Implementation of MPLS L2VPN

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1. Introduction to implementation
The current MPLS L2VPN has not yet formed a formal standard. The IETF's PPVPN (Provider-provisioned Virtual Private Network) working group has developed a number of draft frameworks, the most two of which are called the Martini draft and the Kompella draft:

1.1draft-martini-l2circuit-trans-mpls
The Martini draft defines a way to implement MPLS L2VPN by establishing point-to-point links. It uses the LDP (Label Distribution Protocol) as the signaling protocol to transmit the VC labels of both parties, which is called Martini MPLS L2VPN.

1.2draft-kompella-ppvpn-l2vpn
The Kompella draft defines the establishment of MPLS L2VPN in an end-to-end (CE to CE) manner over an MPLS network. Currently, it uses the extended BGP (Border Gateway Protocol) as the signaling protocol to advertise Layer 2 reachability information and VC labels, called Kompella MPLS L2VPN.

1.3 The MPLS L2VPN service
The MPLS L2VPN service can be implemented by statically configuring VC labels. CCC (Circuit Cross Connect) and SVC (Static Virtual Circuit) are two implementations of statically configured MPLS L2VPN.

2. Introduce the characteristics and implementation methods of these MPLS L2VPNs:
2.1CCC mode MPLS L2VPN
Different from the normal MPLS L2VPN, the CCC uses a layer of labels to transmit user data. Therefore, the use of the LSP (Label Switched Path) is exclusive. The CCC LSP is only used to transmit the data of this CCC connection. It cannot be used for other MPLS L2VPN connections, or for MPLS L3VPN or for carrying ordinary IP packets.

The biggest advantage of this method is that it does not require any label signaling to transmit Layer 2 VPN information, as long as it can support MPLS forwarding, ensuring that operators can interconnect in any case. In addition, since the LSP is dedicated, QoS guarantees can be provided.
There are two types of CCC connections: local and remote.

a) Local connection: A connection established between two local CEs, that is, two CEs are connected to the same PE. A PE is similar to a Layer 2 switch and can be directly exchanged. You do not need to configure a static LSP.
b) Remote connection: The connection established between the local CE and the remote CE, that is, the two CEs are connected to different PEs. You need to configure a static LSP to transfer packets from one PE to another.

2.2SVC mode MPLS L2VPN
SVC (Static Virtual Circuit) is also a static MPLS L2VPN. The signaling protocol is not used in L2VPN information transmission.
The SVC mode is very similar to the MPLS L2PVN in the Martini mode. However, it does not use LDP to transmit Layer 2 VC and link information. Manually configuring VC label information can be a static implementation of the Martini method.

3Martini mode MPLS L2VPN
The Martini MPLS L2VPN focuses on establishing a VC (Virtual Circuit) between two CEs. It is more suitable for sparse two-layer connections, such as star connections.

The Martini method uses VC-TYPE plus VC ID to identify a VC. VC-TYPE indicates the encapsulation type of VC: ATM, VLAN, or PPP; VC ID is used to uniquely identify a VC. In all VCs of the same VC-TYPE, the VC ID must be unique among the entire PE.

A PE that connects two CEs exchanges VC labels through LDP and binds the corresponding CEs through VC IDs. After the LSPs connected to the two PEs are set up successfully, and the label switching and binding of the two parties are complete, a VC is established. The CEs can transmit Layer 2 data through the VC.

In order to exchange VC labels between PEs, the Martini draft extends LDP and adds FEC types of VC FEC (Forwarding Equivalence Class). In addition, since the two PEs that exchange VC labels may not be directly connected, LDP must use a remote peer to establish a session and pass VC FEC and VC labels on this session.

In the Martini mode, the P device does not contain any Layer 2 VPN information because the PE device needs to store a small amount of information such as the mapping between the VC label and the LSP. Therefore, scalability is good. In addition, when a new VC is required, only one single-directional VC connection can be configured on the PE devices at both ends, which does not affect the operation of the network.

4Kompella mode MPLS L2VPN
Different from the Martini mode, the Kompella MPLS L2VPN does not directly operate the connection between the CE and the CE. Instead, it divides the VPNs between the entire carrier network and numbers the CEs within the VPN. To establish a connection between two CEs, you only need to set the CE ID of the local CE and the remote CE on the PE and specify the circuit ID assigned by the local CE to the connection (for example, the VPI/VCI of the ATM).

Kompella mode MPLS L2VPN distributes VC labels by using the BGP extension as a signaling protocol. When assigning labels, the Kompella method uses a label block to assign labels to multiple connections at a time.

You can specify the CE range of a VPN to indicate the maximum number of CEs that can be connected to the current VPN. The system allocates a label block to the CE at a time, and the size of the label block is equal to the CE range. This approach allows the user to assign some extra tags to the VPN for later use. In this short-term, it will cause a waste of label resources, but it has a great advantage, which can reduce the configuration workload when VPN deployment and expansion.

Suppose an enterprise's VPN includes 10 CEs but considering that the enterprise will expand its business, there may be 20 CEs in the future. In this way, the CE range of each CE can be set to 20, and the system will pre-assign labels for the next 10 CEs. When a CE is added to a VPN in the future, the configuration modification is limited to the PE directly connected to the new CE. The other PEs do not need to be modified. This makes the expansion of the VPN very simple.

Similar to MPLS L3VPN, Kompella MPLS L2VPN also uses VPN Target to distinguish different VPNs, which makes VPN networking extremely flexible.

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