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MPLS works by prefixing packets
with an MPLS header, containing one or more 'labels'.
This is called a label stack.
Each label stack entry contains four fields:
• A 20-bit label value.
• A 3-bit Traffic Class field
for QoS (Quality of Service) priority (experimental)
and ECN (Explicit Congestion
Notification).
• A 1-bit bottom of stack flag.
If this is set, it signifies that the current label is the
last in the stack.
• An 8-bit TTL (time to live) field.
Figure : MPLS
header encapsulation for an IP packet
* These MPLS-labeled packets are
switched after a Label Lookup/Switch instead of a lookup into the IP table.
*As mentioned above, when MPLS
was conceived, Label Lookup and Label Switching were faster than a RIB lookup
because they could take place directly within the switched fabric and not the
CPU.
* The entry and exit points of an
MPLS network are called Label Edge Routers (LER), which, respectively, push an
MPLS label onto an incoming packet and pop it off the outgoing packet.
* Routers that perform routing
based only on the label are called Label Switch Routers (LSR).
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