Table of Contents

About Cisco Documentation

What is LSC Redundancy
Benefits of LSC Redundancy
LSC Redundancy Architecture

Operational Modes

LSC Redundancy Levels

Hot Redundancy
Warm Redundancy

How the LSC, ATM Switch, and VSI Work Together
Implementing LSC Redundancy

Partitioning the Resources of the ATM Switch
Implementing the Parallel VSI Model
Adding Interface Redundancy
Implementing Hot LSC Redundancy
Implementing Warm LSC Redundancy

Using LSC Redundancy in Dedicated LSC Mode
Sample LSC Redundancy Configuration

Connections to BPX1:
Connections to BPX2
BPX1 Resource Parameter Settings
LER1 Configuration File
LSC1 Configuration File
LSC2 Configuration File

Feature Note:
LSC Redundancy for IP+ATM Networks

This document describes the Cisco Label Switch Controller (LSC) redundancy architecture that provides IP+ATM networks using MPLS with a level of reliability comparable to the hot-standby redundancy used in router networks but without the difficulty of implementing it. The hot LSC redundancy model provides the fastest reroute recovery time for IP+ATM networks.

• What is LSC Redundancy

• Benefits of LSC Redundancy

• LSC Redundancy Architecture

• LSC Redundancy Levels

• How the LSC, ATM Switch, and VSI Work Together

• Implementing LSC Redundancy

• Using LSC Redundancy in Dedicated LSC Mode

• Sample LSC Redundancy Configuration

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The product number for this document: 71559
The document number for this document: DOC-7811098=

What is LSC Redundancy

In traditional router IP networks, network managers ensure reliability by creating multiple paths through the network from every source to every destination. If a device or link on one path fails, IP traffic uses an alternate path to reach its destination.

Unlike router networks, circuit switch networks like ATM and Frame Relay transfer data by establishing circuits or virtual circuits. To ensure reliability, network managers incorporate redundant switch components: backup backplanes, power supplies, line cards, trunk cards, and so on.

But unlike router networks, switches take some time to reroute traffic when a failure occurs. Switch connection routing software, such as AutoRoute, PNNI, and MPLS, require calculating routes and reprogramming hardware for each connection. That's why router networks can reroute large aggregates of traffic more quickly than most connection-oriented networks.

Cisco's LSC redundancy recognizes that the LSC is the single point of failure for an IP+ATM network. Whether an LSC is an external router such as the Cisco 7204 router or an internal Routing Processor Module (RPM) in a BPX or MGX switch, an LSC is in the critical path for network reliability. If the LSC fails or if the LSC's port adapter goes down, the ATM-LSR also goes down and the entire connected MPLS network loses connections. As a critical component of IP+ATM networks, label switch controllers must be robust, providing continued service despite equipment or software failures, and do so quickly.

Cisco's LSC redundancy is an alternative way to increase reliability in IP networks. This reliability is nearly equivalent to that provided with the use of hot-standby routing processes. But the result is in general terms the same: if the primary controller fails, traffic can be almost instantly routed by a secondary controller. In addition, the Cisco LSC redundancy architecture reroutes traffic much faster than conventional rerouting processes.

LSC redundancy basically consists of:

• Two controllers, such as two MPLS controllers

• The Virtual Switch Interface (VSI)

• Equal-cost multipath IP routing

In essence, two independent MPLS controllers, via VSI, control separate partitions in the IP+ATM switch, creating a set of two identical subnetworks. Multipath IP routing chooses to use both subnetworks equally, leading to identical connections in both subnetworks. If a controller in one subnetwork fails, then multipath IP routing very quickly diverts traffic to the other subnetwork.

LSC redundancy differs from hot-standby redundancy in that the LSCs do not need copies of each other's internal state or database, thus increasing reliability. LSC redundancy is simpler than hot-standby redundancy because it is not necessary to set up new connections when a controller fails. The LSC redundancy architecture requires the same amount of equipment as a network with hot-standby controllers, except that the controllers act independently, rather than in hot-standby mode.

Benefits of LSC Redundancy

By implementing the LSC redundancy model, you eliminate the single point of failure between the LSC and the ATM switch it controls. If one LSC fails, the other LSC takes over and routes the data on the other path. The other benefits of LSC redundancy are now described in more detail.

You can use different models of routers in this LSC redundancy model. For example, one LSC can be a Cisco 7200 series router, and the other LSC can be a Cisco 7500 series router. Using different hardware in the redundancy model reduces the chance that a hardware fault would interrupt network traffic.

