L2TPv3 with IOS

This is a little difficult topic for networking students as sometimes they don’t have work experience and find some scenarios where to use it is not so easy.

L2TPv3 protocol allows us to capture an entire data link layer frame as it enters an interface, encapsulate it and deliver it out to another interface no matter what type of network we there is in between. One of the cases where for sure we need it is when configuring RSPAN and we need the monitored traffic to cross an L3 boundary. Another one could be moving traffic from a VLAN to another edge of a Campus or even extend our LAN network through the Internet.

We will use GNS3 for our lab. Very simple, just 4 routers to test this awesome feature.

Here we have our sample topology. We will use R2 and R3 to create the tunnel. Then R4 and R5 will be the ones that will communicate using L2.

I’ll assume that you already have R2 and R3 sharing routing information. It’s a very simple topology and should take no more than 1 minute to get it working. I configured one Loopback interface in R2 and another one in R3. This loopback interfaces will establish the pseudo-wire.

We need to create the pseudowire-class on each router. For the local interface we will use the loopback as we already mention:

R2

pseudowire-class R2-R3  encapsulation l2tpv3  ip local interface Loopback0

R3

pseudowire-class R3-R2  encapsulation l2tpv3  ip local interface Loopback0

Once done we will configure the physical interfaces. I’ll use FastEthernet0/0. Clear configurations and let’s use xconnect to reference the previously created pseudowire. Xconnect does the magic here, it’s pretty much the same than connecting interfaces F0/0 of routers R1 and R4 with a cross cable.

R2 interfaces

interface FastEthernet0/0  xconnect 192.168.1.2 99 encapsulation l2tpv3 pw-
class R2-R3

R3 interfaces

interface FastEthernet0/0  xconnect 192.168.1.1 99 encapsulation l2tpv3 pw-
class R3-R2

Now it’s time for the verifications. Let’s make sure that the tunnel is established:

Ready, now go to R4 and R5 and configure interfaces as if it were connected with a crossover copper cable. Then check there is communication:

I highly suggest you to make wireshark traffic captures so you’ll have a chance to get a deeper understanding of the underlying protocol we’re using.

Cheers,