L2TPv3 Pseudowhat?

L2TPv3 has been around for a while now, but it seems to be one of those things that not too many people know about.

Typically, when I raise the subject of L2TPv3, I get one of the following reactions:

‘L2TPv3 pseudowhat?'

‘Nobody uses that anymore - it's obsolete, isn't it?'

‘That's a good solution for tunnelling PPP, but we're talking about Ethernet.'

So, for those who aren't really aware of L2TPv3 or what it can do, I thought I'd blog a little on the subject.

The first thing to say about L2TPv3 is that it is not L2TPv2, but it is based on L2TPv2. And the first thing to say about L2TPv2 is that it is neither L2F nor PPTP, but it is based on both of those protocols.

L2TPv2 can be used to tunnel PPP in a remote access VPN configuration, using either a voluntary tunnel mode (between a client workstation/router and a VPN gateway) or compulsory tunnel mode (between two gateways) architecture, as shown in the figure below:

<Figure 1>

L2TPv3 can do everything that L2TPv2 can do, but much more besides.

L2TPv3 can used to build voluntary and compulsory tunnel mode VPNs (the same as L2TPv2 - see figure above).

L2TPv3 pseudowires (emulated circuits) can be used to transport Ethernet, Ethernet VLAN (802.1Q), PPP, HDLC, X.25, ATM (ATM cell-relay and AAL5), and Frame Relay across an IP backbone network. This capability is very similar to that provided by, for example, draft martini (AToM) pseudowires - though you'll typically need an MPLS enabled backbone network for those (you don't really want to transport draft-martini pseudowires over GRE, do you?).

The following figure illustrates some L2TPv3 pseudowires:

<Figure 2>

L2TPv3 can also be used to transport MPLS Layer-3 VPNs across an IP backbone. This can be useful if you are a service provider who has an IP backbone, but doesn't for some reason want to enable MPLS in the core. Instead, you can enable RFC2547bis/RFC 4364 MPLS Layer-3 VPNs on your PE routers, and then use L2TPv3 tunnels to transport the traffic over the IP only backbone network.

L2TPv3 can even be used to transport IPv6 traffic over an IPv4 network. This may be useful when connecting islands of IPv6 in your network.

So, all-in-all L2TPv3 can do quite a lot of different things. And it may just be the thing that you were looking for.

Next time, I'll take a look at configuring L2TPv3.

Mark