Configuring Limit Learning

To limit the number of dynamic MAC addresses that can participate in the network, use the limit-learning option in following command:
configure ports port_list {tagged tag} vlan vlan_name | vlan_list [limit-learning number {action [blackhole | stop-learning]} | lock-learning | unlimited-learning | unlock-learning]
This command specifies the number of dynamically learned MAC entries allowed for these ports in this VLAN (Virtual LAN). The range is 0 to 500,000 addresses.

When the learned limit is reached, all new source MAC addresses are blackholed at the ingress and egress points. This prevents these MAC addresses from learning and responding to ICMP (Internet Control Message Protocol) and ARP packets.

Dynamically learned entries still get aged and can be cleared. If entries are cleared or aged out after the learning limit has been reached, new entries will then be able to be learned until the limit is reached again.

Permanent static and permanent dynamic entries can still be added and deleted using the create fdb and disable flooding ports commands. These override any dynamically learned entries.
For ports that have a learning limit in place, the following traffic still flows to the port:
- Packets destined for permanent MAC addresses and other non-blackholed MAC addresses
- Broadcast traffic
- EDP (Extreme Discovery Protocol) traffic
Traffic from the permanent MAC and any other non-blackholed MAC addresses still flows from the virtual port.
To remove the learning limit, use the unlimited-learning option.
configure ports port_list {tagged tag} vlan vlan_name | vlan_list [limit-learning number {action [blackhole | stop-learning]} | lock-learning | unlimited-learning | unlock-learning]

The MAC limit-learning feature includes a stop-learning argument that protects the switch from exhausting FDB (forwarding database) resources with blackhole entries. When limit-learning is configured with stop-learning, the switch is protected from exhausting FDB resources by not creating blackhole entries. Any additional learning and forwarding is prevented, but packet forwarding is not impacted for existing FDB entries.

Published March 2020prev | next

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