MPLS Labels and Label Spaces

MPLS uses labels from either the platform label space or the interface label space. ATM AAL5 interfaces always use labels from only the interface label space. For every interface using the interface label space, you must define the range available to the router for labels in the interface label space. All other interface types always use labels from only the platform label space. You cannot configure the range for the platform label space.

The platform label space is a large, single, unconfigurable pool of labels that can be shared by the platform—all MPLS interfaces on a given virtual router. By contrast, interface labels enable you to effectively create multiple smaller pools of labels, each used only by a particular interface. When you configure interface labels, you restrict only a given interface to a particular range of labels. Other interfaces in that VR can still use labels from that space unless you restrict them in turn to a different range of interface labels.

In the interface label space, MPLS selects labels from interface resources, a VPI/VCI combination. You configure a VPI range and a VCI range available to the labels. When an upstream LSR requests a label, the downstream LSR allocates a VPI/VCI combination for use as a label between these two peers. Allocating labels on a per interface basis is necessary because the VPI/VCI ranges are limited. This enables you to use the same label on different interfaces without conflict.

When you use the platform label space, the MPLS ingress node places labels in shim headers between the link-layer header and the payload. The shim header includes the following bits (Figure 50):

• Label bits—Twenty bits
• EXP bits—Three bits for class of service information; these bits are variously called the experimental bits, class of service (CoS) bits, or type of service (ToS) bits. The EXP bits are mapped from the IP packet at the ingress node and are mapped back into the IP packet at the egress node.
• S bit—One bit to indicate whether the label is on the bottom of the label stack.
• TTL bits—Eight bits for a time-to-live indicator. The TTL bits are mapped from the IP packet at the ingress node. The TTL bits in the shim header are decremented at each hop. The bits are mapped back into the IP packet at the egress node. See TTL Processing in the Platform Label Space for more information.

  Figure 50: Shim Header

  

If you configure an MPLS interface to use the interface label space, the VPI/VCI combinations are used as labels, so there is no need to place them within a shim header. As the data travels along the LSP, the LSRs examine only the VPI/VCI combination. The shim header is used only to carry the TTL bits to the egress, and is not visible to intermediate LSRs. The ingress node learns the total hop count from signaling and then uses that count to decrement the TTL to the correct final value. The TTL is then carried in the shim header to the egress node without modification, arriving with the correct count.

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