MPLS Label Format

MPLS Label Format
A label identifies the path a packet should travel. The label
values are based on the underlying network layer (such as
Frame Relay, Ethernet or ATM). For example, if an MPLS core is
operating on ATM equipment, VPIs and VCIs can be used
directly as labels. For Frame Relay networks, DLCI values can
be used as labels. A packet traveling through the network may
have more than one label prepended to it, in which case the
series of labels is referred to as a stack. The innermost label is
used for routing to the next hop, while the outer labels are used
for tunnelling a packet through specific end points of the MPLS core.
The generic MPLS label has four fields:
• 20-bit label value
• 3-bit field for QoS priority
• 1-bit bottom of stack flag (If this is set, it signifies the current
label is the last in the stack.)
• 8-bit TTL (time to live) field

MPLS Label Distribution
Just as IP routers exchange routing information with each other,
MPLS routers (LSRs and LERs) exchange information with each
other using the Label Distribution Protocol (LDP).
A combination of UDP and TCP is used by LDP for exchanging
information with peer devices.
Four types of LDP messages are used:
1. Discovery messages
2. Session messages
3. Advertisement messages
4. Notification messages
Using discovery messages, the LSRs announce their presence
in the network by sending hello messages periodically. This hello
message is transmitted as a UDP packet. When a new session
must be established, the hello message is sent over TCP. Apart
from the discovery message, all other messages are sent over
TCP. The session, advertisement and notification messages are
used for exchanging label information and setting up linked switched paths (LSPs).

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