Isolated LSP Computation
Isolated LSP computation enables a device to compute isolated primary and hot-standby label switched paths (LSPs) using the disjoint algorithm and constrained shortest path first (CSPF) algorithm simultaneously.
Background
Most live IP radio access networks (RANs) use ring topologies and have the access ring separated from the aggregation ring. To improve the end-to-end and inter-ring LSP reliability, many IP RAN carriers require isolated primary and hot-standby LSPs. The CSPF algorithm does not meet this reliability requirement, because CSPF is a metric-based path computing algorithm that may compute two intersecting LSPs. Specifying explicit paths can meet this reliability requirement; this method, however, does not adapt to topology changes. Each time a node is added to or deleted from the IP RAN, operators must configure new explicit paths, which is time-consuming and laborious. To resolve these problems, you can configure isolated LSP computation.
Figure 4-27 illustrates an IP RAN on which an MPLS TE tunnel is established between a cell site gateway (CSG) on the access ring and a radio service gateway (RSG) on the aggregation ring. The MPLS TE tunnel implements the end-to-end virtual private network (VPN) service. To improve the network reliability, this network requires the constraint-based routed label switched path (CR-LSP) hot standby feature and isolated primary and hot-standby LSPs.
Without the isolated LSP computation feature, CSPF on this network will compute CSG -> ASG1 -> ASG2 -> RSG as the primary LSP. This LSP does not have an isolated hot-standby LSP. However, two isolated LSPs exist on this network: CSG -> ASG1 -> RSG and CSG -> ASG2 -> RSG. With the isolated LSP computation feature, the disjoint and CSPF algorithms work simultaneously to get the two isolated LSPs.
Implementation
Isolated LSP computation is implemented by both the disjoint and CSPF algorithms. This feature computes primary and hot-standby LSPs simultaneously and cuts off overlapping paths of the two LSPs to get two isolated LSPs. In the example shown in Figure 4-28, before isolated LSP computation is configured, CSPF computes LSRA -> LSRB -> LSRC -> LSRD as the primary LSP and LSRA -> LSRC -> LSRD as the hot-standby LSP if path overlapping is allowed. These two LSPs intersect, so that they do not meet the reliability requirement.
After isolated LSP computation is configured, the disjoint and CSPF algorithms compute LSRA -> LSRB -> LSRD as the primary LSP and LSRA -> LSRC -> LSRD as the hot-standby LSP. These two LSPs do not intersect, so that they meet the reliability requirement.
Isolated LSP computation is a best-effort technique. If the disjoint and CSPF algorithms cannot get isolated primary and hot-standby LSPs or two isolated LSPs do not exist, the device uses the primary and hot-standby LSPs computed by CSPF.
The disjoint algorithm cannot work together with the following features: explicit path, and hop limit. Therefore, before you configure isolated LSP computation, check that all those features are disabled. Otherwise, the device does not allow you to configure isolated LSP computation. After you configure isolated LSP computation, the device does not allow you to configure any of those features, either.
After you configure isolated LSP computation, the shared risk link group (SRLG), if configured, becomes ineffective.
When an affinity is configured, the disjoint algorithm takes effect only when affinities of the primary and backup LSPs are the same or no affinities are configured for the primary and backup LSPs.
Usage Scenario
Isolated LSP computation applies to networks on which Resource Reservation Protocol - Traffic Engineering (RSVP-TE) tunnels and the hot standby feature are configured.
Benefits
Isolated LSP computation offers the following benefits to carriers:
Improves the network reliability.
Reduces the maintenance workload.
