Basic Concepts; Mpls Te Implementation - HP HSR6800 Configuration Manual

Basic concepts

LSP tunnel—On an LSP, after packets are labeled at the ingress node, the packets are forwarded
based on label. The traffic is transparent to the transits nodes on the LSP. In this sense, an LSP can
be regarded as a tunnel.
MPLS TE tunnel—Rerouting and transmission over multiple paths might involve multiple LSP
tunnels. A set of such LSP tunnels is called a TE tunnel.

MPLS TE implementation

MPLS TE accomplishes the following functions:
•
Static Constraint-based Routed LSP (CR-LSP) processing—Creates and removes static
CR-LSPs. The bandwidth of a static CR-LSP must be configured manually.
•
Dynamic CR-LSP processing—Handles three types of CR-LSPs: basic CR-LSPs, backup
CR-LSPs, and fast rerouted CR-LSPs.
Static CR-LSP processing is simple. Dynamic CR-LSP processing is more complex and involves
four phrases: advertising TE attributes, calculating paths, establishing paths, and forwarding
packets.
Advertising TE attributes
MPLS TE must be aware of dynamic TE attributes of each link on the network, which is achieved by
extending link state-based IGPs, such as OSPF and IS-IS.
OSPF and IS-IS extensions add to link states such TE attributes as link bandwidth, color, among
which maximum reservable link bandwidth and non-reserved bandwidth with a particular priority are
most important.
Each node collects the TE attributes of all links on all routers within the local area or at the same level
to build up a TE database (TEDB).
Calculating paths
Link state-based routing protocols use SPF to calculate the shortest path to each network node.
In MPLS TE, the CSPF algorithm is used to calculate the shortest, TE compliant path to a node. It is
derived from SPF and makes calculations based on the following conditions:
•
Constraints on the LSP to be set up with respect to bandwidth, color, setup/holding priority,
explicit path and other constraints. They are configured at the LSP ingress.
•
TEDB
CSPF first prunes TE attribute incompliant links from the TEDB and then performs SPF calculation to
identify the shortest path to an LSP egress.
Establishing paths
When setting up LSP tunnels, you can use two types of signaling: CR-LDP and RSVP-TE. Both can
carry constraints such as LSP bandwidth, some explicit route information, and color and deliver the
same function.
They are different in that CR-LDP establishes LSPs using TCP while RSVP-TE uses raw IP.
RSVP is a well-established technology in terms of its architecture, protocol procedures and support
to services. CR-LDP is an emerging technology with better scalability.
Both CR-LDP and RSVP-TE are supported on your device.
Forwarding packets
Packets are forwarded over established tunnels.
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