When sending IP traffic over MPLS, the IP
TTL may be copied into the MPLS TTL at ingress to the LSP and then overwritten
Figure 14.1 Encapsulation of an IP packet in MPLS
IP
packet
L2
FCS
Layer 2
header
MPLS
header
Figure 14.2 The MPLS label stack entry
EXP S TTL Label
424 Chapter 14
with the new MPLS TTL at egress (this is known as the uniform model and makes
the MPLS LSP visible to IP traceroute), or alternatively the MPLS TTL may be set
to an operator defined value??”generally 225??”at ingress (this is known as the pipe
model and makes the MPLS LSP appear to be a single hop to IP traceroute). TTL
processing for MPLS is documented in RFC 3443.
?– MPLS supports label stacking, where multiple label stack entries are prepended to
a single packet. This is useful in creating hierarchy to scale networks or to separate
administrative domains, and in separating forwarding through the network from
service multiplexing at a given endpoint. You will see that in the Ethernet over
MPLS case, a packet typically has at least two labels??”one used to reach the egress
router and one used to identify the specific Ethernet service at the egress router.
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