Cisco ASR 920 series

X, Site 1, will build a spanning tree on the switches at that site without considering convergence parameters
based on Customer X's switch in Site 2. This could result in the topology shown in figure below.
Figure 1: Layer 2 Protocol Tunneling
Figure 2: Layer 2 Network Topology without Proper Convergence
In a service-provider network, you can use Layer 2 protocol tunneling to enhance the creation of EtherChannels
by emulating a point-to-point network topology. When you enable protocol tunneling (PAgP or LACP) on
the service-provider switch, remote customer switches receive the PDUs and can negotiate the automatic
creation of EtherChannels.
Carrier Ethernet Configuration Guide (Cisco ASR 920 Series)
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Ethernet Virtual Connections Configuration
Configuring IEEE 802.1Q Tunneling and Layer 2 Protocol Tunneling Using EFPs