What is the difference between high availability and disaster recovery? Where do RTO and RPO fit?
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High availability keeps a service running through the failure of a component inside a site, automatically and usually within seconds: redundant links, devices and power. Disaster recovery restores the service after you lose a whole site or something equally large, and is measured by RTO (how long until it is back) and RPO (how much data you can afford to lose). A fully redundant data center does not help when the building is flooded, so you need both.
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| High availability | Disaster recovery | |
|---|---|---|
| Protects against | One device, link, power supply or supervisor | Loss of a site, region or major system |
| Failover | Automatic, seconds or less | Often a decision, minutes to hours |
| Measured by | Availability percentage | RTO and RPO |
| Network examples | Dual core, FHRP, ECMP, dual ISP | Second data center or cloud region, routing and DNS plan to move users |
The network's part in DR
- Capacity at the DR site and WAN paths so branches can reach it.
- An address strategy: advertise the same prefixes from the DR site, or re-address and change DNS with short TTLs, or use global load balancing.
- Configuration backups, IPAM and diagrams stored outside the primary site.
- Regular DR tests. A plan that has never been run is a hope, not a plan.
Backups are part of DR, not HA. Restoring a config from backup can take an hour; a standby device takes seconds.
Traps
- Using HA and DR as synonyms.
- Defining RPO as a time to recover. RPO is data loss measured in time; RTO is time to recover.
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