Configuration Guide User guide

564 FastIron Configuration Guide
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Flow-based MAC address learning
Flow-based MAC address learning
NOTE
Flow-based MAC address learning is supported on FastIron X Series and Brocade FCX Series
devices. It does not apply to FastIron WS Series devices. However, on Brocade FCX Series devices,
this feature is enabled by default. There is no command to enable or disable it. Therefore, the CLI
commands in this section apply to FastIron X Series devices only.
This section describes flow-based MAC address learning and how to enable it on a FastIron X Series
switch.
Flow-based learning overview
With regular MAC address learning, when a new MAC address is learned, it is programmed in the
same location (hardware index) in all packet processors in a FastIron Layer 2 or Layer 3 switch.
There are multiple packet processors (one per port region) in a compact switch, and in each
module in a chassis-based switch. With regular MAC address learning, MAC addresses are global,
meaning the hardware MAC table is identical across all packet processors.
With the introduction of flow-based MAC address learning, when a new source MAC address is
learned, it is programmed only in the source packet processor (the processor that received the
packet). The destination MAC address gets added to other packet processors on demand,
whenever a traffic flow that needs it is detected. With flow-based MAC address learning, the MAC
address is programmed in different hardware locations and the hardware MAC table is different
across all packet processors.
The benefits of flow-based learning
With global MAC address learning, all MAC addresses are programmed in all packet processors,
even though they may not be required and are never used by all packet processors. Global MAC
address learning wastes some space in the hardware MAC table and limits the number of
supported MAC addresses to 16K.
With flow-based MAC address learning, MAC addresses are learned and programmed selectively,
only in the packet processors that need them. Since the MAC addresses are distributed across
several packet processors, flow-based learning frees up space in the hardware MAC table and
increases the number of supported MAC addresses from 16K to 32K.
How flow-based learning works
When a packet processor, for example, PP 1, receives an incoming packet with source MAC
address X, it sends a new address message to the CPU. The system learns MAC address X by
adding it to the software MAC table in the CPU, then programming it in the hardware MAC table in
the source packet processor, in this case PP 1. If the MAC address is learned on a trunk port, the
MAC address is also programmed on all of the packet processors that have ports in the same trunk
group.
When another packet processor, let call it PP 2, receives an incoming packet and the packet
destination MAC address matches source MAC address X, it floods the packet in hardware as an
unknown unicast packet and copies the packet to the CPU. The system locates the MAC address in
the software MAC table, then programs the MAC address in the hardware MAC table in PP 2. If the