3Com 8807 Configuration Manual page 512

512 C 47: MPLS A
HAPTER RCHITECTURE
Figure 123 Label distribution process
Ingress Ingress
A A
LSP2 LSP2
E E
Label map message Label map message
MPLS edge router (LER) MPLS edge router (LER)
For the label distribution mentioned previously, there are two modes: DoD and
DU. The main difference between these two modes is that the label mapping
messages are distributed actively or passively.
In DoD mode, the label is distributed in this way: the upstream LSR sends label
request message (containing FEC descriptive information) to the downstream LSR,
and the downstream LSR distributes label for this FEC, and then it sends the
bound label back to the upstream LSR through label map message. The time when
the downstream LSR feeds back the label map message depends on whether this
LSR uses independent label control mode or sequential label control mode. When
the sequential label control mode is used by the downstream LSR, the label map
message is sent back to its upstream LSR if only it has received the label map
message from its downstream LSR. And when the independent label control mode
is used by the downstream LSR, then it will send label map message to its
upstream LSR immediately, no matter whether it has 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 123,
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.
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.
B B
Label request message Label request message
F F
G G
LDP session LDP session
LSP1 LSP1
C C
Egress Egress
D D
MPLS core router (LSR) MPLS core router (LSR)
H H