3Com S7902E, Руководство по настройке

Страница: 1442 / 2621

1-8
The QoS classification in Diff-Serv is very similar to the MPLS label distribution mechanism. In fact, the
MPLS-based Diff-Serv is implemented by integrating the DS distribution into the MPLS label
distribution.
MPLS Configuration Basics
Label Distribution and Management
In MPLS, the label that an LSR uses for a FEC is assigned by the downstream LSR. The downstream
LSR then informs the upstream LSR of the assignment. That is, labels are advertised in the upstream
direction.
Label advertisement mode
Two label advertisement modes are available:
z
Downstream on demand (DoD): In this mode, an LSR distributes a label binding to another LSR
only when it receives a label request from the LSR.
z
Downstream unsolicited (DU): In this mode, an LSR does not wait for any label request before
distributing a label binding.
An upstream LSR and its downstream LSR must use the same label advertisement mode; otherwise,
no LSP can be established normally. For more information, refer to
LDP Label Distribution .
Currently, the S7900E series supports only the DU mode.
Label distribution control mode
There are two label distribution control modes:
z
Independent: In this mode, an LSR can advertise label bindings upstream at anytime. A
consequence of this mode is that an LSR may have advertised a label binding to the upstream LSR
when it receives a binding from its downstream LSR.
z
Ordered: In this mode, an LSR advertises its label binding for a FEC upstream only when it
receives a label binding from the next hop for the FEC or it is the egress of the FEC.
Label retention mode
Label retention mode dictates how to process a received label binding that is not useful at the moment.
There are two label retention modes:
z
Liberal: In this mode, an LSR keeps any received label binding regardless of whether the binding is
from its next hop for the FEC or not.
z
Conservative: In this mode, an LSR keeps only label bindings that are from its next hops for the
FECs.
In liberal mode, an LSR can adapt to route changes quickly; while in conservative mode, there are less
label bindings for an LSR to advertise and keep.