Juniper JUNOSE, Configuration Manual

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Figure 48: Label Switching
Any packet can carry multiple labels. The labels are stacked in a last-in-first-out order.
Each LSR forwards packets based on the outermost (top) label in the stack. An LSR
pushes a label onto the stack when it prepends the label to a packet header. It pops
the label when it pulls the label off the stack and compares it with the forwarding
table. On determining the label for the next segment of the LSP, the LSR pushes the
new label on the stack. A label swap consists of a pop, lookup, and push.
When the egress router, such as LSR 4 in Figure 48 on page 211, receives a packet,
it may perform two lookups: it looks up the label and determines that the label must
be popped, then it does another lookup based on the exposed header to determine
where to forward the packet. This behavior is known as
ultimate hop popping , and
was the only possible action for the JUNOSe implementation before Release 7.3.0.
Beginning with JUNOSe Release 7.3.0, an alternative behavior, known as penultimate
hop popping (PHP), is the default when RSVP-TE is the signaling protocol. Beginning
with JUNOSe Release 8.1.0, PHP is also the default when LDP is the signaling protocol.
PHP reduces the number of lookups performed by the LER. In PHP, the LER requests
its upstream neighbor (the penultimate hop) to pop the outermost label and send
just the packet to the LER. The LER then performs only the lookup for the packet.
The request to perform PHP is signaled by the LER when it includes an implicit null
label in the label mapping message that it sends to its upstream neighbor. The implicit
null label never appears in the encapsulation.
You can still achieve ultimate hop popping by configuring the egress router to
advertise an explicit null label to its upstream neighbor. This advertisement, performed
by LDP or RSVP-TE, ensures that all MPLS packets traversing the LSP to the egress
router include a label. Alternatively, you can configure the egress router to advertise
real (non-null) labels, and achieve the same result.
Regardless of whether the LSR advertises the implicit null label to achieve PHP on
an upstream neighbor, if the LSR receives a PHP request from a downstream neighbor,
then the LSR does perform the PHP for its neighbor.
MPLS Label Stacking
Figure 49 on page 212 shows an LSP that uses label stacking. The ingress node, LSR
1, receives an unlabeled data packet and prepends label
d to the packet. LSR 2
receives the packet, removes label d and uses it as an index in its forwarding table
to find the next label, prepending label e to the packet. LSR 3 removes label e and
prepends label s (negotiated with LSR 5) to the packet. LSR 3 pushes label x on top
of label s . LSR 4 pops the top (outermost) label, x , and pushes label r on top of label
s . LSR 5 pops label r , determines that it must pop label s , and pushes label z on the
MPLS Label Switching and Packet Forwarding
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Chapter 2: MPLS Overview