Layer 2 Networking

Let’s take a step back and review what exactly a Layer 2 network is. This chapter introduces a lot of new topics related to Layer 2 switching in the MX, and it’s critical that you have an expert understanding of the underlying protocols.

Specifically, we’ll take a look at Ethernet. A Layer 2 network, also known as the data link layer in the seven-layer Open Systems Interconnection (OSI) model, is simply a means to transfer data between two adjacent network nodes. The feature we’re most interested in is virtual local area networks (VLANs) and how they’re processed.

A bridge domain is simply a set of interfaces that share the same flooding, filtering, and forwarding characteristics. A bridge domain and broadcast domain are synonymous in definition and can be used interchangeably with each other.

Ethernet II

By default, an Ethernet frame isn’t aware of which VLAN it’s in as there’s no key to unique identify this information. As the frame is flooded, filtered, or forwarded, it’s done so within the default bridge domain of the interface. Let’s take a look at the format of a vanilla Ethernet II frame.

Ethernet II frame format

Figure 2-2. Ethernet II frame format

There are seven fields in an Ethernet II frame: preamble, start frame delimiter (SFD), destination address (DA), source address (SA), type, data, and frame check sequence (FCS). Let’s take a closer look at each.


This eight-octet field ...

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