Network Availability Flashcards
Network Availability
o Measure of how well a computer network can respond to connectivity and
performance demands that are placed upon it
High Availability
▪ Availability is measured by uptime
▪ Five nines of availability (99.999%)
▪ Maximum of 5 minutes of downtime per year
● Availability
o Concerned with being up and operational
● Reliability
o Concerned with not dropping packets
● Mean Time to Repair (MTTR)
o Measures the average time it takes to repair a network
device when it breaks
● Mean Time Between Failures (MTBF)
o Measures the average time between failures of a device
Redundant Network with Single Points of Failure
Link Redundancy (Multiple connections between devices)
● Internal Hardware Redundancy (Power supplies and NICs)
Redundant Network with No Single Points of Failure
Link Redundancy (Multiple connections between devices)
● Redundancy of Components (Switches and Routers)
Hardware Redundancy
▪ Takes many forms
▪ Devices with two network interface cards (NICs), hard drives, or internal
power supplies
Often found in strategic network devices
● Routers, Switches, Firewalls, and Servers
● Not often found in clients due to costs and administrative
overhead involved in management
o Active-Active
▪ Multiple NICs are active at the same time
▪ NICs have their own MAC address
▪ Makes troubleshooting more complex
o Active-Passive
▪ One NIC is active at a time
▪ Client appears to have a single MAC address
Network Interface Card Teaming
Using a group of network interface cards for load balancing and failover
on a server or other device
Layer 3 Redundancy
▪ Clients are configured with a default gateway (router)
● If the default gateway goes down, they cannot leave the subnet
● Layer 3 Redundancy occurs with virtual gateways
o Hot Standby Router Protocol (HSRP)
▪ Proprietary first-hop redundancy by Cisco
▪ Allows for active router and standby router
▪ Creates virtual router as the default gateway
o Virtual Router Redundancy Protocol (VRRP)
▪ IETP open-standard variant of HSRP
▪ Allows for active router and standby router
▪ Creates virtual router as the default gateway
o Gateway Load Balancing Protocol (GLBP)
▪ Proprietary first-hop redundancy by Cisco
▪ Focuses on load balancing over redundancy
▪ Allows for active router and standby router
▪ Creates virtual router as the default gateway
o Link Aggregation Control Protocol (LACP)
▪ Achieves redundancy by having multiple links
between devices
▪ Load balancing occurs over multiple links
▪ Multiple links appear as single logical link
Multipathing
Creates more than one physical path between the server and its storage
devices for better fault tolerance and performance
Design Considerations
Where will redundancy be used?
● Module (or Parts) Redundancy
● Chassis Redundancy
▪ What software redundancy features are appropriate?
▪ What protocol characteristics affect design requirements?
▪ What redundancy features should be used to provide power to an
infrastructure device?
▪ What redundancy features should be used to maintain environmental
conditions?
Best Practices
Examine the technical goals
▪ Identify the budget to fund high availability features
▪ Categorize business applications into profiles
● Each requires a certain level of availability
▪ Establish performance standards for high-availability solutions
● Performance standards will drive how success is measured
▪ Define how to manage and measure the high-availability solution
● Metrics help quantify success to decision makers
o Remember…
▪ Existing networks can be retrofitted, but it reduces the cost by integrating
high availability practices and technologies into your initial designs
Cold Sites
An available building that does not have any hardware or software in
place or configured
▪ While recovery is possible, it is going to be slow and time-consuming
o Warm Sites
An available building that already contains a lot of the equipment
▪ Restoral time is between 24 hours and seven days
Hot Sites
An available building that already has the equipment and data in place
and configured
▪ Minimal downtime and with nearly identical service levels maintained
Cloud Site
Allows for the creation of a recovery version of an organization’s
enterprise network in the cloud
● Recovery Time Objective (RTO)
o Time and service level within which a business process
must be restored after a disaster to avoid unacceptable
consequences
o How much time did it
take to recover after the notification of a business process
disruption?