You can migrate from a standalone LSC to a redundant LSC and back again without affecting network operations. Because the LSCs work independently, you can add a redundant LSC without interrupting the other LSC.

LSC Redundancy Architecture

The architecture is distinguished by two main features:

• Multiple controllers share the resources of the same switch, creating two independent IP networks

• The resulting subnetworks are both linked at the edge Label Switch Routers (LSR)

Consider a basic IP network of switches with one MPLS controller (or a hot-standby pair of them) and MPLS Edge Label Switch Routers (LSR) feeding the edge of the network.

The LSC redundancy architecture adds to this basic network two independent controllers of the same type (such as MPLS), enabled by the Virtual Switch Interface (VSI) to control two separate partitions on the same IP+ATM switch. The pair of controllers on the switch form two separate MPLS control planes for the network that effectively create two independent parallel IP subnetworks.

Provided that the two independent MPLS controllers on each switch have identical shares of the switch's resources and link capacity, the two subnetworks are identical. The two identical, parallel IP subnetworks exist on virtually the same equipment that would otherwise support only one IP network.

Note   Each control plane and partition might have a redundant pair of controllers, but these are coupled. Note that the two independent controllers must be of the same type. Also, the equipment must have sufficient connection capacity for the doubled-up connections.

The LSC redundancy solution differs from hot-standby redundancy in that the MPLS controllers need not have copies of each other's internal state.

The second feature of the LSC redundancy architecture is the linkage of the two parallel subnetworks on the same physical ATM-LSR at the edge.

This LSC redundancy network might use the Open Shortest Path First (OSPF) protocol with equal-cost multipath or a similar IP routing protocol with multipath capability. Because there are two identical, parallel IP subnetworks, there are at least two equally good paths from every edge LSR to every other edge LSR, one in each subnetwork.

OSPF equal-cost multipath chooses to distribute traffic evenly across both sets of paths (and hence both subnetworks). Because of this, MPLS sets up two identical sets of connections for the two MPLS control planes. IP traffic is shared evenly across the two sets of connections, across both control planes.

LSC Redundancy works with any routing protocol. For example, you can use:

• OSPF

• Intermediate System to Intermediate System (IS-IS)

• Tag Distribution Protocol (TDP)

• Label Distribution Protocol (LDP)

If there were a failure in one MPLS controller in one switch, some paths in one of the subnetworks would no longer work. If there were only one subnetwork, there would be an undesirable interruption in passing data while other switches break connections and reroute them around the failed node.

However, because all connections are mirrored in the secondary subnetwork, there are already alternative paths for the traffic without the need to establish new links. All that is required is for multipath routing to detect the failure of one set of paths and to divert the traffic onto the remaining good paths. Because connections on the other paths have already been set up, the interruption to traffic flow is much smaller than if new connections were required.

Operational Modes

The LSC redundancy architecture supports these operational modes:

• Transparent Mode
  The primary and backup LSCs have identical images and startup configurations. Thus when one controller fails, the other takes over seamlessly. However, this mode has a higher risk of software failure because both controllers use the same algorithms.

• Upgrade Mode
  You can upgrade the redundant system and change resources of the switch without rebooting the system. You can use this mode to change the resources between different partitions of the slave ATM switch.

• Nontransparent Mode
  You can have different images on the primary and backup LSCs, thereby reducing the risk of both systems suffering the same configuration problem.

• Experimental Mode
  You can use the backup subnetwork to test images in a real environment.

LSC Redundancy Levels

You can configure the LSC controller to provide two levels of redundancy, providing either equal cost routing (Hot Redundancy) or unequal-cost routing (Warm Redundancy).

In both cases, the two LSCs:

• Use VSI to control two separate partitions of the same IP+ATM switch

• Run in parallel with independent LDPs

Hot and warm redundancy differ in the following ways:

• Hot redundancy uses both paths to route traffic. You set up both paths to use equal cost multipath routing, so that traffic is load balanced between the two paths. As a result, hot redundancy uses twice the number of MPLS label VCs as warm redundancy.

• Warm redundancy uses only one path at a time. You set up the paths so that one path has a higher cost than the other. Traffic only uses one path and the other path is a backup path.

Hot Redundancy

The hot redundancy level (Figure 1) provides the fastest rerouting.

The backup MPLS controller provisions connections in tandem with the primary controller. Both controllers are active or "hot" at all times, giving each destination two independent paths, each path generated by one of the two controllers.

The two partitions on the switch must be configured with equal bandwidth and cross-connect space. Also, both LSCs must run the same routing protocols.

