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Things You Need to Know About High Availability

Components for High Availability

  • Objective of high availability is to prevent outages and minimize downtime
  • Achieving high-availability integrates multiple components:
    • Redundancy
    • Technology
    • People
    • Processes
    • Tools

Redundancy

  • Redundancy is used to reduce or limit the effects of failure
  • Design of redundancy attempts to eliminate single points of failure
    • avoid single causes of failure
    • use geographic diversity and path diversity
    • use dual devices and links
    • use dual WAN providers
    • as appropriate implement dual data centers, dual CO facilities, and dual power substations
  • Design of redundancy needs to trade off cost versus benefit
    • hours of downtime compared to the cost of redundancy, planning, etc

Technology

  • Cisco Nonstop Forwarding (NSF)
  • Stateful Switch Over (SSO)
  • Catalyst 3750 series switches and Cisco StackWise technology

Technologies for detecting failures and triggering failover

  • Monitoring
  • IP SLAs and object tracking

Other technologies

  • Fast-routing convergence

People

  • Staff work habits and skills can impact high availability
  • Good skills and ongoing technical training are needed
  • Communication and documentation are important
  • People are not given the time to do the job right to cut corners
  • Staff team should align with services

Processes

  • Build repeatable processes
  • Use labs appropriately
  • Use meaningful change controls
  • Manage operation changes

Tools

  • Monitor availability and key statistics for devices and links
  • Good documentation is a powerful tool

Resiliency for High Availability

     Network-level resiliency

  • Redundant links
  • Redundant devices

     System-level resiliency

  • Integrated hardware resiliency
  • Redundant power supply
  • Stackable switches

     Management and monitoring

  • Deletion of failure

Network-Level Resiliency

  • Link redundancy
  • Fast convergence
  • Power redundancy
  • Monitoring
  • SNMP
  • Syslog
  • IP SLA
  • Time synchronization via NTP

Network Monitoring

Syslog

SNMP — versions 1, 2 and 3

  • switch (config) # access-list 100 permit IP 10.1.1.0 0.0.0.255 any
  • switch (config) # snmp-server community Cisco RO 100
  • switch (config) # snmp-server community xyz123 RW 100
  • switch (config) # snmp-server trap 10.1.1.50           

SLA

  • switch (config) # ip sla Monitor 11
  • switch (config-sla) # type echo prot ipIcmpEcho 10.1.1.1 source-int fa0/1
  • switch (config-sla) # frequency 10
  • switch (config) # ip sla monitor schedule 11 life forever start-time now
  • switch (config) # track 1 ip sla 11 reachability

HSRP

High-availability for default gateways

HSRP defines a standby group of routers with one router as the active one

      Active Router

  • Responds to ARP request of the default gateway with the MAC address of the virtual router
  • Assumes the active forwarding of packets for the virtual router
  • Sends hello messages
  • Knows the virtual router IP address

      Standby Router

  • Listens for periodic hello messages
  • Assumes the active forwarding of packets if no hello messages heard from active router
    • SwitchA (config) # interface vlan10
    • SwitchA (config-if) # ip address 10.1.10.2 255.255.255.0
    • SwitchA (config-if) # standby 10 10.1.10.1
    • SwitchB (config) # interface vlan10
    • SwitchB (config-if) # ip address 10.1.10.3 255.255.255.0
    • SwitchB (config-if) # standby 10 10.1.10.1

HSRP Priority and Preemption

  • The device with the highest priority in an HSRP group becomes
  • the active router
  • The default priority is 100
  • In the case of a tie, router with the highest configured IP address will become active
  • Preemption enables the higher priority device to become active
    • Switch (config) # interface vlan10
    • Switch (config-if) # ip address 10.1.1.2 255.255.255.0
    • Switch (config-if) # standby 10 10.1.1.1
    • Switch (config-if) # standby 10 priority 110
    • Switch (config-if) # standby 10 preempt

HSRP and STP

  • Configured active router should be the same as STP root bridge

HSRP Versions

HSRPv1 (default)

  • Group number from 0 to 255
  • Virtual MAC address 0000.0C07.ACXX (XX=HSRP group)
  • Hello packets sent to multicast address 224.0.0.2
  • Default timers:  Hello=3 sec     Hold time=10 sec
  • Preempt delay supported
  • Authentication supported

HSRPv2

  • Group number from 0 to 4095
  • Virtual MAC address 0000.0c9F.FXXX (XXX=HSRP group)
  • Hello packets sent to multicast address 224.0.0.102
  • Default timers:  Hello=3 sec     Hold time=10 sec
  • Preempt delay supported
  • Authentication supported
  • HSRPv2 and HSRPv1 have different packet formats
  • Same version should be configured on all devices in HSRP group

HSRP Interface Tracking

  • Configure the standby group
  • Configure priority
  • Configure preempt on all devices within the HSRP group
  • Configure the tracked interfaces and decrement (default decrement is 10)
    • Switch (config)# interface vlan10
    • Switch (config-if) # ip address 10.1.1.2 255.255.255.0
    • Switch (config-if) # standby 10 10.1.1.1
    • Switch (config-if) # standby 10 priority 110
    • Switch (config-if) # standby 10 preempt
    • Switch (config-if) # standby 10 track fa0/23 20
    • Switch (config-if) # standby 10 track fa0/24

Tracking Options

  • Track number interface can verify the protocol or IP routing capability
  • Track number ip route can verify network reachability
  • Track number list can define complex condition
  • Track number rtr can verify parameters such as IP SLA

VRRP

  • Open Standard – RFC 3768
  • 255 groups max
  • 1 Active, several backup routers
  • Virtual IP address can be the same as the real IP address of one of the group members
  • Sends hellos on multicast address 224.0.0.18
  • Can track objects only
  • Authentication no longer supported

GLBP

  • Cisco proprietary 2005
  • 1024 groups max
  • 1 Active Virtual Gateway (AVG)
  • Several Active Virtual Forwarders (AVF)
  • Virtual IP of AVG is different than real IP address
  • Sends hellos to multicast address 224.0.0.102
  • Can track objects only
  • Default timers:  hello=3sec    hold time=10sec
  • Authentication supported
  • Preempt delay supported
  • Can be configured to track interfaces (interface=object)
  • GLBP uses weights to determine whether router can act as AVF
    • Switch(config)# track 90 interface fa0/24 line-protocol
    • Switch(config)# track 91 interface fa0/23 line-protocol
    •  Switch(config)# interface vlan10
    • Switch(config-if)# ip address 10.1.10.2 255.255.255.0
    • Switch(config-if)# glbp 1 10.1.10.1
    •  Switch(config-if)# glbp 1 weighting 110 lower 85 upper 105
    • Switch(config-if)# glbp 1 weighting track 90 decrement 10
    • Switch(config-if)# glbp 1 weighting track 91 decrement 20

Make sure you can configure:                            

  • SLA’s                                                                    
  • HSRP                                                                    
    • Priority                                                                      
    • Preempt                                                                     
    • Interface Tracking                                                     
  • GLBP                                                                     
    • Priority                                                                        
    • Preempt                                                                       
    • Object Tracking                                                                       
    • VRRP         

Understand:

Header             

  • Cisco Switching Methods:
  • Process Switching
  • Fast Switching
  • CEF

High Availability – all components

  • Redundancy
  • Technology
  • People
  • Processes
  • Tools

Make sure you are familiar with output of:

  • Show ip cef
  • Show ip cef VLAN __ detail
  • Show adjacency
  • Show cef drop
  • Show standby brief
  • Show standby
  • show ip sla statistics
  • sh ip arp
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