
146
a.
Set up an AC: Configure the link layer protocol on a PE and the connected CE to set up a
link layer connection (such as a PPP connection) between the PE and the CE.
b.
Bind the AC to the VC: For most link layer protocols, you bind the AC to the VC by binding
the PE's Layer 3 interface connected to the CE to the VC. For Ethernet links, you can bind
the Layer 3 interface to the VC or bind a service instance to the VC. After the binding, the PE
forwards packets received from the AC to the bound VC, and forwards packets received
from the bound VC to the AC.
Packet forwarding process
MPLS L2VPN implements transparent transmission of Layer 2 user packets over an MPLS or IP
backbone by encapsulating user packets with a VC label and a tunnel tag .
•
The tunnel tag is used to transfer packets from one PE to another. If the public tunnel is an LSP
tunnel or an MPLS TE tunnel, the tunnel tag is an MPLS label. If the public tunnel is a GRE
tunnel, the tunnel tag is the GRE header and the transport protocol packet header.
•
The VC label identifies a Layer 2 connection to a CE. When receiving a packet from the
backbone, a PE forwards the packet to a CE identified by the VC label.
Figure 37 Packet forwarding process
L2 PDU: Layer 2 protocol data unit.
T represents a tunnel tag. V represents a VC label.
As shown in
, MPLS L2VPN forwards packets in the following steps:
1.
After PE 1 receives a Layer 2 packet from CE 1, it adds a VC label to the packet according to
the VC bound to the AC, searches for the public tunnel, adds a tunnel tag to the packet, and
then forwards the packet to the P device.
2.
The P device forwards the packet to PE 2 according to the tunnel tag.
3.
After PE 2 receives the packet from the public tunnel, it identifies the VC to which the packet
belongs according to the VC label of the packet, deletes the tunnel tag and the VC label from
the packet, and then forwards the resulting packet to CE 2 through the AC bound to the VC.
This packet forwarding process is not applicable to the CCC mode of MPLS L2VPN. For more
information about the CCC mode of MPLS L2VPN, see "
Local connection operation
Local connection establishment
To set up a local MPLS L2VPN connection between two CEs:
1.
Set up ACs:
L2PDU
L2PDU
CE 1
PE 1
CE 2
PE 2
P
L2PDU
V
T
L2PDU
V
T