H3C Technologies H3C S7500E Series Switches - Manual (Page 85)

3-3
Basic Concepts of MPLS TE
LSP tunnel
On an LSP, after packets are labeled at the ingress node, the packets are forwarded based on label.
The traffic thus is transparent to the transits nodes on the LSP. In this sense, an LSP can be regarded
as a tunnel.
MPLS TE tunnel
Reroute and transmission over multiple paths may involve multiple LSP tunnels. A set of such LSP
tunnels is called a traffic engineered tunnel (TE tunnel).
MPLS TE Implementation
MPLS TE mainly accomplishes two functions:
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Static Constraint-based Routed LSP (CR-LSP) processing to create and remove static CR-LSPs.
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Dynamic CR-LSP processing to handle three types of CR-LSPs: basic CR-LSPs, backup
CR-LSPs and fast rerouted CR-LSPs.
Static CR-LSP processing is simple, while dynamic CR-LSP processing 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. This 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 Shortest Path First (SPF) to calculate the shortest path to each
network node.
In MPLS TE, the Constraint-based Shortest Path First (CSPF) algorithm is used. It is derived from
SPF and makes calculation based on two conditions:
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Constraints on the LSP to be set up with respect to bandwidth, color, preemption/holding priority,
explicit path and other constraints. They are configured at the LSP ingress.
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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 may 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 using raw IP.
RSVP is a well-established technology in terms of its architecture, protocol procedures and support to
services; while CR-LDP is an emerging technology with better scalability.