LTM Virtual Servers, Pools, Nodes, and Load Balancing Methods
Virtual server types and matching
The LTM virtual server domain tests how BIG-IP selects a destination, chooses a pool member, and reports state. A Standard virtual server is full proxy and supports HTTP, SSL, persistence, and iRules. Performance (Layer 4) and FastL4 virtual servers trade feature depth for throughput. Forwarding IP, Stateless, Reject, and Internal/Network virtual servers serve specific routing or control cases.
A virtual server matches on destination address and port. * matches any address; a port of 0 is a wildcard port. More specific matches generally win, but overlapping wildcard configurations are a common source of unexpected pool selection.
Pools, nodes, and members
A node is an IP address; a pool member is a node plus a port. Member state can be up, down, checking, or unchecked. A node monitor affects all members on that node. Forced offline and disabled are administrative states and are not the same as a monitor failure. Pool member ratio and priority influence selection, but only for methods that honor them. Priority group activation keeps traffic on the highest-priority group with available members.
Load-balancing methods
- Round Robin: rotates through members; ratio-aware.
- Least Connections: sends to the member with the fewest current connections.
- Observed: uses a composite of connections and sessions.
- Predictive: uses the observed trend, not just the current count.
- Fastest: uses measured response time.
- Dynamic Ratio: uses SNMP-derived weights.
- Least Sessions: uses session table counts.
- Weighted Least Connections: combines connection count and ratio.
Worked example
A pool has members A, B, and C with ratios 3, 1, and 1 using Round Robin. Over five new connections the expected distribution is three to A and one each to B and C. If A is marked down by its monitor, the ratio is recalculated across the remaining members, so B and C share traffic. If A is only forced offline, that is an administrative action, not a monitor result, but it still removes A from selection.
Traps
Do not read unchecked as down: an unchecked member can still receive traffic. A node marked down takes every member on that node out of service. Ratio is not a universal setting for every algorithm. A pool with no available members returns an error rather than silently using a down member.