Specifications

CHAPTER 5

Implementing High Availability

Network Level Resiliency

Redundant links were discussed in Chapter 2. STP blocks a redundant link by default so that they are in an active/backup

configuration. Etherchannels enables multiple links to be active. If a failure occurs they distribute traffic across the

remaining links.

Configure your devices for fast convergence to avoid traffic drops when a link fails. RSTP is preferred over 802.1D STP

because it provides faster failover. Use routing protocols such as EIGRP that have fast convergence times. You might need

to tune the Layer 2 and Layer 3 protocol timers.

For accurate monitoring statistics, it is important that network clocks are synchronized. Use NTP for this. Syslog, SNMP,

and IP SLA are some tools that help you monitor and track your network’s resiliency. They are discussed in more detail

in a future section.

Fast Failover

When measuring network resiliency, you must consider how long it takes for failover and convergence at all layers of the

OSI stack, not just Layers 1–3. Table 5-1 outlines some of the typical convergence times.

Table 5-1 Convergence Times for Network Components

Network Component Convergence Time

Rapid Spanning Tree Subsecond for minor failures, 1–2 seconds for major failures.

Etherchannel Approximately 1 second to redirect traffic to a different link in the channel.

First Hop Redundancy Protocols Default of 10 seconds. Recommended tuning of hello time to 1 second and hold time to

such as HSRP, VRRP, or GLBP 3 second yields a 3 second convergence time.

Routing Protocols Subsecond for OSPF and EIGRP with recommended tuning of timers.

Switch Service Modules Typically 3–5 seconds. Exception is Cisco Application Control Engine (ACE) with 1

second failover in active/active configuration.

Computer/Server TCP Stacks 9-second session teardown for Windows, longer for other OSs.

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CCNP SWITCH 642-813 Quick Reference by Denise Donohue

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