Benefits and Drawbacks of L2TPv3-Based L2VPNs

For service providers, some of the main benefits and drawbacks of deploying L2TPv3-based L2VPNs are as follows:

• L2TPv3 requires only an IP-enabled backbone. It does not require that the service provider backbone network be MPLS enabled.
• L2TPv3 pseudowires enable service providers to consolidate legacy networks with newer IP-enabled networks.

For example by moving customer ATM and Frame Relay circuits from legacy networks to L2TPv3 pseudowires over an IP-enabled backbone, the service provider can decommission those legacy networks and, therefore, substantially reduce capital and operational expenditure (the service provider now only has to maintain a single network infrastructure, rather than multiple network infrastructures).

• Even after it decommissions legacy networks, a service provider can maintain revenues derived from legacy services, while developing newer services such as Ethernet MAN/WAN service offerings.

For enterprises, on the other hand, the main advantages and disadvantages of deploying L2TPv3-based L2VPNs are as follows:

• When using L2TPv3 pseudowires to connect sites, an enterprise can have complete control of routing. L2TPv3 pseudowires transport Layer 2 protocols, and so enterprise CE devices such as routers can maintain routing protocol adjacencies and exchange routing information over a service provider network. CE devices do not exchange routing information with service provider devices.

  In contrast, if an enterprise chooses to connect sites using an MPLS Layer 3 VPN (L3VPN), CE devices exchange routing information with PE devices, and PE routers participate in the enterprise's routing.

• L2TPv3 pseudowires can transport non-IP protocols such as IPX and AppleTalkthis is again because L2TPv3 transports Layer 2 protocols such as Ethernet, PPP, HDLC, ATM, and Frame Relay, and as such, L2TP Control Connection Endpoints (LCCEs) are completely unaware of Layer 3 protocols that might be transported by those Layer 2 protocols. Other VPN types such as MPLS L3VPNs do not have this capability natively.

  So, L2TPv3 pseudowires are one solution for connecting sites requiring non-IP connectivity.

• L2TPv3 can be used to help migrate networks to IP version 6 (IPv6) using the Cisco L2TPv3 IPv6 demultiplex capability. An enterprise or service provider can use this capability to transparently connect islands of IPv6 over an IPv4 network, while not interrupting regular IPv4 routing.

You should now have a pretty good understanding of the benefits and drawbacks of L2TPv3-based L2VPNs. The sections that follow cover L2TPv3 pseudowire operation, pseudowire configuration, and pseudowire design and deployment considerations.

L2TPv3 Pseudowire Operation