Note that although the most commonly used FEC is the Address Prefix,
which maps packets to routes in the IP routing table based on their Layer 3 forwarding
information, packets may be mapped to FECs based on Layer 1, Layer 2, or Layer 4
forwarding information. The use of Layer 1 or Layer 2 information to map packets to
FECs is a key building block in the pseudowire architecture that will be presented in
???Pseudowires,??? later in this chapter.
The MPLS label is a 20-bit value that is included within a 32-bit ???label stack entry,???
as defined in RFC3032 and as shown in Figure 14.2.
The fields in the MPLS label stack entry are used as follows:
?– Each LSR along the LSP decrements the TTL (Time to Live) by one when forwarding
a packet. If the TTL reaches zero, the packet is discarded. The TTL is used to
prevent packets from recirculating when loops are created as a result of inconsistent
router forwarding tables, such as during a routing transient??”the period
between a change in network topology and the point where all routers have learned
the new topology. The TTL field is also used for traceroute where a sequence of
packets is sent with increasing TTL.
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