72
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.
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, 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
