Extreme Networks BlackDiamond 6800 MPLS, Installation And User Manual

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MPLS Module Installation and User Guide 3-5
Overview of MPLS
binding for the FEC. The label mapping message then follows the routed path back to
the ingress LSR, and a label binding is provided by each LSR along the path. LSP
establishment is complete when the ingress LER receives the label mapping message.
Conversely, using DU mode, an LSR may distribute label bindings to LSRs that have
not specifically requested them. These bindings are distributed using the label mapping
message, as in downstream-on-demand mode. From an LDP message perspective, the
primary difference using DU mode is the lack of a preceding label request message.
Architecturally, the difference is more significant, because the DU mode is often
associated with a topology-driven strategy, where labels are routinely assigned to
entries as they are inserted into the routing database. In either case, an LSR only uses a
label binding to switch traffic if the binding was received from the current next hop for
the associated FEC.
Both label advertisement modes can be concurrently deployed in the same network.
However, for a given adjacency, the two LSRs must agree on the discipline. Negotiation
procedures specify that DU mode be used when a conflict exists. Label request
messages can still be used when MPLS is operating in unsolicited mode.
The Extreme LDP implementation supports DU mode only. RSVP-TE, by definition, is
DoD.
Label Retention Modes
MPLS provides two modes for label retention:
•
Conservative
• Liberal
Using conservative label retention mode, an LSR retains only the label-to-FEC mappings
that it currently needs (mappings received from the current next hop for the FEC).
Using liberal retention mode, LSRs keep all the mappings that have been advertised to
them. The trade-off is memory resources saved by conservative mode versus the
potential of quicker response to routing changes made possible by liberal retention (for
example, when the label binding for a new next hop is already resident in memory).
The Extreme MPLS implementation supports liberal label retention, only.