H3C Technologies H3C S6800 Series Switches - Manual (Page 67)

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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 reserve
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.
Advertising TE attributes
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).
Calculating paths
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. 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 at least 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 at least one bit whose value
is 1, and the lowest four bits must be 0.
• Setup priority and holding priority
If MPLS TE cannot find a qualified path for an MPLS TE tunnel, it can remove an existing MPLS TE
tunnel and preempt its bandwidth to set up the new MPLS TE tunnel.
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