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MPLS TE Overview 1347
Basic Concepts of MPLS
TE
LSP tunnel
On an LSP, the nodes make forwarding decision for labeled packets 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:
■
Static constraint-based routed LSP (CR-LSP) processing to create and remove
static CR-LSPs. The bandwidth of LSPs 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
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:
■
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.
■
TEDB
What CSPF does to identify the shortest path to an LSP egress is first pruning TE
attribute incompliant links from the TEDB and then performing SPF calculation
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.