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3Com Switch 8800 Configuration Guide Chapter 35 MPLS Architecture
35-7
received the returned label map message from its downstream LSR. Usually, the
upstream LSR selects the downstream LSR according to the information in its routing
table. In
Figure 35-4, LSRs on the way along LSP1 use the sequential label control
mode, and the LSR F on LSP2 uses independent label control mode.
In DU mode, the label is distributed in the following way: when LDP session is
established successfully, the downstream LSR will actively distribute label map
message to its upstream LSR. And the upstream LSR saves the label map information
and processes the received label map information according to the routing table.
II. LSP loop control
While establishing LSP in MPLS domain, it is also necessary to prevent the presence of
path loop. Then, such two methods as maximum hop count and path vector can be
used.
The maximum hop count method refers to that the hop-count information is contained
in the message bound with the forwarding label, and the value pluses one for each hop.
When the value exceeds the threshold value, it is considered that a loop presents, and
the process for establishing LSP is terminated.
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.
35.3.4 LSP Tunnel and Hierarchy
I. 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.
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
Figure 35-5 LSP tunnel