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
Resilient network technologies
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
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
Analyze technical goals and constraints.
Determine the availability budget for the network.
Create application profiles for business applications.
Define availability and performance standards.
Create an operations support plan.
- Within the Enterprise Edge functional area, the following must be considered for high availability:
Service level agreement
Routing protocol convergence