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F5 LTM Module 3 Virtual Servers & pools

Node is an IP. Member is IP:port. Pool is the algorithm. Virtual Server is the listener. Standard VS means two TCP sessions. Browser refresh is not an LB test.

20 min read · L2 primary · Quiz at end

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Lessons · F5 LTM series · Module 3

F5 LTM recorded course · 7 modules

Same lab numbers on every page: client 198.51.100.50, VIP 192.0.2.100, Self IPs 192.0.2.10 / 10.20.20.10, members 10.20.20.101–103.

  1. Hub · Course map
  2. M1 · Fundamentals & admin
  3. M2 · Networking & traffic flow
  4. M3 · Virtual Servers & pools ← you are here
  5. M4 · Profiles, SNAT, SSL
  6. M5 · Monitors, iRules, policies
  7. M6 · High availability
  8. M7 · Troubleshooting

Next → M4 · Profiles, SNAT, SSL

Recorded course + workbooks: My Courses · syllabus F5 LTM / GTM / ASM

Publishing the application

Module 2 made the box reachable. Module 3 publishes an application: Node → Pool Member → Pool → Virtual Server. Ticket: “VIP is up, pool is empty, users get a reset.” Someone created a Virtual Server without a pool, or named the VIP and thought that was load balancing.

Hero · VIP to pool
Virtual IP in front of a three-member pool
The VIP is only a listener. The pool is the set of IP:port services. Confusing those two words in a bridge call wastes an hour.
Quick answer

A node is an IP. A pool member is IP:port. A pool is the group plus algorithm plus monitor. A Virtual Server matches destination IP + port + protocol (and VLAN) and then uses that pool. Standard VS = full proxy, two TCP sessions.

Object hierarchy

Flow 1 · objects
Node10.20.20.101Member101:443PoolWEB_POOLVS192.0.2.100:443

Build bottom-up: nodes/members, then pool, then VS. Deleting a node still in a pool will refuse.

Say this out loud

Clients send to the Virtual Server destination. BIG-IP does not “become” 10.20.20.101. It selects a member and builds a server-side connection.

Load-balancing methods

MethodPicksUse when
Round RobinNext member in orderEqual servers, easy lab proof
RatioWeighted membersUnequal capacity
Least ConnectionsMember with fewest live connectionsLong-lived sessions, mixed object sizes
Priority GroupHigher group first until min-membersN+M: keep backups idle until primaries die
Fastest / Observed / PredictiveDynamic samplesOnly after you understand the metric — Module 3 PDF flags these as advanced
Decision · Path A vs Path B
Decision node splitting two load-balancing paths
Round Robin is the lab default. Least Connections is the usual production HTTP default when members are equal. Do not mix them in one sentence in an interview.

Also set Slow Ramp Time so a member just marked up does not eat every new connection, and connection limits so one server cannot be drowned. Priority Group Activation needs a minimum-active-members number or the backup group never engages.

Runbook — first HTTPS (or HTTP lab) Virtual Server

Side A · node and pool

  1. Create or confirm nodes

    GUI: Local Traffic → Nodes → Node List. A node can exist without being in a pool; a member cannot.

  2. Create the pool

    Local Traffic → Pools → Pool List → Create. Name WEB_POOL. Add 10.20.20.101–103 with the real service port.

  3. Attach a monitor later

    Module 5 builds the HTTP monitor. For this module a TCP monitor is enough to prove L4. Do not pretend ICMP means the app works.

TMSH · pool from Module 3 PDF
tmsh create ltm pool WEB_POOL monitor tcp load-balancing-mode round-robin members add { 10.20.20.101:80 10.20.20.102:80 10.20.20.103:80 }
tmsh list ltm pool WEB_POOL
tmsh show ltm pool WEB_POOL members
https://192.168.100.10/tmui/Control/jspmap/tmui/locallb/pool/create
Training mock · not live

Local Traffic > Pools > Pool List > Create

New Pool

WEB_POOL
tcp (upgrade to HTTP in Module 5)
Round Robin
10.20.20.101:80, 10.20.20.102:80, 10.20.20.103:80

Source: F5-BIG-IP-LTM-Module-3.pdf and F5 cert Lab 1.

Side B · Virtual Server

https://192.168.100.10/tmui/Control/jspmap/tmui/locallb/virtual_server/create
Training mock · not live

Local Traffic > Virtual Servers > Virtual Server List > Create

New Virtual Server

vs_web_http
Standard
192.0.2.100 / 32
80
TCP
WEB_POOL
Auto Map (if return path needs it — Module 4)

More-specific VS wins: longer mask, explicit port, explicit VLAN. A :443 VS will not catch :8443.

TMSH · Virtual Server
tmsh create ltm virtual vs_web_http destination 192.0.2.100:80 ip-protocol tcp pool WEB_POOL source-address-translation { type automap }
tmsh list ltm virtual vs_web_http
tmsh show ltm virtual vs_web_http

Side C · verify without lying to yourself

Browser refresh is not a load-balancing test — it reuses the TCP connection. Open a new connection per request (curl in a loop, or disable keep-alive).

Proof
curl -sv --http1.0 http://192.0.2.100/ | head
tmsh show ltm virtual vs_web_http
tmsh show ltm pool WEB_POOL members
tcpdump -nni 0.0 host 192.0.2.100
Ops · pool health tiles
Virtual server and pool health tiles on a monitor
One red member is not a down VIP. A down VIP is no VS match, empty pool, or no listener on that port.

Runtime after VS match

Flow 2 · client to member
MatchIP+port+protoProfileslater M4LB pickalgorithmMembernew TCP

Standard VS = two sessions. FastL4 is a different type — do not mix the mental model.

Traps + proof

FailureSymptomFix
Said 'the VIP' meaning the poolWrong object in the change windowName VS, pool, member separately
Port mismatchClient 443, VS 8443, no statstmsh list ltm virtual destination
No pool attachedVS enabled, RST or timeoutDefault pool field
Browser keep-aliveAlways hits one membercurl --http1.0 in a loop
Member :80 vs app :443Monitor up, HTTP garbageMember port is the service port, not the VIP port
You are done with Module 3 when

Knowledge check

Object words matter on a bridge call.

Q1

A pool member is:

Correct: b. Hierarchy in Module 3.
Q2

Standard Virtual Server is:

Correct: b. FastL4 is a different type.
Q3

Priority Groups are for:

Correct: b. Set minimum active members.
Q4

Client hits :443 but VS is :8443:

Correct: b. Match is IP+port+protocol.
Q5

Best lab proof of Round Robin:

Correct: b. Keep-alive reuses one server.
Q6

GUI path to create the pool:

Correct: c. PDF runbook.

Sources

Related: Course hub · Syllabus · My Courses · F5 LTM interview