Configure your multicast WAN for OTV

It is easy to find design and configuration guides about OTV implementations on Nexus 7000 switches, ASR and CSR routers. But it is much harder to find some information about the requirements for your WAN.

Please read my previous blog posts about OTV here, here, here and here. I’ll cover the OTV device configurations in those posts. But for now, lets start with the DCI WAN for OTV.

First of all, there are two OTV deployment options:

  • Unicast mode
  • Multicast mode

The WAN requirements in unicast mode are simple: deliver unicast connectivity between the join interfaces of all OTV edge devices. This is just a simple straight forward configuration, I will not cover this in this blog post.

The multicast deployment is a bit harder to configure and requirements are less easier to find. This blog post will cover the required WAN configuration in a multicast deployment. In this particular scenario, we use dark fiber / DWDM connections as DCI to get a more clear understanding about the requirements and configuration.

First, a drawing to get a view on this deployment scenario:

OTV WAN multicast  layout

OTV WAN multicast layout

This blog will provide you with the most easiest way to get your OTV multicast deployment up and running. There are some more finetune options available, but that will not be covered in this blog.

First thing you see in this layout is the fact that all join interfaces of the OTV edge devices (aka  wan faced interfaces) are connected to each other. This is a requirement for all implementations: all join interfaces need IP connectivity to all other join interfaces.

As described before, this scenario covers only a multicast deployment scenario. So, lets take a look at the OTV part of the configuration. The join interface configuration will look like this:

interface TenGigabitEthernet0/1/0
 mtu 1542
 ip address
 ip pim sparse-mode
 ip igmp version 3

There are three important configuration items on this interface:

  • We increase the MTU to at least 1542
  • PIM spare-mode for creating the multicast tree
  • igmp

That is it for the OTV edge device. Lets take a look at the WAN switch configuration.

In our scenario, all three interfaces on all the WAN switches are routed interfaces. This is done to eliminate spanning-tree timers and get a full-routed topology. All three routed interfaces on all the WAN switches will look like this:

interface TenGigabitEthernet1/1/1
 description To-ASR1-JoinIntf
 no switchport
 ip address
 ip pim sparse-mode

The IP address is different for all interfaces, but that makes sense 🙂

To get things going, routing is required. The choice for the routing protocol is up to you. As long as all the join interface can connect to each other. Keep in mind: fast convergence will reduce the OTV downtime during a dark fiber failure. Please choose your routing protocol carefully! It is recommended to join the join-interface in the routing protocol, to get a faster re-convergence, but this is not required. A default static route on the OTV edge pointing to the WAN switch will get you going.