The result is that the edge LSRs have multiple routes to the same destination and request multiple labels. If one controller fails, only one of the two paths fails; the secondary controller already has the labels established and immediately provides an active backup path to handle the traffic with no time lost for rerouting or setting up labels.

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Note   By placing two LSCs on an ATM switch, they become two logically separate ATM-LSRs, which is what the "Logical equivalent" is showing. It's important to clearly distinguish between an LSC and an ATM-LSR: an LSC is not an ATM-LSR, it is merely part of one.

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Warm Redundancy

Warm redundancy (Figure 2) is more flexible but less robust than hot redundancy.

Both LSCs are active on the switch, but to avoid overburdening the single VC, one controller is designated "primary" with least-cost path and the other is designated "backup" with higher cost. The edge LSRs have only one route over the primary (least-cost) LSC.

The backup controller, configured with higher routing cost, creates only one physical path for each destination. If the primary controller fails, the backup controller creates new paths for the destinations.

The advantage of warm redundancy is that the switch's resources are conserved because only one partition is in use at a given time. Unlike hot redundancy, the two partitions on the switch need not be configured for equal bandwidth and cross-connect space. Also, the two controllers can run different routing protocols. Thus, warm redundancy offers additional flexibility

The disadvantage of warm redundancy is that there is no way for the backup processor to maintain existing connections during the changeover. For the backup controller to reestablish the path takes at least one reroute time to become active. This also incurs LDP negotiation/bind time.

How the LSC, ATM Switch, and VSI Work Together

The LSC and slave ATM switch have these characteristics:

• The LSC runs all of the control protocols

• The ATM switch forwards the data

• Each physical interface on the slave ATM switch maps to an XtagATM interface on the LSC. Each XtagATM interface has a dedicated LDP session with a corresponding interface on the edge. The XtagATM interfaces are mapped in the routing topology and the ATM switch behaves as a router.

• The LSC can also function as an edge LSR. The data for the edge LSR passes through the control interface of the router.

If a component on the LSC fails, the ATM switch's IP switching function is disabled. The stand-alone LSC is the single point of failure.

The VSI implementation includes these characteristics:

• The VSI allows multiple, independent control planes to control a switch. The VSI ensures that the control processes (SS7, MPLS, PNNI, and so on) can act independently of each other by using a VSI slave process to control the resources of the switch and apportion them to the correct control planes.

• In MPLS, each physical interface on the slave ATM switch maps to an XtagATM interface on the LSC through the VSI. In other words, physical interfaces are mapped to their respective logical interfaces.

• The routing protocol on the LSC generates route tables entries. The master sends connection requests and connection release requests to the slave.

• The slave sends the configured bandwidth parameters for the ATM switch interface to the master in the VSI messages. The master includes the bandwidth information in the link state topology. You can override these bandwidth values by manually configuring the bandwidth on the XtagATM interfaces.

Implementing LSC Redundancy

To make an LSC redundant, you perform these basic steps:

• Partition the resources of the slave ATM switch

• Implement a parallel VSI model

• Assign redundant LSCs to each switch

• Create redundant LSRs

Partitioning the Resources of the ATM Switch

In the LSC redundancy model, two LSCs control different partitions of the ATM switch. When you partition the ATM switch for LSC redundancy, follow these guidelines:

• Make the MPLS partitions identical. If you create two partitions, make sure both partitions have the same amount of resources. (You can have two MPLS VSI partitions per switch.) Use the cnfrsrc command to configure the partitions.

• If the partitions are on the same switch card, perform these steps:

  • Create different control VCs for each partition.
    For example, there can be only one (0, 32) control VC on the XtagATM interface. To map two XtagATM interfaces on the same ATM switch interface, use a different control VC for the second LSC. Use the tag-switching atm control-vc command.

  • Create the LVC on the XtagATM interfaces using nonintersecting VPI ranges.
    Use the tag-switching atm vpi command.

• Specify the bandwidth information on the XtagATM interfaces. Normally, this information is read from the slave ATM switch. When you specify the bandwidth on the XtagATM interface, the value you enter takes precedence over the switch-configured interface bandwidth.

• Configure the logical channel number (LCN) ranges for each partition according to the expected number of connections.

Implementing the Parallel VSI Model

The parallel VSI model means that the physical interfaces on the ATM switch are shared by more than one LSC. For example:

Figure 3 shows four ATM physical interfaces mapped as four XtagATM interfaces at LSC1 and LSC2. Each LSC is unaware that the other LSC is mapped to the same interfaces. Both LSCs are active all the time. The ATM switch runs the same VSI protocol on both partitions.

