MPLS EXP | Marking Tools

MPLS is a tunneling technology that prepends labels to IP packets to enable more efficient packet switching and virtual private networking. More than one MPLS label can be prepended, or “stacked,” onto an IP packet.

Note

Typically, two labels are used in most MPLS VPN scenarios (one to identify each customer’s traffic and another to perform local switching with a service provider’s cloud); however, in some scenarios, such as MPLS Traffic Engineering, three or more labels are used.

MPLS labels contain 3 bits for CoS marking, which are referred to as the MPLS Experimental (EXP) bits. The possible values of the MPLS EXP bits for CoS are the same as those for 802.1Q/p CoS. Figure 1 shows the MPLS EXP bits within an MPLS label.

 
Figure 1: MPLS EXP bits within an MPLS label

Because MPLS labels include three bits for QoS marking, it is possible to tunnel DiffServ—that is, preserve Layer 3 DiffServ markings through a service provider’s MPLS VPN cloud while still performing re-marking (through MPLS EXP bits) within the cloud to indicate in- or out-of-contract traffic.

RFC 3270 defines three distinct modes of MPLS DiffServ tunneling:

• Uniform Mode: Generally used when the customer and service provider share the same DiffServ domain, as in the case of an enterprise deploying its own MPLS VPN core. The provider can re-mark at either Layer 2 (MPLS EXP) or at Layer 3 (IPP/DSCP). The key point is that in uniform mode, the Layer 3 marking value that a packet has when exiting the MPLS VPN might be different from the value it had when entering the VPN.
• Short-Pipe Mode: Both Pipe and Short-Pipe modes preserve DiffServ transparency; that is, under these modes, Layer 3 packet marking values never change as a packet enters, transits, and exits the MPLS VPN. The key difference between Short-Pipe and Pipe mode relates to the final egress queuing policies as the packet exits the VPN. In Short-Pipe mode, these queuing policies are based on the customer’s Layer 3 markings.
• Pipe Mode: Identical to Short-Pipe mode, but with the final egress queuing policies based on the service provider’s Layer 2 markings.

The enterprise network administrator needs to understand these basic differences between MPLS DiffServ Tunneling modes, especially as they relate to how traffic can be marked and re-marked over the MPLS VPN.