Extreme Networks BlackDiamond 6800 MPLS Installation And User Manual page 49

Figure 3-5 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 a MPLS unicast packet. A different Ethertype value
(x8848) is used to identify MPLS multicast packets.
MAC DA
Figure 3-5: MPLS unicast frame on Ethernet
Figure 3-6 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.
MAC DA
Figure 3-6: MPLS unicast frame on tagged Ethernet VLAN
The approach of the shim header encapsulation is similar for Packet over SONET (PoS)
interfaces running PPP. For PoS interfaces running PPP, the MPLS shim header follows
the PPP Protocol ID (PID) field. A PID of x0281 is used to indicate MPLS unicast, while
a PID of x0283 identifies MPLS multicast.
MPLS can also take advantage of technologies that can carry labels in the link layer
header. For example, MPLS labels can be carried in the VPI/VCI fields of ATM cell
headers. Frame Relay provides another example; an MPLS label can be carried in the
DLCI field.
For more detailed information on MPLS encapsulations, see RFC 3032, MPLS
Label Stack Encoding.
MPLS Module Installation and User Guide
Ethertype
MAC SA
x8847
Ethertype
MAC SA
x8100
MPLS remainder
label stack of frame
Ethertype
VLAN tag
x8847
MPLS Layer
MPLS_03
MPLS remainder
label stack of frame
MPLS_04
3-9