Operation Manual – MPLS Basics
H3C S9500 Series Routing Switches
Chapter 1 MPLS Basics Configuration
1-3
z
TTL: Time to live (TTL) for the label. Eight bits in length. This field has the same
meaning as that for an IP packet.
Similar to the VPI/VCI in ATM and the DLCI in frame relay, an MPLS label functions as
a connection identifier. If the link layer protocol has a label field like VPI/VCI in ATM or
DLCI in frame relay, the MPLS label is encapsulated in that field. Otherwise, it is
inserted between the data link layer header and the network layer header as a shim. As
such, an MPLS label can be supported by any link layer protocol.
Figure 1-2
shows the place of a label in a packet.
Figure 1-2
Place of a label in a packet
Note:
Currently, the S9500 series does not support the cell mode.
III. LSR
Label switching router (LSR) is a fundamental component on an MPLS network. All
LSRs support MPLS.
IV. LSP
Label switched path (LSP) means the path along which a FEC travels through an MPLS
network. Along an LSP, two neighboring LSRs are called upstream LSR and
downstream LSR respectively. In
Figure 1-3
, R2 is the downstream LSR of R1, while
R1 is the upstream LSR of R2.
