Forwarding Labeled Packets - 3Com 8800 Configuration Manual

3Com Switch 8800 Configuration Guide
with labels, distributes label binding messages, establishes and maintains label
forwarding table.
The network consisting of LSRs is called MPLS domain. The LSR that is located at the
edge of the domain is called edge LSR (LER, Labeled Edge Router). It connects an
MPLS domain with a non-MPLS domain or with another MPLS domain, classifies
packets, distributes labels (as ingress LER) and distracts labels (as egress LER). The
ingress LER is termed as ingress and egress LER as egress.
The LSR that is located inside the domain is called core LSR, which provides functions
such as label swapping and label distribution. The labeled packets are transmitted
along the LSP (Label Switched Path) composed of a series of LSRs.
MPLS edge router (LER) MPLS edge router (LER) MPLS edge router (LER) MPLS edge router (LER)
Figure 35-3 MPLS basic principle

35.3.2 Forwarding Labeled Packets

At the ingress, the packets entering the network are classified into FECs according to
their characteristics. Usually, packets are classified into FECs according to the IP
address prefix or host address. Packets in the same FEC pass through the same path
(that is, LSP) in MPLS area. LSR assigns a short label of fixed length for the incoming
FEC packet, and then forwards it through the corresponding interface.
On the LSR along the LSP, the mapping table of the import/export labels has been
established (the element of this table is referred to as Next Hop Label Forwarding Entry
(NHLFE)). When the labeled packet arrives, LSR only needs to find the corresponding
NHLFE from the table according to the label and replace the original label with a new
one, and then forwards the labeled packet. This process is called Incoming Label Map
(ILM).
At the ingress, MPLS specifies a FEC for a specific packet, and the following routers
only need to forward the packet by label switching, therefore this method is much
simpler than general network layer forwarding and increases the forwarding speed.
Ingress Ingress Ingress Ingress
35-5
Chapter 35 MPLS Architecture
LSP LSP LSP LSP
Egress Egress Egress Egress
MPLS core router (LSR) MPLS core router (LSR) MPLS core router (LSR) MPLS core router (LSR)