User guide

3-8 MPLS Module Installation and User Guide
Configuring the MPLS Module
MPLS Layer
MPLS can be thought of as a shim-layer between layer 2 and layer 3 of the protocol
stack. MPLS provides connection services to layer-3 functions while making use of
link-layer services from layer-2. To achieve this, MPLS defines a shim header that is
inserted between the link layer header and the network layer header of transmitted
frames. The format of a 32-bit MPLS shim header is illustrated in Figure 3-3.
Figure 3-3: MPLS shim header
MPLS Label Stack
The MPLS shim header is also referred to as a label stack, because it can contain multiple
entries. Each entry contains the following fields:
• 20-bit label
• 3-bit experimental (EXP) field
The EXP field can be used to identify different traffic classes to support the DiffServ
QoS model.
• 1-bit bottom-of-stack flag
The bottom-of-stack bit is set to 1 to indicate the last stack entry.
• 8-bit Time-To-Live (TTL) field.
The TTL field is used for loop mitigation, similar to the TTL field carried in IP
headers.
The format of an MPLS label stack containing two entries is shown in Figure 3-4.
Figure 3-4: MPLS label stack
MPLS_01
Label
(20 bits)
EXP
(3 bits)
bottom-of-stack
(1 bits)
TTL
(8 bits)
MPLS_0
2
Label 1 EXP TTL
bottom-of-stack
= 0
bottom-of-stack
= 1
Label 2 TTLEXP