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.

set interfaces lt-0/0/0 unit 100 encapsulation ethernet
set interfaces lt-0/0/0 unit 100 peer-unit 200
set interfaces lt-0/0/0 unit 100 family inet address 192.168.0.100/24

set interfaces lt-0/0/0 unit 200 encapsulation ethernet
set interfaces lt-0/0/0 unit 200 peer-unit 100
set interfaces lt-0/0/0 unit 200 family inet address 192.168.0.200/24

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 192.168.0.0/24 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.

set routing-instances VR100 instance-type virtual-router
set routing-instances VR100 interface lt-0/0/0.100
set routing-instances VR100 interface lo0.100

set routing-instances VR200 instance-type virtual-router
set routing-instances VR200 interface lt-0/0/0.200
set routing-instances VR200 interface lo0.200

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.

root@TestSRX# run ping 192.168.0.100 routing-instance VR200
PING 192.168.0.100 (192.168.0.100): 56 data bytes
64 bytes from 192.168.0.100: icmp_seq=0 ttl=64 time=1.879 ms
64 bytes from 192.168.0.100: icmp_seq=1 ttl=64 time=3.480 ms
<SNIP>

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.

root@TestSRX# run show route                  

VR100.inet.0: 5 destinations, 5 routes (5 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

10.0.0.100/32      *[Direct/0] 00:41:33
                    > via lo0.100
10.0.0.200/32      *[OSPF/10] 00:00:28, metric 1
                    > to 192.168.0.200 via lt-0/0/0.100
192.168.0.0/24     *[Direct/0] 00:38:21
                    > via lt-0/0/0.100
192.168.0.100/32   *[Local/0] 00:38:21
                      Local via lt-0/0/0.100
224.0.0.5/32       *[OSPF/10] 00:01:18, metric 1
                      MultiRecv

VR200.inet.0: 5 destinations, 5 routes (5 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

10.0.0.100/32      *[OSPF/10] 00:00:28, metric 1
                    > to 192.168.0.100 via lt-0/0/0.200
10.0.0.200/32      *[Direct/0] 00:41:33
                    > via lo0.200
192.168.0.0/24     *[Direct/0] 00:38:21
                    > via lt-0/0/0.200  
192.168.0.200/32   *[Local/0] 00:38:21
                      Local via lt-0/0/0.200
224.0.0.5/32       *[OSPF/10] 00:01:18, metric 1
                      MultiRecv

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.

root@TestSRX# ping source 10.0.0.100 routing-instance VR100 10.0.0.200
PING 10.0.0.200 (10.0.0.200): 56 data bytes
64 bytes from 10.0.0.200: icmp_seq=0 ttl=64 time=1.463 ms
64 bytes from 10.0.0.200: icmp_seq=1 ttl=64 time=1.443 ms
<SNIP>

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 like to lean my head to the left, hit it with the palm of my right hand, and document what knowledge falls out.

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1 comment for “Junos – Logical Tunnel Interfaces with Virtual Routers

  1. joydeep adhikary
    October 22, 2013 at 05:26

    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

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