DNS and Global Traffic Manager
DNS and Global Traffic Manager
This domain tests GTM, now BIG-IP DNS: listeners, wide IPs, pools, pool members, load-balancing methods, iQuery, sync groups, and DNS express. Questions focus on how a DNS query is resolved to a member and how health and topology change the answer.
Core rules
- A GTM listener listens on UDP and TCP port 53. A wide IP matches a DNS name and optional aliases and contains pools.
- Pool members are virtual servers or IP-and-port addresses. GTM monitors them; iQuery shares status between GTM and LTM.
- Load-balancing methods include round robin, ratio, global availability, topology, least connections, and QoS or RTT. Topology uses topology records to prefer or return specific pools or members.
- A wide IP can have a fallback method and a fallback pool for when no preferred pool is available.
- Sync groups replicate GTM configuration and iQuery state. A device that is not in sync may answer with stale member status.
- DNS express serves zones from GTM without a back-end DNS server; it requires zone transfers or zone imports.
Worked example
A client queries app.example.com. The GTM listener receives the query; the wide IP matches with a topology rule that sends European clients to the EU pool. The EU pool has two members, both up; least connections selects member 2. GTM returns member 2 address in the A record. If member 2 fails its monitor, iQuery marks it down and the next query goes to member 1. If the EU pool has no up members, the fallback pool answers.
Traps
- Confusing a GTM pool member with an LTM pool member; GTM members are typically virtual servers.
- Assuming GTM load balancing replaces LTM load balancing; both can apply in sequence.
- Forgetting that topology rules are evaluated before the pool load-balancing method.
- Overlooking sync-group status and iQuery connectivity when answers look stale.
- Using DNS express without a valid zone transfer or imported zone data.