MPLS TE Class-Based Tunnel Selection (CBTS)

Class-based tunnel selection (CBTS) is an MPLS Traffic Engineering (TE) feature where you can forward traffic with certain EXP values onto different TE tunnels. You can assign one or more EXP values to a tunnel and assign a TE tunnel as the default. This way, you don’t have to assign all EXP values separately.

CBTS is a local mechanism on the headend router and only works for TE tunnels between the same headend and tailend router. We call this “set” of TE tunnels a bundle. There is one master tunnel that bundles the member tunnels.

Mpls Te Cbts Master Member Tunnel Bundle

CBTS is not a routing solution. You still need something like a static route or autoroute to send traffic down the master tunnel interface. When the headend router receives traffic in the master tunnel interface, it will automatically select the correct member tunnel interface based on the EXP value of the packet. That’s the only thing CBTS does.

Packets can enter the headend router through multiple incoming interfaces, and not all packets may be marked correctly if you deal with different customers. When you configure MQC to (re)mark packets on the headend router, CBTS will use the new EXP values to decide what TE tunnel to use.

Configuration

Let’s see how we configure CBTS. This is the topology I’ll use:

Mpls Te Cbts Topology

Routers PE1, P1, P2, and PE2 run MPLS TE. I have two TE tunnels from PE1 to PE2.  I use IOSv Software (VIOS-ADVENTERPRISEK9-M), Version 15.9(3)M4 on all routers.

  • Configurations
  • CE1
  • CE2
  • P1
  • P2
  • PE1
  • PE2

Want to take a look for yourself? Here you will find the startup configuration of each device.

Let’s say we use tunnel 1 for VoIP traffic and tunnel 2 for all other traffic. You would probably use an explicit path option in a production network so that the tunnels use two different paths through the network. For this lab, we won’t need this.

There are two things we need to configure:

  • Assign EXP values to member tunnel interfaces.
  • Create a new master tunnel interface and add the member tunnel interfaces.

Let’s configure our two member tunnels so that packets with EXP value 5 end up in tunnel 1 and everything else ends up in tunnel 2:

PE1(config)#interface Tunnel 1
PE1(config-if)#tunnel mpls traffic-eng exp 5
PE1(config)#interface Tunnel 2
PE1(config-if)#tunnel mpls traffic-eng exp default

Now we configure the master TE tunnel:

PE1(config)#interface Tunnel 10
PE1(config-if)#ip unnumbered Loopback0
PE1(config-if)#tunnel mode mpls traffic-eng
PE1(config-if)#tunnel destination 6.6.6.6
PE1(config-if)#tunnel mpls traffic-eng autoroute announce
PE1(config-if)#tunnel mpls traffic-eng exp-bundle master
PE1(config-if)#tunnel mpls traffic-eng exp-bundle member Tunnel1
PE1(config-if)#tunnel mpls traffic-eng exp-bundle member Tunnel2

The configuration of the master tunnel interface is the same as the member tunnel interfaces. We turn this tunnel interface into a master tunnel with the tunnel mpls traffic-eng exp-bundle master command. We add the two member tunnel interfaces with the tunnel mpls traffic-eng exp-bundle member command.

I also enable autoroute on the master tunnel interface. That’s all you need to configure.