H3C Technologies H3C Intelligent Management Center - Manual (Page 158)

In Figure 34 , tunnel 1 is a static tunnel and tunnel 2 is a dynamic tunnel.
MPLS TE implementation
MPLS TE mainly accomplishes the following functions:
• Static Constraint-based Routed LSP (CR-LSP) processing to create and remove static CR-LSPs. The
bandwidth of a static CR-LSP must be configured manually.
• 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 phases: 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 to calculate the shortest, TE
compliant path to a node. It is derived from SPF and makes calculation based on two conditions:
• Constraints on the LSP to be set up with respect to bandwidth, color, setup/hold priority, explicit
path and other constraints. They are configured at the LSP ingress.
• TE database (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
The head-end node establishes a tunnel to the tail-end node using a signaling protocol called RSVP TE.
It can carry constraints such as LSP bandwidth, some explicit route information, and color.
Forwarding packets
Packets are forwarded over established tunnels.
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