Riverstone Networks WICT1-12 - Manual (Page 310)

17-2 Riverstone Networks RS Switch Router User Guide Release 8.0
MPLS Architecture Overview MPLS Configuration
17.1 MPLS ARCHITECTURE OVERVIEW
A forwarding equivalence class (FEC) is a group of IP packets that are forwarded over the same path with the same
forwarding treatment. Examples of FECs include:
• unicast packets whose destination address matches a specified IP address prefix
• unicast packets whose destination address matches a specified IP address prefix and whose type of
service (ToS) bit matches a specified value
• multicast packets with the same source and destination addresses
In “traditional,” non-MPLS networks, each router maps a packet to an FEC based on the destination IP address in the
packet’s network layer header. Each FEC is then mapped to a next hop. At each hop, the packet is examined and
assigned an FEC and a next hop.
In an MPLS network, packets are forwarded in one direction across a label switched path (LSP), as shown in
Figure 17-1 . Labels are the mapping of network layer routing to data link layer switched paths. With MPLS, the
assignment of a specific packet to an FEC is only done once—at the first router in the path, the ingress label switching
router (LSR). Only the ingress LSR
1
in the path needs to analyze the layer 3 header information. If the destination
address in the incoming packet matches an entry in the routing table, the appropriate label is applied to the packet and
the packet is forwarded on to the next hop. At subsequent hops, intermediate or transit LSRs in the path need to only
look at the packet label to forward the packet through the MPLS network; no further examination of the packet’s
network layer header is required.
Figure 17-1 MPLS label switched path
At the egress LSR at the other edge of the path, the label is stripped off. The packet is then forwarded to its next
destination using information in the IP forwarding table. There is one ingress router and one egress router for each LSP.
Note that a router cannot be both an ingress and egress LSR for the same LSP.
17.1.1 Labels
An MPLS label is a 20-bit integer between 0 and 1048575 that identifies a particular FEC. The label is encapsulated
in the packet’s layer-2 header, as shown in
Figure 17-2 .
 ,QJUHVV DQG HJUHVV /65V DUH VRPHWLPHV DOVR UHIHUUHG WR DV ODEHO HGJH URXWHUV /(5V 
Ingress Egress to packet incoming
Label
label assigned label switched label removed
destination
Switching
Router
Label
Switching
Router packet
MPLS Network
Label
Switching
Router Label
Switching
Router
Transit Transit