Mpls Label Stacking - Juniper JUNOSE 11.2.X BGP AND MPLS Configuration Manual

JunosE 11.2.x BGP and MPLS Configuration Guide

MPLS Label Stacking

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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 219, 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.
Figure 49 on page 221 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 empty stack. Finally, the
egress node, LSR 6, removes label z and determines where to forward the packet outside
the MPLS domain.
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