No relevant resource is found in the selected language.

This site uses cookies. By continuing to browse the site you are agreeing to our use of cookies. Read our privacy policy>Search

Reminder

To have a better experience, please upgrade your IE browser.

upgrade

NE20E-S2 V800R010C10SPC500 Feature Description - MPLS 01

This is NE20E-S2 V800R010C10SPC500 Feature Description - MPLS
Rate and give feedback:
Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).
Path Calculation Component

Path Calculation Component

Intermediate System to Intermediate System (IS-IS) or Open Shortest Path First (OSPF) uses shortest path first (SPF) to calculate the shortest paths between nodes. MPLS TE uses constrained shortest path first (CSPF) to calculate the optimal path to a specific node. CSPF, which is derived from SPF, is an algorithm that supports constraints.

Related Concepts

Path Calculation Component involves the following concepts:
Table 4-5 Related Concepts

Concept

Description

Bandwidth

Bandwidth values are planned based on services that are to pass through a tunnel. The configured bandwidth is reserved on each node through which a tunnel passes.

Affinity attribute

An affinity is a 32-bit vector, configured on the ingress of a tunnel. It must be used together with a link administrative group attribute.

After a tunnel is configured with an affinity, a device compares the affinity with the administrative group value 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 OR administrative group value is not 0.

  • The result of the ExcludeAny affinity OR the administrative group value is 0.

IncludeAny equals the result of the affinity attribute OR the subnet mask; ExcludeAny equals IncludeAny OR the subnet mask; the administrative group value equals the administrative group value OR the subnet mask.

The following rules apply:
  • If some bits in a mask are 1s, at least one bit in the administrative group is 1 and the corresponding bit in the affinity must be 1. If some bits in the affinity are 0s, the corresponding bits in the administrative group cannot be 1.

    For example, an affinity is 0x0000FFFF and its mask is 0xFFFFFFFF. The higher-order 16 bits in the administrative group of available links are 0 and at least one of the lower-order 16 bits is 1. This means the administrative group attribute ranges from 0x00000001 to 0x0000FFFF.

  • If some bits in a mask are 0s, the corresponding bits in the administrative group are not compared with the affinity bits.

    For example, an affinity is 0xFFFFFFFF and its mask is 0xFFFF0000. At least one of the higher-order 16 bits in an administrative group attribute is 1 and the lower-order 16 bits can be 0s and 1s. This means that the administrative group attribute ranges from 0x00010000 to 0xFFFFFFFF.

NOTE:

Understand specific comparison rules before deploying devices of different vendors because the comparison rules vary with the vendor.

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 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. By specifying a strict explicit path, the most accurate path is provided for a CR-LSP.

    Figure 4-5 Strict explicit path

    For example, a CR-LSP is set up between LSRA and LSRF on the network shown in Figure 4-5. LSRA is the ingress, and LSRF is the egress. "X strict" specifies the LSR that the CR-LSP must travel through. For example, "B strict" indicates that the CR-LSP must travel through LSRB, and the previous hop of LSRB must be LSRA. "C strict" indicates that the CR-LSP must travel through LSRC, and the previous hop of LSRC must be LSRB. The procedure repeats. A path with each node specified is provided for the CR-LSP.

  • Loose explicit path

    A loose explicit path contains specified nodes through which a CR-LSP must pass. Other routers that are not specified can also exist on the CR-LSP.

    Figure 4-6 Loose explicit path

    For example, a CR-LSP is set up over a loose explicit path between LSRA and LSRF on the network shown in Figure 4-6. LSRA is the ingress, and LSRF is the egress. "D loose" indicates that the CR-LSP must pass through LSRD and LSRD and LSRA may not be directly connected. This means that other LSRs may exist between LSRD and LSRA.

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

  • Traffic engineering database (TEDB)

NOTE:

A TEDB can be generated only after Interior Gateway Protocol (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:

  1. Links that do not meet tunnel attribute requirements in the TEDB are excluded.

  2. SPF calculates the shortest path to a tunnel destination based on TEDB information.

NOTE:

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.

When several links have the same proportion of used bandwidth to the maximum reservable bandwidth (for example, the links do not use the reserved bandwidths or the same bandwidth is used on every link), the link discovered first is selected, irrespective of whether most-fill or least-fill is configured.

For example, CSPF removes links marked blue and links each with bandwidth of 50 Mbit/s based on tunnel constraints and uses other links each with bandwidth of 100 Mbit/s to calculate a path for an MPLS TE tunnel on the network shown in Figure 4-7. The constraints include the destination LSRE, the affinity of black, bandwidth of 80 Mbit/s, and a transit node LSRH.
Figure 4-7 Process of link removal
CSPF calculates a path shown in Figure 4-8 in the same way SPF would calculate it.
Figure 4-8 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.

Translation
Download
Updated: 2019-01-02

Document ID: EDOC1100055471

Views: 8032

Downloads: 7

Average rating:
This Document Applies to these Products
Related Documents
Related Version
Share
Previous Next