Mpls Layer; Mpls Label Stack - Extreme Networks BlackDiamond 6800 MPLS Installation And User Manual

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.
Label
(20 bits)
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.
bottom-of-stack
Label 1 EXP
Figure 3-4: MPLS label stack
3-8
EXP bottom-of-stack
(3 bits) (1 bits)
TTL Label 2
= 0
TTL
(8 bits)
MPLS_01
bottom-of-stack
EXP
= 1
MPLS Module Installation and User Guide
TTL
MPLS_02