o Use either a hot site or a cloud site for low RTO situations
● Recovery Point Objective (RPO)
o Interval of time during a disruption before data lost
exceeds the BCP’s maximum allowable threshold or
tolerance
Backup and Recovery
▪ Full
● Complete backup is the safest and most comprehensive; Time
consuming and costly
▪ Incremental
● Backup only data changed since last backup
▪ Differential
● Only backups data since the last full backup
▪ Snapshots
● Read-only copy of data frozen in time (VMs)
Uninterruptible Power Supply (UPS)
Provides emergency power to a load when the input power source or
main power fails
▪ Great for short duration power outages (less than 15 minutes)
o Power Distribution Unit (PDU)
Distributes electric power, especially to racks of computers and
networking equipment located within a data center
▪ PDUs combined with a UPS or a generator can provide power during a
blackout
Need for Quality of Service (QoS)
▪ Networks carry data, voice, and video content
▪ Convergence of media on the network requires high availability to ensure
proper delivery
▪ Optimizing the network to efficiently utilize the bandwidth to deliver
useful solutions to network users is crucial to success and cost savings
Quality of Service (QoS)
▪ Enables strategic optimization of network performance for different
types of traffic
● Identifies types of traffic needing priority
● Determines how much bandwidth required
● Efficiently uses WAN link’s bandwidth
● Identifies types of traffic to drop during network congestion
▪ For example:
● Voice (VoIP) and Video should have higher priority levels (less
latency)
Categories of QoS
Delay
● Time a packet travels from source to destination
● Measured in milliseconds (ms)
▪ Jitter
● Uneven arrival of packets
● Especially harmful in VoIP
▪ Drops
● Occurs during link congestion
● Router’s interface queue overflows and causes packet loss
Categorization of Traffic
▪ Determine network performance requirements for various traffic types
(Voice, Video, Data)
▪ Categorize traffic into specific categories:
● Low delay
o Voice
o Streaming Video
● Low priority
o Web browsing
o Non-mission critical data
▪ Document your QoS policy and make it available to your users
Ways of Categorizing Traffic
Best Effort
● Does not truly provide
QoS to that traffic
● No reordering of packets
● Uses FIFO (first in, first
out) queuing
▪ Integrated Services (IntServ or
Hard QoS)
● Makes strict bandwidth
reservations
● Reserves bandwidth by signaling devices
▪ Differentiated Services (DiffServ or Soft QoS)
● Differentiates between multiple traffic flows
● Packets are “marked”
● Routers and switches make decisions based on those markings
Congestion Management
▪ When a device receives traffic faster than it can be transmitted, it buffers
the extra traffic until bandwidth becomes available
● Called queuing
▪ Queuing algorithm empties the packets in specified sequence and
amount
▪ Queuing algorithms types
● Weighted fair queuing
● Low-latency queuing
● Weighted round-robin
Congestion Avoidance
▪ Newly arriving packets would be discarded if the device’s output queue
fills to capacity
▪ Random Early Detection (RED) is used to prevent this from occurring
● As the queue fills, the possibility of a discard increases until it
reaches 100%
● If at 100%, all traffic of that type is dropped
● RED instead drops packets from selected queues based on
defined limits
▪ If TCP traffic, it will be retransmitted
▪ If UDP, it will simply be dropped
Policing and Shaping
Policing
● Typically discards packets that exceed a configured rate limit
(speed limit)
● Dropped packets result in retransmissions
● Recommended for higher-speed interfaces
▪ Shaping
● Buffers (delays) traffic exceeding configured rate
● Recommended for slower-speed interfaces
Link Efficiency: Compression
Packet payload is compressed to conserve bandwidth
▪ VoIP payload can be reduced by 50%
● Payload size from 40 bytes to 20 bytes
▪ VoIP header can be reduced by 90-95%
● Uses RTP header compression (cRTP)
● Header size goes from 40 bytes to 2 to 4 bytes
▪ Utilized on slower-speed links to make most of limited bandwidth
Link Efficiency: LFI
Link Fragmentation & Interleaving (LFI)
▪ Fragments large data packets and interleaves smaller data packets
between the fragments
▪ Utilized on slower-speed links to make the most of limited bandwidth
