Packet Forwarding; Mpls L2Vpn Implementation - 3Com 3C13636 Configuration Manual

3Com Router 3000 Ethernet Family
Configuration Guide
Control word: It is unnecessary to transmit the L2 frame as a whole in transmitting
12VPN packets over MPLS network. All required is to distract the L2 frame header at
the ingress and to add it at the egress. But for AAL5 and FR encapsulation at the link
layer, extra information needs to be contained in L2 frame header. Control word is
raised in this context to bear necessary information, which has been negotiated
between ingress and egress.
Tunnel label (exterior-layer label): It may be MPLS label or GRE label, to transmit
packets from one PE to another.
VC label (interior-layer label): It is the bottom-layer label to identify a PE-CE link. It has
different meaning in Martini mode and in Kompella MODE. No such label is available
for MPLS L2VPN in CCC mode.
These encapsulation modes at link layer are available for MPLS L2VPN: ATM AAL5,
ATM cell relay, FR, Cisco HDLD, PPP, VLAN, Ethernet etc. The nodes in a same VPN
currently are required to use identical encapsulation mode.

4.1.3 Packet Forwarding

MPLS L2VPN defines CE, PE and P in the same way with BGP/MPLS VPN, and their
fundamentals are similar. Both of them make use of label stack to implement
transparent transmission of subscriber packets over MPLS networks. First tunnels
should be established between PEs either manually or through signaling protocol.
When the interface connecting a PE and a CE enters UP state, PE allocates VC labels
for the packets received and then stick tunnel labels to them. When they reach the
remote PE, it removes tunnel labels and then forwards them to the desired CEs
according the VC labels.
Figure 4-3 illustrates the label stack changes in the process of packet transmission.
CE 1 CE 1
L2 PDU L2 PDU
L2 PDU : Link layer packet L2 PDU : Link layer packet
T: Tunnel label T: Tunnel label
V: VC label V: VC label
T': The outer label will be repl a ced in the forwarding process T': The outer label will be repl a ced in the forwarding process
Figure 4-3 Process an L2VPN label stack

4.1.4 MPLS L2VPN Implementation

By far, no official MPLS L2VPN standard has come into being. Provider-provisioned
Virtual Private Network (PPVPN) workgroup of IETF has proposed several guideline
drafts, among which, Martini and Kompella drafts are the most commonly known drafts.
By March of 2002, these two drafts were respectively called:
PE 1 PE 1
P P
T V L2 PDU T V L2 PDU
T' V L2 PDU T' V L2 PDU
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Chapter 4 MPLS L2VPN Configuration
PE 2 PE 2
CE 2 CE 2
L2 PDU L2 PDU