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17-4 Riverstone Networks RS Switch Router User Guide Release 8.0
MPLS Architecture Overview MPLS Configuration
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swap the label at the top of the label stack with a new label, then add ( push ) a new label onto
the label stack
Label stacks allow for hierarchical routing operations: for example, packets can be routed within an ISP network as
well as at a higher, domain level. This allows MPLS packets to be tunneled through backbone networks. For more
information about using MPLS tunneling, see
Section 17.1.5, "MPLS Tunnels."
After a packet is labeled, the packet is forwarded through the network by switching the incoming label value with an
outgoing label value at each router. A router that receives a labeled packet checks the label value to determine the next
hop for the packet. A label value is relevant only to a particular hop between LSRs; in other words, a label value is
significant only to two connected LSRs.
Label values 0 through 15 are reserved and have the following meanings:
Table 17-2 is a summary of the label operations supported on the RS. These label operations are described in the
following sections.
Table 17-1 Reserved label values
Label Value Meaning
0 IPv4 explicit null label. When it is the only label entry (i.e., there is no label stacking), it
indicates that the label is popped upon receipt. For example, if the LSP is for IPv4 traffic only,
the egress router can signal the penultimate router to use 0 as the final hop label.
1 Router alert label. Packets received with this label value are sent to the CPU for processing.
2 IPv6 explicit null label. When it is the only label entry (i.e., there is no label stacking), it
indicates that the label is popped upon receipt. For example, if the LSP is for IPv6 traffic only,
the egress router can signal the next to last, or penultimate, router to use 2 as the final hop label.
3 Implicit null label. Used in LDP or RSVP packets to request that the label be popped by the
upstream router (penultimate hop label popping). This label should not appear in encapsulation
and should not be used in a data packet.
4-15 Unassigned.
Table 17-2 MPLS label operations supported on the RS
Label bindings:
per interface
per router
if RSVP is used
if LDP is used
Label distribution: Downstream unsolicited—FEC-label bindings are distributed to peers when the RS
is ready to forward packets in the FEC.
Label retention: Liberal—RS stores all labels learned from peers.
Label advertising: Ordered—RS advertises FEC-to-label bindings only when it has previously
received a label for the FEC from the FEC next-hop or when it is an egress router
for the FEC.