3com 8807 사용자 가이드
MPLS Architecture
513
The path vector method refers to that the path information is recorded in the
message bound with the forwarding label, and, for every hop, the corresponding
router checks if its ID is contained in this record. If not, the router adds its ID into
the record; and if yes, it indicates that a loop presents and the process for
establishing LSP is terminated.
message bound with the forwarding label, and, for every hop, the corresponding
router checks if its ID is contained in this record. If not, the router adds its ID into
the record; and if yes, it indicates that a loop presents and the process for
establishing LSP is terminated.
LSP Tunnel and
Hierarchy
LSP tunnel
MPLS supports LSP tunnel technology. On an LSP path, LSR Ru and LSR Rd are
both the upstream and the downstream for each other. However, the path
between LSR Ru and LSR Rd may not be part of the path provided by routing
protocol. MPLS allows establishing a new LSP path <Ru R1...Rn Rd> between LSR
Ru and LSR Rd, and LSR Ru and LSR Rd are respectively the starting point and
ending point of this LSP. The LSP between LSR Ru and LSR Rd is referred to as the
LSP tunnel, which avoids the traditional encapsulated tunnel on the network layer.
If the route along which the tunnel passes and the route obtained hop by hop
from routing protocol is consistent, this tunnel is called hop-by-hop routing
tunnel. And if the two routes are not consistent, then the tunnel of this type is
called explicit routing tunnel.
both the upstream and the downstream for each other. However, the path
between LSR Ru and LSR Rd may not be part of the path provided by routing
protocol. MPLS allows establishing a new LSP path <Ru R1...Rn Rd> between LSR
Ru and LSR Rd, and LSR Ru and LSR Rd are respectively the starting point and
ending point of this LSP. The LSP between LSR Ru and LSR Rd is referred to as the
LSP tunnel, which avoids the traditional encapsulated tunnel on the network layer.
If the route along which the tunnel passes and the route obtained hop by hop
from routing protocol is consistent, this tunnel is called hop-by-hop routing
tunnel. And if the two routes are not consistent, then the tunnel of this type is
called explicit routing tunnel.
Figure 124 LSP tunnel
As shown in Figure 124, LSP <R2 R21 R22 R3> is a tunnel between R2 and R3.
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.
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.
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.
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.
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.
R1
R2
R3
R4
R21
R22
Layer 1
Layer 2
R1
R2
R3
R4
R21
R22
R1
R2
R3
R4
R21
R22
Layer 1
Layer 2