L4 LB drops packets due to session exhaustion

Products

• VMware NSX

Issue/Introduction

Symptoms:

Impact new traffic through L4 LB.

Environment

VMware NSX-T Data Center
VMware NSX-T Data Center 3.x

Cause

L4 LB has session limit for each size. You can check session limit as follows.

root@edge:~# grep l4_sessions /config/vmware/edge/lb/etc/lbs_*.conf
/config/vmware/edge/lb/etc/lbs_large.conf:l4_sessions        2100000;
/config/vmware/edge/lb/etc/lbs_medium.conf:l4_sessions        525000;
/config/vmware/edge/lb/etc/lbs_small.conf:l4_sessions        105000;
/config/vmware/edge/lb/etc/lbs_xlarge.conf:l4_sessions        4200000;

L4 LB cannot accept new session if session table is full. The following is example of reaching to session limit for small size.

edge> get load-balancer ########-####-####-####-##########ff session-tables | count l4lb
Tue Jul 05 2022 UTC 04:53:39.558
Number of lines that match pattern 'l4lb': 105000

Resolution

Please consider changing LB size if session limit is not enough to handle traffic.