MPLS can label packets by using the existing layer 2 header or an encapsulation header that carries the MPLS label. During LSP negotiation, the LSRs in an MPLS domain agree on a labeling method. Labels have only local meaning; that is, meaning for two LSR peers. Each pair of LSRs—consisting of a label originator and a label acceptor—must use a label distribution protocol to agree on the label-to-FEC binding.
Because of the local label assignment, packet labels typically change at each segment in the LSP path, as shown in Figure 48. 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. LSR 2 prepends label e to the packet. LSR 3 does the same thing, removing label e and prepending label u. Finally, the egress node, LSR 4, removes label u and determines where to forward the packet outside the MPLS domain.
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, 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.
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