Dynamic CRLSP establishment

Dynamic CRLSPs are dynamically established as follows:

1. An IGP advertises TE attributes for links.

2. MPLS TE uses the CSPF algorithm to calculate the shortest path to the tunnel destination.

   The path must meet constraints such as bandwidth and explicit routing.

3. A label distribution protocol (such as RSVP-TE) advertises labels to establish CRLSPs and reserves bandwidth resources on each node along the calculated path.

Dynamic CRLSPs adapt to network changes and support CRLSP backup and fast reroute, but they require complicated configurations.

MPLS TE uses extended link state IGPs, such as OSPF and IS-IS, to advertise TE attributes for links.

TE attributes include the maximum bandwidth, maximum reservable bandwidth, non-reserved bandwidth for each priority, and the link attribute. The IGP floods TE attributes on the network. 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).

Based on the TEDB, MPLS TE uses the Constraint-based Shortest Path First (CSPF) algorithm, an improved SPF algorithm, to calculate the shortest, TE constraints-compliant path to the tunnel destination.

CSPF first prunes TE constraints-incompliant links from the TEDB, and then it performs SPF calculation to identify the shortest path (a set of LSR addresses) to an egress. CSPF calculation is usually performed on the ingress node of an MPLS TE tunnel.

TE constraints include the bandwidth, affinity, setup and holding priorities, and explicit path. They are configured on the ingress node of an MPLS TE tunnel.

• Bandwidth

  Bandwidth constraints specify the class of service and the required bandwidth for the traffic to be forwarded along the MPLS TE tunnel. A link complies with the bandwidth constraints when the reservable bandwidth for the class type is greater than or equal to the bandwidth required by the class type.

• Affinity

  Affinity determines which links a tunnel can use. The affinity attribute and its mask, and the link attribute are all 32-bit long. A link is available for a tunnel if the link attribute meets the following requirements:

  • The link attribute bits corresponding to the affinity attribute's 1 bits whose mask bits are 1 must have a minimum of one bit set to 1.

  • The link attribute bits corresponding to the affinity attribute's 0 bits whose mask bits are 1 must have no bit set to 1.

  The link attribute bits corresponding to the 0 bits in the affinity mask are not checked.

  For example, if the affinity attribute is 0xFFFFFFF0 and its mask is 0x0000FFFF, a link is available for the tunnel when its link attribute bits meet the following requirements:

  • The highest 16 bits each can be 0 or 1 (no requirements).

  • The 17^th through 28^th bits must have a minimum of one bit whose value is 1.

  • The lowest four bits must be 0.

• Setup priority and holding priority

  If MPLS TE cannot find a qualified path to set up an MPLS TE tunnel, it removes an existing MPLS TE tunnel and preempts its bandwidth.

  MPLS TE uses the setup priority and holding priority to make preemption decisions. For a new MPLS TE tunnel to preempt an existing MPLS TE tunnel, the setup priority of the new tunnel must be higher than the holding priority of the existing tunnel. Both setup and holding priorities are in the range of 0 to 7. A smaller value represents a higher priority.

  To avoid flapping caused by improper preemptions, the setup priority value of a tunnel must be equal to or greater than the holding priority value.

• Explicit path

  Explicit path specifies the nodes to pass and the nodes to not pass for a tunnel.

  Explicit paths include the following types:

  • Strict explicit path—Among the nodes that the path must traverse, a node and its previous hop must be directly connected. Strict explicit path precisely specifies the path that an MPLS TE tunnel must traverse.

  • Loose explicit path—Among the nodes that the path must traverse, a node and its previous hop can be indirectly connected. Loose explicit path vaguely specifies the path that an MPLS TE tunnel must traverse.

  Strict explicit path and loose explicit path can be used together to specify that some nodes are directly connected and some nodes have other nodes in between.

After calculating a path by using CSPF, MPLS TE uses a label distribution protocol to set up the CRLSP and reserves resources on each node of the path.

The device supports the label distribution protocol of RSVP-TE for MPLS TE. Resource Reservation Protocol (RSVP) reserves resources on each node along a path. Extended RSVP can support MPLS label distribution and allow resource reservation information to be transmitted with label bindings. This extended RSVP is called RSVP-TE.

For more information about RSVP, see "Configuring RSVP."