Exam F5CAB5 Topic 4 Question 50 Discussion

Actual exam question for F5's F5CAB5 exam
Question #: 50
Topic #: 4
A BIG-IP Administrator configured the following virtual server to pass traffic on all addresses and ports.
After configuration is completed, the BIG-IP Administrator notices that the virtual server is unable to pass traffic.
ltm virtual forwarding_any_vs {
destination 0.0.0.0:any
ip-forward
mask 255.255.255.255
profiles {
fastL4 { }
}
serverssl-use-sni disabled
source 0.0.0.0/0
translate-address disabled
translate-port disabled
}
Which part of the configuration is the cause of the issue? (Choose one answer)

Suggested Answer: B Vote an answer

This virtual server is intended to function as aforwarding (IP-forwarding) virtual server, which is commonly used for routing or firewall-style deployments where BIG-IP forwards traffic transparently without load balancing or address translation. For a forwarding virtual server to match and passall traffic, the destination must be configured as 0.0.0.0:anywith a mask of 0.0.0.0, not 255.255.255.255.
The configured mask 255.255.255.255 represents a/32 host mask, which restricts the virtual server to matching traffic destined only for the exact IP address 0.0.0.0. Since 0.0.0.0 is not a valid routable destination for normal traffic, no packets will ever match the virtual server, causing it to pass no traffic at all.
This is a well-documented BIG-IP behavior:
* destination 0.0.0.0:any
* mask 0.0.0.0
together define acatch-all forwarding virtual server.
The destination itself (Option A) is correct for a forwarding VS, and disabling address translation (Option C) is expected and required for IP-forwarding mode. Therefore, the incorrect subnet mask is the sole reason the virtual server is not functioning as expected.

by Michael at Aug 03, 2026, 05:25 AM

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