3Com Switch 8800 Configuration Guide
Chapter 35 MPLS Architecture
35-8
As shown in Figure 35-5, LSP <R2 R21 R22 R3> is a tunnel between R2 and R3.
II. Multi-layer label stack
In MPLS, a packet may carry multiple labels which are in the form of stack. Operations
to the stack follow the “last in first out” principle and it is always the labels at the top of
the stack that decide how to forward packets. Pushing label indicates to add a label into
a outgoing packet, then the depth of the label stack is the former one plus 1, and the
current label of the packet changes to the newly added one; popping a label indicates to
remove a label form a packet, then the depth of the packet is the former one minus 1,
and the current label of the packet changes to the label of its underlayer.
Multiple-layer label stack is used in LSP tunnel. When a packet travels in LSP tunnel,
there will be multiple layers for the label of the packet. Then, at the ingress and egress
of each tunnel, it is necessary to implement pushing and popping operation for the label
stack. For each pushing operation, the label will be added with one layer. And there is
no depth limitation for the label stack from MPLS.
The labels are organized according to the principle of “last in first out” in the label stack,
and MPLS processes the labels beginning from the top of the stack.
If the depth of the label stack for a packet is m, it indicates that the label at the bottom of
that stack is level 1 label, and the label at the top of the stack is level m label. A packet
with no label can be regarded as a packet with empty label stack, that is, the depth of its
label stack is 0.
35.4 MPLS and other Protocols
35.4.1 MPLS and Routing Protocols
When LDP establishes LSP in hop-by-hop mode, the next hop is determined by using
the information, which is usually collected by such routing protocols as IGP, BGP in
each LSR route forwarding table, on the way. However, LDP just uses the routing
information indirectly, rather than associates with various routing protocols directly.
On the other hand, although LDP is the special protocol for implementing label
distribution, it is not the sole protocol for label distribution. The existing protocols such
as BGP, RSVP, after being extended, can also support MPLS label distribution. For
some MPLS applications, it is also necessary to extend some routing protocols. For
example, the application of MPLS VPN requires extending the BGP protocol, thus the
BGP protocol can propagate VPN routing information.
35.5 MPLS Application
35.5.1 MPLS VPN
To transmit data stream of private network on public network, traditional VPN uses
tunnel protocols like GRE, L2TP, and PPTP. LSP itself is a tunnel on public network, so
