The format of an MPLS label stack containing two MPLS shim header entries is shown in MPLS Label Stack.
MPLS Unicast Frame on Ethernet illustrates the format of a unicast MPLS frame on an Ethernet link. The MAC addresses are those of the adjacent MPLS router interfaces. The x8847 Ethertype value indicates that the frame contains an MPLS unicast packet. A different Ethertype value (x8848) is used to identify MPLS multicast packets.
MPLS Unicast Frame on Tagged Ethernet VLAN shows the format of a unicast MPLS frame that contains an 802.1Q VLAN tag. In both cases, the Ethertype values no longer identify the network layer protocol type. This implies that, generally, the protocol type must be inferable from the MPLS label value(s). For example, when only one type of protocol is carried on a given LSP.
Note
For more detailed information on MPLS encapsulations, see RFC 3032, MPLS Label Stack Encoding.