Junos – Logical Tunnel Interfaces with Virtual Routers

There are a few ways to leak routes in and out of virtual routers in Junos. On the list is a cool feature called the logical tunnel interface.

So, what am I talking about?  One way to separate traffic on a router is to use virtual routers (VRs) so that you wind up with multiple routing tables on the same router.  This separate traffic, but you will usually (read: always) have a demand to get traffic from one VR to another.  There are a few different way to do that (see rib-group, instance-import, next-table, et al.), but one really cool way to do it is through logical tunnel interfaces.

The logical tunnel (lt-0/0/0) interface is a special little guy that allows you to connect its units to each other.  The result is similar to connecting an Ethernet cable from one physical interface to another. With a little configuration, these guys provide a point-to-point interface that you can include in your routing setup.  Let’s look at an example.

The encapsulation lines are pretty straightforward. In this case, I want the link to appear as an Ethernet interface, but you can choose frame-relay, vlan, bridging, and others.

The peer unit lines tell what unit is connected to what other unit. Each lt-0/0/0 unit is a point-to-point link, so you have to tell the router what’s on the other end of the link (sorry…no multiaccess here). In this case, I want unit 100 to connect to unit 200, so I configure both units with the appropriate peer unit.

Of course, we can all figure out that we’re using IPv4 on these new units as well. In this case, I’ve put both interfaces on the network (how original!).

Now that we have our interfaces configured, we need to put these interfaces into the correct VR. Don’t forget the security zone, too, if you’re running in flow mode. That’s beyond the scope here, though.

I put lo0.100 and lo0.200 in there to have something to advertise. You’ll see that in a second.

Now, let’s see if we can ping across.

Woot! It works. Now we can treat these new interaces as if they are regular ole Ethernet. Since I’m not ready to try and blog about IS-IS, let’s just use the standard OSPF. I’m not going to go through the steps to configure OSPF, but here’s the routing table after all the interfaces are included.

Look! OSPF routes! Sweet. Just to keep my OCD at bay, let’s ping from the loopback of one VR to the loopback of the other.

Well, look at that.  It works again.

Let’s look back at the topic we’re discussing, though.  If we use OSPF between the VRs, we need to make sure our routing design allows us to filter routes between the VRs; the risk is that you may wind up having all the routes from each VR advertised to the other.  Kind of defeats the purpose, eh?  Running BGP between the VRs might be an option that allows you to control what routes go in and out.  Statics might be the answer, as well.  As long as you can filter the advertisements, you wind up with a pretty elegant solution for sharing routes between VRs.

Send any Marshmallow Peeps questions my way.

Aaron Conaway

I shake my head around sometimes and see what falls out. That's what lands on these pages.

More Posts

Follow Me:

4 comments for “Junos – Logical Tunnel Interfaces with Virtual Routers

  1. joydeep adhikary
    October 22, 2013 at 5:26 am

    Nice article but i am unable to get the logical tunnel interface up on ex 4550 core switch running 12.3R3.4.
    Not sure if logical tunnels r supported on ex series swicthes

  2. hasky
    July 18, 2014 at 3:30 am

    Really nice, but first you must configure tunneling at chassis level:
    set chassis fpc 1 pic 0 tunnel-services bandwidth 10g

  3. deadElk
    October 25, 2014 at 12:30 pm

    for bgp to work you’ll need packet-mode on both lt-0/0/0 interfaces

  4. sara
    January 26, 2016 at 9:48 am


    is this config possible on MX-series ?


Leave a Reply

Your email address will not be published. Required fields are marked *