Riverstone Networks WICT1-12 - Manual (Page 313)

Riverstone Networks RS Switch Router User Guide Release 8.0 17-5
MPLS Configuration MPLS Architecture Overview
17.1.2 Label Binding
As mentioned previously, in a non-MPLS network the assignment or binding of a packet to an FEC is based solely on
the destination IP address in the packet header. In an MPLS network, packets that belong to the same FEC follow the
same path, although more than one FEC can be mapped to a single LSP. At the ingress LSR, the assignment of a packet
to an FEC can be influenced by external criteria and not just by the information contained in the packet header. For
example, the following forwarding criteria can also be used to determine label assignment:
• destination unicast routing
• traffic engineering
• whether the packet is a multicast packet
• virtual private network (VPN) configuration
• quality of service (QoS)
The RS supports the following types of FEC-to-label bindings:
• Label bindings can be associated with interfaces. A separate pool of label values is defined for each
interface on which MPLS is enabled.
• Label bindings for the router as a whole can be made from a single “global” pool of label values.
Labels that are distributed on different interfaces cannot have the same value.
The label distribution protocol used determines whether the label bindings are assigned on a per-interface or per-router
basis. See "Label Distribution Protocols" for more information.
17.1.3 Label Distribution and Management
Before the ingress LSR can label incoming packets for a specific FEC and forward the packets, the LSP must be set
up. The binding of the label to the FEC is advertised to neighboring LSRs to establish the LSP. In
Figure 17-4 , packets
sent from R1 to R2 for a particular FEC use the same label binding. In this relationship, R1 is the upstream LSR, while
R2 is the downstream LSR. The downstream LSR determines the binding of a label to an FEC and informs the
upstream LSR of the binding through a label distribution protocol (discussed later in this section).
Figure 17-4 Label binding distribution
There are two ways an LSR can distribute label bindings:
• An LSR can explicitly request a label binding for a particular FEC from its next hop—this is called
downstream on demand label distribution.
• An LSR can also distribute label bindings to other LSRs even if it has not been explicitly requested
to do so—this is called downstream unsolicited label distribution. In downstream unsolicited mode,
FEC-label bindings are distributed to peers when an LSR is ready to forward packets in the FEC.
Downstream
LSR Upstream
LSR
Packets
R1 R2
Label Binding