Introduction to Layer 2 Protocol Transparent Transmission

Definition

Layer 2 protocol transparent transmission is a Layer 2 tunneling technology that transparently transmits BPDUs between private networks at different locations over a specified tunnel on a public Internet Service Provider (ISP) network.

Purpose

Leased lines of ISPs are often used to establish Layer 2 networks. As a result, private user networks can be located at two sides of ISP networks. In Figure 21-1, User A has two networks: network1 and network2. The two networks are connected through the ISP network. When network1 and network2 run the same Layer 2 protocol (such as MSTP), Layer 2 protocol packets from network1 and network2 must be transmitted through the ISP network to perform Layer 2 protocol calculation (for example, calculating a spanning tree). Generally, the destination MAC addresses in Layer 2 protocol packets of the same Layer 2 protocol are the same. For example, the MSTP PDUs are BPDUs with the destination MAC address 0180-C200-0000. Therefore, when a Layer 2 protocol packet reaches an edge device on the ISP network, the edge device cannot identify whether the Layer 2 protocol packet comes from a user network or the ISP network and sends the Layer 2 protocol packets to the CPU to calculate a spanning tree.

In Figure 21-1, devices on user network1 build a spanning tree together with PE1 but not with devices on user network2. As a result, the Layer 2 protocol packets on user network1 cannot traverse the ISP network to reach user network2.

Figure 21-1  Transparent transmission of Layer 2 protocol packets on the ISP network