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TR7Enterprise Application Delivery & Security Platform
What is TTL; on which TR7 screens does it appear, and with what meaning?
Short answer
TTL (Time To Live) is a number that specifies for how long, or for how many steps, a value is considered valid; its meaning changes according to where it appears. In DNS, it is the period for which a record can be kept in the cache of resolvers (RFC 1035): in TR7 GTM, it is entered per record in seconds between 0 and 31,536,000, and the default is 3600 (TTL). The Cache TTL field of the DNS service, on the other hand, is not announced to clients; it is the period of the service's own cache (default 60 seconds). At the IP layer, TTL is the maximum number of hops a packet can pass through; in TR7, it is entered in the range 1–255 for a VXLAN interface (TTL). The TTL column in the AGS Sessions and Lockout lists, in turn, shows the time remaining until the session or the lock ends.
Where It Is Used in TR7
| Where | Field / label | What for |
|---|---|---|
| GTM → GTM Services → service name → Records → Add | TTL (0–31,536,000 s, default 3600) | The period the record stays in resolver caches; it appears in the list in a shortened form (e.g. 1h) |
| Same screen, SOA record | Soa Default TTL (default 3600) (record type specific fields) | The minimum field of the SOA record; the cache period of negative answers (NXDOMAIN, NODATA) |
| GTM → GTM Services → Service Management → Add / Edit | With Cache on, Cache TTL (default 60 seconds; seconds, minutes, hours) | The DNS service's own cache period |
| GTM → GTM Services → service name → Records (Datacenter mode) | Selection Criterion = TTL | While DC Selection = Client based, one of the criteria for sorting the data centers by their network proximity to the client |
| Network → Interfaces → Add (VXLAN) | TTL (1–255, optional) | The hop limit of the VXLAN tunnel packets |
| Left Monitor Panel → AGS-Enabled vServices → vService → Sessions | TTL column (hidden by default; added from the column selector) | The time remaining until the session ends |
| Left Monitor Panel → AGS-Enabled vServices → vService → Lockout | TTL column | The time remaining until the lock ends |
Meanings
| Context | What it measures | Unit |
|---|---|---|
| DNS record (TTL) | The period the answer stays in resolver caches | Seconds |
| DNS service (Cache TTL) | The service's own cache period; it is not announced to clients | Seconds, minutes, hours |
| IP packet (VXLAN TTL) | The maximum number of hops the packet can pass through | Hops |
| AGS lists | The time remaining until the session or the lock ends | Duration |
Common Misconceptions
- "Lowering the TTL propagates the change instantly." The old record in resolver caches continues to be used until the old TTL period expires; before an address change, the TTL is lowered and a period as long as the old TTL is allowed to pass.
- "The record TTL and Cache TTL are the same setting." The record TTL is the period announced to clients together with the answer; Cache TTL manages only the DNS service's own cache.
- "A short TTL is always better." A short TTL makes the new answer propagate quickly after a failover, but it leads to more frequent queries for the same name; the TTL is selected according to the need for switchover speed.
How it is done in the documentation
