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CCDP Self-Study: Designing High-Availability Services

Chapter Description

Cisco IOS high-availability technologies provide network redundancy and fault tolerance. Reliable network devices, redundant hardware components with automatic failover, and protocols like Hot Standby Router Protocol (HSRP) are used to maximize network uptime. This chapter will help you get a handle on high-availability technologies.


In this chapter, you learned the following key points:

  • Enterprises implement high availability to meet the following requirements:

    • Ensure that mission-critical applications are available

    • Improve employee and customer satisfaction and loyalty

    • Reduce reactive IT support costs, resulting in increased IT productivity

    • Reduce financial loss

    • Minimize lost productivity

  • Availability is a measurable quantity. The factors that affect availability are MTTR and MTBF. Decreasing MTTR and increasing MTBF increase availability. Using the following equation results in a percentage that indicates availability (99.999 percent is a common goal):

    Availability = MTBF / (MTBF + MTTR) 
  • A Cisco high-availability solution has the following requirements:

    • Reliable, fault-tolerant network devices

    • Device and link redundancy

    • Load balancing

    • Resilient network technologies

    • Network design

    • Best practices

  • One approach to building highly available networks is to use extremely fault-tolerant network devices throughout the network. Fault-tolerant network devices must have redundant key components, such as supervisor engine, routing module, power supply, and fan. Redundancy in network topology and provisioning multiple devices and links is another approach to achieving high availability. Each approach has its own benefits and drawbacks.

  • Cisco IOS Software provides the following Layer 3 redundancy features:

    • HSRP or VRRP

    • Fast routing protocol convergence

    • EtherChannel technology

    • Load sharing

    • CEF

  • The Cisco spanning-tree implementation provides a separate spanning-tree domain for each VLAN called PVST+. RSTP as specified in 802.1w supersedes STP specified in 802.1D, but remains compatible with STP. RSTP shows significant convergence improvement over the traditional STP. RST's advantage is experienced when the inter-switch links (connections) are full-duplex (dedicated/point-to-point), and the access port connecting to the workstations are in PortFast mode. MST allows you to map several VLANs to a reduced number of spanning-tree instances because most networks do not need more than a few logical topologies.

  • To design high-availability services for an enterprise network one must answer the following types of questions:

    • Where should module and chassis redundancy be deployed in the network?

    • What software reliability features are required for the network?

    • What protocol attributes need to be considered?

    • What high-availability features are required for circuits and carriers?

    • What environmental and power features are required for the network?

    • What operations procedures are in place to prevent outages?

  • To fully determine the benefit of device, chassis, and link redundancy, one should discover the answers to the following questions:

    • Will the solution allow for load sharing?

    • Which components are redundant?

    • What active-standby fault detection methods are used?

    • What is the MTBF for a module? What is the MTTR for a module? Should it be made redundant?

    • How long does it take to do an upgrade?

    • Are hot swapping and online, insertion and removal (OIR) available?

  • Cisco Systems recommends implementing the following software features:

    • Protect gateway routers with HSRP or VRRP

    • Implement resilient routing protocols, such as EIGRP, OSPF, IS-IS, RIPv2, BGP

    • Use floating static routes and access control lists to reduce load in case of failure

  • Consider protocol attributes such as complexity to manage and maintain, convergence, hold times, and signal overhead

  • Because the carrier network is an important component of the enterprise network and its availability, careful consideration of the following points about the carrier network is essential:

    • Understand the carrier network

    • Consider multihoming to different vendors

    • Monitor carrier availability

    • Review carrier notification and escalation procedures to reduce repair times

  • The general network design conclusions with respect to high availability are

    • Reduce complexity, increase modularity and consistency

    • Consider solution manageability

    • Minimize the size of failure domains

    • Consider protocol attributes

    • Consider budget, requirements, and areas of the network that contribute the most downtime or are at greatest risk

    • Test before deployment

  • Cisco has developed a set of best practices for network designers to ensure high availability of the network. The five-step Cisco recommendations are

Step 1

Analyze technical goals and constraints.

Step 2

Determine the availability budget for the network.

Step 3

Create application profiles for business applications.

Step 4

Define availability and performance standards.

Step 5

Create an operations support plan.

  • Within the Enterprise Edge functional area, the following must be considered for high availability:
    • Service level agreement

    • Link redundancy

    • Load balancing

    • Policy-based routing

    • Routing protocol convergence