Adding Interface Redundancy

To ensure reliability throughout the LSC redundant network, you can also implement:

• Redundant interfaces between the edge LSR and the ATM-LSR.
  Most edge LSRs are collocated with the LSCs. Creating redundant interfaces between the edge LSRs and the ATM-LSRs reduces the chance of a disruption in network traffic by providing parallel paths.

• Redundant virtual trunks and VP tunnels between slave ATM switches.
  To ensure hot redundancy between the ATM switches, you can create redundant virtual trunks and VP tunnels. See Figure 4.

Implementing Hot LSC Redundancy

Hot redundancy provides instant failover to the other path when an LSC fails. When you set up hot redundancy, both LSCs are active and have the same routing costs on both paths. To ensure that the routing costs are the same, run the same routing protocols on the redundant LSCs.

In hot redundancy, the LSCs run parallel and independent Label Distribution Protocols (LDPs). At the edge LSRs, when the LDP has multiple routes for the same destination, it requests multiple labels. It also requests multiple labels when it needs to support Class of Service (CoS). When one LSC fails, the labels distributed by that LSC are removed.

• Redundant physical interfaces between the edge LSR and the ATM-LSR to ensure reliability in case one physical interface fails.

• Redundant interfaces or redundant VP tunnels between the ATM switches.

• Slave ATM switches, such as the BPX 8650, can have redundant control cards and switch fabrics. If redundant switch fabrics are used and the primary switch fails, the other switch fabric takes over.

• Redundant LSCs.

• The same routing protocol running on both LSCs. (You can have different label distribution protocols.)

Implementing Warm LSC Redundancy

Virtually any configuration of switches and LSCs that provides hot redundancy can also provide warm redundancy. You can also switch from warm to hot redundancy with little or no change to the links, switch configurations, or partitions.

To achieve warm redundancy, you need only redundant LSCs. You do not necessarily need to run the same routing protocols or distribution protocols on the LSCs.

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Note   You can use different routing protocols on parallel LSCs. However, you do not get instant failover. The failover time includes the time it takes to reroute the traffic, plus the LDP bind request time. If the primary routing protocol fails, the secondary routing protocol finds new routes and creates new label virtual circuits (LVCs). An advantage to using different routing protocols is that the ATM switch uses fewer resources and offers more robust redundancy.

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If you run the same routing protocols, you specify a higher cost for the interfaces on the backup LSC. This causes the data to use only the lower-cost path. This also saves resources on the ATM switch, because the edge LSR requests LVCs only through the lower-cost LSC. When the primary LSC fails, the edge LSR uses the backup LSC and creates new paths to the destination. Creating new paths requires reroute time and LDP negotiation time.

Using LSC Redundancy in Dedicated LSC Mode

Normally, LSCs include edge LSRs. In the "dedicated" LSC mode, the LSC removes edge LSR functionality. In Cisco 6400 NRP-based LSCs, the dedicated LSC mode of operation provides an opportunity for the LSC to be scaled. To achieve the edge LSR functionality, the LSC creates a label switch path (LSP) for each destination in the route table.

Sample LSC Redundancy Configuration

The sample configuration settings shown in this section assumes a network topology with two BPX switches (BPX1 and BPX2). Each BPX is connected to its own edge label switch router (LER) and each BPX supports two LSCs in separate partitions.

The diagram in Figure 5 indicates the connections to support two active independent controllers on each BPX switch with two independent paths for each destination; that is, hot redundancy.

Connections to BPX1:

LSC1           4.4

atm port      atm3/0

LSC2           4.1

atm port      atm5/0

LER1          5.1

Trunk port       4.8

Virtual trunk port         4.5.1 3.5.1

atm port      atm5/0/0

Connections to BPX2

LSC3         1.3

atm port       atm3/0/0

LSC4          1.2

atm port      atm3/0/0

LER2         3.2

Trunk Port       1.1

Virtual Trunk Port      1.5.1 2.5.1

atm port       atm3/0

BPX1 Resource Parameter Settings

Use the cnfrsrc command to configure all VSI and AutoRoute resources. The following dsprsrc command screens show the recommended settings the BPX1 side of the basic topology. Naturally, depending on your network, your will need to adjust the resource parameters to maximize efficiency.

Note that the configuration for BPX2 and its LER2, LSC3 and LSC4 are almost identical to those of the BPX1 but with different addresses for the BPX, the router ATM port, and loopback.

LER1 Configuration File

LSC1 Configuration File

LSC2 Configuration File

Posted: Fri May 4 08:27:31 PDT 2001
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