Palo Alto Networks Certified Network Security Engineer (PCNSE)Core ConceptsHard
A security engineer is performing a packet capture on a Palo Alto Networks firewall and notices that some packets are being dropped due to 'flow_np_sess_alloc_fail'. Which of the following is the most likely cause of this specific drop reason?
- AThe firewall's data plane CPU is overloaded, leading to delayed packet processing.
- BThe firewall is experiencing a high volume of new session creations exceeding its capacity.
- CA Security policy rule is configured with an 'deny' action for the identified traffic.
- DThe packet size exceeds the maximum transmission unit (MTU) of the egress interface.
Show answer & explanationAnswer & explanation
Correct answer: B. The firewall is experiencing a high volume of new session creations exceeding its capacity.
The 'flow_np_sess_alloc_fail' drop reason specifically indicates that the firewall tried to allocate a new session in the Network Processor (NP) but failed because the session table was full or the rate of new session setup exceeded its capacity. This is a common indicator of a session table exhaustion or a session setup rate limit being hit.
Why the other options are wrong
- A. While CPU overload can cause performance issues, this specific drop reason points to session table allocation, not general processing delay.
- C. A deny rule would typically show a 'deny' action in the logs, not a 'flow_np_sess_alloc_fail' drop, which is a resource issue.
- D. MTU issues typically result in fragmentation or 'pkt_len_exceed_mtu' or similar drops, not a session allocation failure.
flow_np_sess_alloc_fail
A Palo Alto Networks packet drop reason indicating that the firewall failed to allocate a new session, often due to the session table being full or exceeding the new session rate limit.
- Indicates session table exhaustion or rate limit.
- Occurs during new session setup.
- Requires investigation into firewall capacity or traffic patterns.
Memory trick: No session allocated? Check your session table, it's overloaded!