In a throwback to the problems I dealt with using AirPlay across VLANs, I recently jumped through similar hoops for Sonos speakers. There are many forum and blog posts out there that describe (or attempt to describe) how to make this work, however all of the ones I read suffered from one or both of these problems:

1. Their instructions had errors (eg, reversing the upstream and downstream interfaces when talking about multicast).
2. They don't have a diagram of traffic flow! Every network engineer knows that a diagram is a must when trying to understand how two systems are talking to each other.

This post will dive deep on what's happening on the wire when a Sonos controller (eg, your mobile phone running the Sonos app) tries to talk with the players (the speakers) on the network. The focus will be how to make this process work when those two devices are in different VLANs.

What you read below works successfully with Sonos Beam, Sonos Sub, and Sonos Move using the Sonos S1 app.

Consider for a moment that you have an application running on a server that needs to push some data out to multiple consumers and that every consumer needs the same copy of the data at the same time. The canonical example is live video. Live audio and stock market data are also common examples. At the re:Invent conference in 2019, AWS announced support for multicast routing in AWS Virtual Private Cloud (VPC). This blog post will provide a walkthrough of configuring and verifying multicast routing in a VPC.

I got an interesting email from Ying Lu who had read my posts on LSM: I am curious about the Ethernet DA and codepoint used for multicast MPLS. Previously, I understand that: Ethernet DA is unicast MAC of nexthop of each replication leg. codepoint is 0x8847 However, looking at RFC5332, I am not so sure... Quote: "Ethernet is an example of a multipoint-to-multipoint data link. Ethertype 0x8847 is used whenever a unicast ethernet frame carries an MPLS packet.

This post is the last one I'm planning in this series on Label Switched Multicast (LSM). The questions & answers below are meant to expand on topics from the previous posts or address topics that weren't mentioned in the previous posts at all.

If you're not familiar with LSM yet then this Q&A likely won't make much sense to you and I recommend you go back and read through the previous posts.

Please post a comment if one of the answers isn't clear or you have additional questions!

This post is going to follow a multicast packet as it moves through a sample MPLS network using Label Switched Multicast (LSM). I'll show how the packet moves through the network by looking at the forwarding tables on different routers and also by doing some packet captures.

This post is part of a series I'm writing on LSM and if you're not already familiar with LSM, I recommend you go back and read the previous posts.

After reading this post you will be able to precisely describe how LSM forwarding works in the data plane and will be able to do some basic troubleshooting.

Let's get into the lab!

In the previous post (Label Switched Multicast - An Introduction) in this series on Label Switched Multicast (LSM) I introduced the concepts behind LSM and draft-rosen, the two most poplar methods for transporting multicast traffic through MPLS Layer 3 VPNs.

In this article I will talk through the configuration of LSM on the PE and P routers and get to the point where two CEs are successfully passing multicast traffic via the MPLS network. All of the configuration examples will be relevant to Cisco IOS.

As was the case in the introduction article in the series, it's best if you already have a good understanding of multicast and MPLS before reading this article.

At the end of this article you'll be able to start configuring your own LSM environment using the configuration samples here as a template.

To the CLI!

There are two common methods for transporting multicast packets within an MPLS-based Layer 3 VPN:

1. Generic Routing Encapsulation (GRE) with Protocol Independent Multicast (PIM) (also known as "draft-rosen")
2. Label Switched Multicast (LSM)

There's also a third method which uses Resource Reservation Protocol-Traffic Engineering (RSVP-TE) but I'm not going to get into that one.

In this first post in a series on LSM, I'll describe how draft-rosen works, how LSM works, and then compare and contrast the two. Subsequent posts will focus solely on LSM.

At the end of this post, you will be able to describe conceptually how the control and data planes work with LSM and what the pros and cons are of LSM as compared to draft-rosen.

I will not be covering any theory on multicast or MPLS and will instead recommend that you be familiar with both topics before reading further.

Here we go!