Types of lsps – Brocade Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide (Supporting R05.6.00) User Manual
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Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide
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How MPLS works
1
In this example, a packet comes into interface 2/1 with label 123. The transit LSR then looks
up this interface-label pair in its MPLS forwarding table. The inbound interface-label pair maps
to an outbound-interface-label pair – in this example, interface 3/1 with label 456. The LSR
swaps label 123 with label 456 and forwards the packet out interface 3/1.
3. The egress LER receives labelled packet, pops label, and forwards IP packet.
When the packet reaches the egress LER, the MPLS label is removed (called popping the
label), and the packet can then be forwarded to its destination using standard hop-by-hop
routing protocols. On signaled LSPs, the label is popped at the penultimate (next to last) LSR,
rather than the egress LER. Refer to
for more information.
Types of LSPs
An LSP in an MPLS domain can be either static or signaled.
Signaled LSPs
Signaled LSPs are configured at the ingress LER only. When the LSP is enabled, RSVP signaling
messages travel to each LSR in the LSP, reserving resources and causing labels to be dynamically
associated with interfaces. When a packet is assigned to a signaled LSP, it follows a
pre-established path from the LSPs ingress LER to its egress LER. This path can be one of the
following:
•
A path that traverses an explicitly specified set of MPLS routers
•
The IGP shortest path across the MPLS domain, determined from local routing tables
•
A traffic-engineered path calculated by the device using constraints such as bandwidth
reservations, administrative groups, and network topology information
For more information, refer to
“How CSPF calculates a traffic-engineered path”
, and
.
Penultimate hop popping
On signaled LSPs, the MPLS label is popped at the next-to-last LSR in the LSP, instead of at the
egress LER. This action is called penultimate hop popping. Penultimate hop popping improves
forwarding efficiency by allowing the egress LER to avoid performing both a MPLS forwarding table
lookup and an IP forwarding table lookup for each packet exiting the LSP. Instead, the MPLS label is
popped at the penultimate (next-to-last) LSR, and the packet is forwarded to the egress LER with no
MPLS encoding. The egress LER, in fact, does not recognize the packet as emerging from an LSP.
illustrates the operation that takes place at the penultimate LSR in an LSP.
FIGURE 3
Penultimate hop popping