Information Technology Reference
In-Depth Information
Example 10-6 Output of traceroute for 192.168.100.6 on R2 (Continued)
2 192.168.35.5 [MPLS: Label 20 Exp 0] 20 msec
192.168.45.5 [MPLS: Label 20 Exp 0] 32 msec
192.168.35.5 [MPLS: Label 20 Exp 0] 28 msec
3 192.168.56.6 40 msec 32 msec *
Forwarding Labeled Packets
Three types of operations can be performed on a received packet:
Push —Push, sometimes called imposition , is an operation performed by an LSR
to create or add a label stack to a packet. This operation is often performed on an
unlabeled packet or a labeled packet where the next-hop information indicates that a
new label or labels should be inserted. Push is typically performed on an ingress LER.
Pop —Pop is performed when a labeled packet is received and the next-hop informa-
tion indicates that one or all label stack entries should be removed. Label popping,
also called label disposition , is typically performed on a penultimate-hop LSR or an
egress LER.
Swap —When a labeled packet arrives, the top label is replaced by a another label.
Label swapping is typically performed on a core LSR.
With the preceding three operations, four packet-switching paths are available:
IP to IP —An incoming IP packet is switched to an outgoing IP packet. This is
conventional IP routing and switching.
IP to label —An unlabeled packet is pushed with one or more labels and is switched
to an LSR.
Label to IP —A labeled packet is popped, and a conventional IP packet is delivered.
Label to label —An incoming label is swapped to an outgoing label, or the top label
is popped, and one or more labels are available for forwarding.
To perform the preceding switching functions, many components must cooperate in a
seamless fashion. Figure 10-4 shows the interaction of the Forwarding Information Base
(FIB) and Label Forwarding Information Base (LFIB). Arrows indicate the directions of
changes triggered from one component to another. As mentioned in Chapter 2, entries in the
RIB provide the prefix entries in FIB. IP packets can be forwarded when the recursion is
fully resolved and the encapsulation string is created in the adjacency table.
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