Introduction to L2VPN Interworking

Overview of L2VPN Interworking

L2 transport over MPLS and IP already exists for like-to-like ACs, like Ethernet-to-Ethernet or PPP-to-PPP. L2VPN Interworking would be building on this functionality by allowing disparate ACs to be connected. An interworking function is going to facilitate the translation between the different encapsulations of L2. Only the following interworking combinations are going to be supported:

• ATM-to-Ethernet - Bridged interworking
• ATM-to-Ethernet - Routed interworking
• Frame Relay-to-Ethernet - Bridged interworking
• HDLC-to-Ethernet - Bridged and Routed interworking
• PPP-to-Ethernet - Routed interworking

L2VPN Interworking Modes

L2VPN interworking works in either Ethernet (bridged) module or IP (routed) module. L2VPN interworking doesn’t go to support Ethernet VLAN (Type 4) module. If you are appearing for Lab exam of Cisco, you would require to have a good prep course, like it is being offered by the SPOTO. You could specify the modules in the following ways:

• If using the older legacy CLI commands, you could use the internetworking {ethernet | ip } command in the mode of pseudowire-class configuration.
• If using the newer L2VPN protocol-based CLI commands, you could use the interworking {ethernet |ip } command in the mode of xconnect configuration.

The interworking command would be going to causes the ACs to be terminated locally. The two keywords would be performed to the following functions:

• The ethernet keyword would be causing Ethernet frames to be extracted from the AC and sent over the pseudowire. Ethernet end-to-end transmission would be resumed. AC frames that are not Ethernet would be dropped. In the case of VLAN, the VLAN tag is going to be removed, leaving an untagged Ethernet frame.
• The ip keyword would be causing IP packets to be extracted from the AC and sent over the pseudowire. AC frames that don’t contain IPv4 packets would be dropped.

The following sections would be able to explain more about Ethernet as well as IP interworking modes:

• Ethernet or Bridged Interworking
• IP or Routed Interworking

Ethernet or Bridged Interworking

Ethernet interworking which is also known as bridged interworking. Ethernet frames are considered to be the bridge across the pseudowire. The CE routers could be natively bridging Ethernet or it could be routing by utilizing a bridged encapsulation model, like the BVI (Bridge Virtual Interface) or RBE (Routed Bridge Encapsulation). The PE routers would be operating in Ethernet like-to-like modules. 

IP or Routed Interworking

IP interworking which is also known as routed interworking. The CE routers which would encapsulate the IP on the link between the CE router as well as PE router. A new VC type would be utilized to signal the IP pseudowire in MPLS. Translation between the L2 as well as IP encapsulations across the pseudowire would be required. Special consideration needs to be given to the address resolution as well as routing protocol operation because these are going to be handled differently on different L2 encapsulations. Therefore, address resolution should be terminated on the PE router. End-to-end address resolution is not going to be supported. Routing protocols operate differently over broadcast as well as point-to-point media. For Ethernet, the CE routers must either use static routing or configure the routing protocols so as to treat the Ethernet side as a point-to-point network. In routed interworking, IP packets that would be needed to get extracted from the ACs are sent over the pseudowire. The pseudowire works in the IP Layer 2 transport like-to-like module. The interworking function at NSP’s (network service provider) end performs that would be required adaptation based on the AC technology. Non-IPv4 packets would be dropped. So, there were certain things which you would be needed to know regarding the L2VPN Internetworking. If you are pursuing a Lab exam of Cisco, you should check out the courses, which would be offered by the SPOTO.