Brocade Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide (Supporting R05.6.00) - Manual (Page 29)

Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide 5
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How MPLS works
1
The following sections describe these basic MPLS concepts:
• How packets are forwarded through an MPLS domain
• The kinds of Label Switched Paths (LSPs) that can be configured on a device
• The components of an MPLS label header
How packets are forwarded through an MPLS domain
An MPLS domain consists of a group of MPLS-enabled routers, called LSRs (Label Switching
Routers). In an MPLS domain, packets are forwarded from one MPLS-enabled router to another
along a predetermined path, called an LSP (Label Switched Path). LSPs are one-way paths
between MPLS-enabled routers on a network. To provide two-way traffic, the user configures LSPs
in each direction.
The LSRs at the headend and tailend of an LSP are known as LERs (Label Edge Routers). The LER
at the headend, where packets enter the LSP, is known as the ingress LER. The LER at the tailend,
where packets exit the LSP, is known as the egress LER. Each LSP has one ingress LER and one
egress LER. Packets in an LSP flow in one direction: from the ingress LER towards the egress LER.
In between the ingress and egress LERs there may be zero or more transit LSRs. A device enabled
for MPLS can perform the role of ingress LER, transit LSR, or egress LER in an LSP. Further, a
device can serve simultaneously as an ingress LER for one LSP, transit LSR for another LSP, and
egress LER for some other LSP.
Figure 1 depicts an MPLS domain with a single LSP consisting of three LSRs: an ingress LER, a
transit LSR, and an egress LER.
FIGURE 1 Label switching in an MPLS domain
Label switching in an MPLS domain works as described below.