packet are described as follows:
Tunnel label (the outer label) is an MPLS label or a GRE label. It is used to transmit
a packet from one PE to anther.
VC label (the inner label) is a lower layer label used to identify the links between
PEs and CEs. Packets of MPLS L2VPNs implemented through circuit cross connect
(CCC) do not contain this label.
Data in MPLS L2VPN can be encapsulated as Ethernet or VLAN packets at the data
link layer. At present, data of different nodes in a VPN must be encapsulated as the
same type of packets.
Packet Forwarding In an MPLS L2VPN, CE, PE, and P operate in the same way as those in a BGP/MPLS
VPN. That is, they also forward packets in an MPLS network transparently by using
label stacks. To forward packets in an MPLS L2VPN, tunnels must be established
first between PEs (this can be achieved by either manual configuration or signaling
protocols). When the interfaces connecting PEs and CEs are up, PEs insert VC
labels for packets sent by CE, and then mark them with tunnel labels. On receiving
these packets, the remote PEs strip off the tunnel labels and send the packets to
the corresponding CEs according to their VC labels.
Table 565 illustrates changes of the label stack of a packet during the forwarding.
Figure 148 Label stack processing of MPLS L2VPN
Implementation At present, the official standard for MPLS L2VPN has not been established yet.
However, the PPVPN (Provider-provisioned Virtual Private Network) group of IETF
(Internet Engineering Task Force) defines multiple framework drafts, two of which
are commonly used. They are known as Martini draft and Kompella draft. Till May,
2005, they are depicted in the following documents respectively:
Tunnel label VC label
Label Stack
L2 PDU
Tunnel label VC label
Label Stack
L2 PDU
Tunnel label VC label
Label Stack
L2 PDU
Tunnel label VC label
Label Stack
L2 PDU
L2 PDU
L2 PDU T V
L2 PDU V
L2 PDU
L2 PDU: Data link layer packet
T: Tunnel label
V: VC label
CE1
PE1 P PE2
CE2
L2 PDU
L2 PDU T V
L2 PDU V
L2 PDU
L2 PDU: Data link layer packet
V:
CE1
PE1 P PE2
CE2
L2 PDU
L2 PDU T V
L2 PDU V
L2 PDU
L2 PDU: Data link layer packet
T: Tunnel label
V: VC label
CE1
PE1 P PE2
CE2
L2 PDU
L2 PDU T V
L2 PDU V
L2 PDU
L2 PDU: Data link layer packet
V:
CE1
PE1 P PE2
CE2