Riverstone Networks WICT1-12 - Manual (Page 314)

17-6 Riverstone Networks RS Switch Router User Guide Release 8.0
MPLS Architecture Overview MPLS Configuration
Label Distribution Protocols
An LSP is defined by the set of labels from the ingress LSR to the egress LSR. When an LSR assigns a label to an
FEC, it must let other LSRs in the path know about the label and its meaning. Label distribution protocols help to
establish the LSP by providing a set of procedures that LSRs can use to distribute labels. Specifically, label distribution
protocols allow an LSR to request a label from a downstream LSR so that it can bind the label to a specific FEC. The
downstream LSR responds to the request from the upstream LSR by sending the requested label.
The RS supports the following protocols for label distribution:
• Label distribution protocol (LDP) is an IETF-defined protocol for LSRs to distribute labels and their
meanings to LDP peers. LDP assigns labels from a single pool of labels on a router. To establish an
LSP, LDP does not need to rely on routing protocols at every hop along the LSP. LDP allows the
establishment of “best effort” LSPs; it does not provide traffic engineering mechanisms. LDP is
required for tunneling of layer-2 frames across MPLS networks, as described in
Section 17.6,
"Configuring L2 Tunnels." For more information about configuring LDP on the RS, see
Section 17.4, "LDP Configuration."
• Resource reservation protocol (RSVP) is a protocol that allows channels or paths to be reserved for
high bandwidth transmissions. RSVP assigns labels on a per-interface basis. RSVP is used for traffic
engineering, which is often required in core or backbone networks where resources are not always
available; see
Section 17.7, "Traffic Engineering." For more information about configuring RSVP
on the RS, see Section 17.3, "RSVP Configuration."
The LSP must be set up before packets can be forwarded through the MPLS network. Figure 17-5 shows how labels
are created and distributed to create an LSP.
Figure 17-5 LSP creation and packet forwarding
1. Downstream routers distribute label bindings to upstream neighbors.
2. Packet P1 is received at router R1.
3. Router R1 inserts the appropriate label into the packet and forwards it to the next hop. The packet
label is examined, replaced and forwarded through the MPLS network. (If R1 does not find a match
for this FEC, the packet is forwarded using IP forwarding table information.)
4. The label is removed and forwarded using information in the IP forwarding table at R4.
to packet incoming
destination packet P1
R1
R2 R3 R4
2
1
label binding
3
4
Upstream LSR Downstream LSR
label binding
packet forwarding with labels