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The "%SYS-3-SYS_MEMLOW:Mbuf usage exceeded 90%" error message appears in Catalyst 6500 and 4500 series switches with Cisco CatOS


Core issue

The message looks similar to this:

%SYS-3-SYS_MEMLOW: [chars] usage exceeded 90%

This indicates that memory usage has exceeded 90 percent. [chars] can be memory cluster (NMP operating system memory) or mbuf (memory buffer). If memory usage is mbuf, the control plane frame buffer usage, which includes Spanning Tree BPDUs, CDP, PAgP, DISL, and all other administrative protocols as well as SNMP and NTP type protocols in the administrative VLAN bound for the NMP CPU, has exceeded 90 percent.

The administrative vlan is the management vlan, which is used to Telnet to the switch and manage it remotely. The best practice is to keep this VLAN out of users data traffic. The error message you see not only points to traffic at the management VLAN, but also deals with memory (DRAM) problems.

In addition to this, you can also check these scenarios:

  • If the ip redirect is enabled, this causes the switch to dynamically learn gateways to the destinations that it never supposed to reach. 
  • The ip unreachable is enabled, which tells the switch to answer all ICMP messages. 


If the message indicates mbuf usage exceeded 90 percent, try to decrease traffic on the administrative VLAN in order to prevent future packet loss.

Create a separate VLAN for the users and for the administration of the switches in order to do this. This way all broadcasts and traffic flooding that can happen in the user vlan does not hit the management interface.

In order to disable ICMP redirect messages, use the set ip redirect disable command. Then you can erase the dynamically created table with clear ip route all command.

In order to disable ICMP unreachable messages, use the set ip unreachable disable command.