Path Calculation Component

IS-IS or OSPF uses SPF to calculate the shortest paths between nodes. MPLS TE uses CSPF to calculate the optimal path to a specific node. CSPF is derived from SPF and supports constraints.

Related Concepts

The path calculation component involves the following concepts.

**Table 1** Related concepts
Concept	Description
Tunnel bandwidth	Tunnel bandwidth needs to be planned and configured based on services to be transmitted through a tunnel. When the tunnel is established, the configured bandwidth is reserved on each node on the tunnel, implementing bandwidth assurance.
Affinity	An affinity is a 128-bit vector that describes the links to be used by a TE tunnel. It is configured and implemented on the tunnel ingress, and used together with a link administrative group attribute to manage link selection. After a tunnel is assigned an affinity, a device compares the affinity with the administrative group attribute during link selection to determine whether a link with specified attributes is selected or not. The link selection criteria are as follows: The result of the IncludeAny affinity value ANDed with the administrative group value is not 0. The result of the ExcludeAny affinity value ANDed with the administrative group value is 0. IncludeAny = the affinity attribute value ANDed with the subnet mask value; ExcludeAny = (–IncludeAny) ANDed with the subnet mask value; the administrative group value= the administrative group value ANDed with the subnet mask value. The following rules apply: If some bits in the mask are 1, at least one bit in an administrative group attribute is 1 and its corresponding bit in the affinity attribute must be 1. If the bits in the affinity attribute are 0s, the corresponding bits in the administrative group cannot be 1. If some bits in a mask are 0s, the corresponding bits in an administrative group attribute are not compared with the affinity bits. Figure 1 uses a 16-digit attribute value as example to describe how the affinity works. Figure 1 Attribute value The mask of the affinity determines the link attributes to be checked by the device. In this example, the bits with the mask of 1 are bits 11, 13, 14, and 16, indicating that these bits need to be checked. The value of bit 11 in both the affinity and the administrative group attribute of the link is 0 (not 1). In addition, the values of bits 13 and 16 in both the affinity and the administrative group attribute of the link are 1. Therefore, the link matches the affinity of the tunnel and can be selected for the tunnel. NOTE: Understand specific comparison rules before deploying devices of different vendors because the comparison rules vary with vendors. A network administrator can use the link administrative group and affinities to control the paths over which MPLS TE tunnels are established.
Explicit path	An explicit path is used to establish a CR-LSP. Nodes to be included or excluded are specified on this path. Explicit paths are classified into the following types: Strict explicit path A hop is directly connected to its next hop on a strict explicit path. A strict explicit path precisely controls the path of an LSP. Figure 2 Strict explicit path On the network shown in Figure 2, an LSP needs to be set up over a strict explicit path between LSRA and LSRF. LSRA is the ingress, and LSRF is the egress. "InterfaceB strict" indicates that the LSP must travel through LSRB, and the previous hop of LSRB must be LSRA. "InterfaceC strict" indicates that the LSP must travel through LSRC, and the previous hop of LSRC must be LSRB. The same is true for the other nodes. In this way, a path with each node specified is provided for the LSP. Loose explicit path A loose explicit path contains specified nodes through which an LSP must pass. Nodes that are not specified can also exist on the LSP. Figure 3 Loose explicit path On the network shown in Figure 3, an LSP needs to be set up over a loose strict explicit path between LSRA and LSRF. LSRA is the ingress, and LSRF is the egress. "D Loose" indicates that the LSP must pass through LSRD but there can be other LSRs between LSR D and LSR A. They do not need to be directly connected.
Hop limit	Hop limit is a condition for path selection during CR-LSP establishment. Similar to the administrative group and affinity attributes, a hop limit defines the number of hops that a CR-LSP allows.

CSPF Fundamentals

CSPF works based on the following parameters:

Tunnel attributes configured on an ingress to establish a CR-LSP
TEDB

A TEDB can be generated only after IGP TE is configured. On an IGP TE-incapable network, CR-LSPs are established based on IGP routes, but not CSPF calculation results.

CSPF Calculation Process

The CSPF calculation process is as follows:

Links that do not meet tunnel attribute requirements in the TEDB are excluded.
SPF calculates the shortest path to a tunnel destination based on TEDB information.

CSPF attempts to use the OSPF TEDB to establish a path for a CR-LSP by default. If a path is successfully calculated using OSPF TEDB information, CSPF completes calculation and does not use the IS-IS TEDB to calculate a path. If path calculation fails, CSPF attempts to use IS-IS TEDB information to calculate a path.

CSPF can be configured to use the IS-IS TEDB to calculate a CR-LSP path. If path calculation fails, CSPF uses the OSPF TEDB to calculate a path.

CSPF calculates the shortest path to a destination. If there are several shortest paths with the same metric, CSPF uses a tie-breaking policy to select one of them. The following tie-breaking policies for selecting a path are available:

Most-fill: selects a link with the highest proportion of used bandwidth to the maximum reservable bandwidth, efficiently using bandwidth resources.
Least-fill: selects a link with the lowest proportion of used bandwidth to the maximum reservable bandwidth, evenly using bandwidth resources among links.
Random: selects links randomly, allowing LSPs to be established evenly over links, regardless of bandwidth distribution.

The Most-fill and Least-fill modes are only effective when the difference in bandwidth usage between the two links exceeds 10%, such as 50% of link A bandwidth utilization and 45% of link B bandwidth utilization. The value is 5%. At this time, the Most-fill and Least-fill modes do not take effect, and the Random mode is still used.

On the network shown in Figure 4, except the blue links and the links marked a specific bandwidth value, all the links are of black and have the bandwidth of 100 Mbit/s. In this topology, an MPLS TE tunnel needs to be established. The constraints on this tunnel are: The destination is LSRE, the bandwidth is 80 Mbit/s, the affinity is black, and a transit node is LSRH. The lower part of Figure 4 shows the topology in which links that do not meet the constraints are removed.

Figure 4 Process of link removal

CSPF calculates a path shown in Figure 5 in the same way SPF would calculate it.

Figure 5 CSPF calculation result

Differences Between CSPF and SPF

CSPF is dedicated to calculating MPLS TE paths. It has similarities with SPF but they have the following differences:

CSPF calculates the shortest path between the ingress and egress, and SPF calculates the shortest path between a node and each of other nodes on a network.
CSPF uses metrics such as the bandwidth, link attributes, and affinity attributes, in addition to link costs, which are the only metric used by SPF.
CSPF does not support load balancing and uses three tie-breaking policies to determine a path if multiple paths have the same attributes.