102Configuring BPDU tunnelingIntroduction to BPDU tunnelingAs a Layer 2 tunneling technology, BPDU tunneling enables Layer 2 protocol packets fromgeographically dispersed customer networks to be transparently transmitted over specific tunnels acrossa service provider network.BackgroundDedicated lines are used in a service provider network to build user-specific Layer 2 networks. As a result,a user network is broken down into parts located at different sides of the service provider network. Asshown in Figure 36, User A has two devices (CE 1 and CE 2) and both devices belong to VLAN 100.User A’s network is divided into network 1 and network 2, which are connected by the service providernetwork. When a Layer 2 protocol (for example, STP) runs on both network 1 and network 2, the Layer2 protocol packets must be transmitted over the service provider network to implement Layer 2 protocolcalculation (for example, spanning tree calculation). When receiving a Layer 2 protocol packet, the PEscannot determine whether the packet is from the user network or the service provider network, and mustdeliver the packet to the CPU for processing. In this case, the Layer 2 protocol calculation in User A’snetwork is mixed with that in the service provider network, and the user network cannot implementindependent Layer 2 protocol calculation.Figure 36 BPDU tunneling application scenarioWith BPDU tunneling, Layer 2 protocol packets from customer networks can be transparently transmittedover the service provider network in the following workflow:1. After receiving a Layer 2 protocol packet from CE 1, PE 1 in the service provider networkencapsulates the packet, replaces its destination MAC address with a specific multicast MACaddress, and then forwards the packet in the service provider network;2. The encapsulated Layer 2 protocol packet (called bridge protocol data unit, BPDU) is forwardedto PE 2 at the other end of the service provider network, which de-encapsulates the packet, restoresthe original destination MAC address of the packet, and then sends the packet to CE 2.NOTE:BPDU tunneling supports the transparent transmission of the Spanning Tree Protocol (STP) packets.
