Note 
You must be working on the F Series module in your Cisco Nexus 7000 Series chassis to use conversational MAC learning.
You must be working on the F Series module in your Cisco Nexus 7000 Series chassis to use conversational MAC learning.
In traditional MAC address learning, each host learns the MAC address of every other device on the network. When you configure a VLAN for conversational learning, the associated interfaces learn only those MAC addresses that are actively speaking to them. Not all interfaces have to learn all the MAC addresses on an F Series module, which greatly reduces the size of the MAC address tables.
Beginning with Cisco NX-OS Release 5.1 when you use the F Series module, you can optimize the MAC learning process. Conversational MAC learning is configured per VLAN. All FabricPath VLANs always use conversational learning; you can configure CE VLANs for conversational learning on this module also. (See Chapter 4 "FabricPath Forwarding" for more information on CE and FabricPath VLANs.)
The F Series modules have 16 forwarding engines (FEs), and the MAC learning takes place on only one of these FEs. Each FE performs MAC address learning independently of the other 15 FEs on the module. An interface only maintains a MAC address table for the MACs that ingress or egress through that FE; the interface does not have to maintain the MAC address tables on the other 15 FEs on the module.
Conversational MAC address learning, and the 16 forward engines (FEs) on each F Series module result in MAC address tables that are much smaller for FabricPath.
The MAC address learning modes available on the F Series modules are the traditional learning and conversational learning. The learning mode is configurable and is set by VLAN mode.
The following VLAN modes have the following MAC learning modes:
•FabricPath (FP) VLANs—Only conversational MAC learning
FabricPath (FP) VLANs—Only conversational MAC learning
•CE VLANs—Traditional learning by default; you can configure CE VLANs on the F Series module for conversational learning.
CE VLANs—Traditional learning by default; you can configure CE VLANs on the F Series module for conversational learning.
With conversational MAC learning, the interface learns only the source MAC address of an ingressing frame if that interface already has the destination MAC address present in the MAC address table. If the source MAC address interface does not already know the destination MAC address, it does not learn that MAC address. Each interface learns only those MAC addresses that are actively speaking with the interface. In this way, conversational MAC learning consists of a three-way handshake.The interface learns the MAC address only if that interface is having a bidirectional conversation with the corresponding interface. Unknown MAC address are forwarded, or flooded, throughout the network.
This combination of conversational MAC address learning and multiple FEs on each F Series module produces greatly reduced MAC address tables on each F Series module.
For CE VLANs, you can configure conversational learning per VLAN on the F Series module using the command-line interface (CLI). CE VLANs use traditional MAC address learning by default.Traditional MAC learning is not supported on FabricPath VLANs with Cisco Release NX-OS 5.1 or higher.
Figure 2-2 shows the allowed FabricPath and CE ports on the M and F Series modules and the allowed FP and CE VLANs.
Figure 2-2 FP and CE VLAN Examples
Switching Using FabricPath
The FabricPath hierarchical MAC address scheme and conversational learning result in much smaller, conversational learning MAC tables within the FabricPath network. Within the FabricPath network, the system uses Layer 2 IS-IS to transmit topology information. The interfaces on the edge of the network, which use conversational MAC address learning, do not have to learn all the MAC addresses in the network (see Figure 2-3).
Figure 2-3 FabricPath Ports Use Only the FabricPath Header to Switch Frames
MAC mobility is expedited using the FabricPath hierarchical MAC addresses. That is, when you want to move a host and keep its same MAC address and VLANs, only the interfaces at the edge of the FabricPath network track this change. Within the FabricPath network, the FabricPath interfaces update their tables with only the outer MAC addresses (ODA and OSA) that have changed from the FabricPath encapsulation.
See Chapter 3 "FabricPath Interfaces", for information on FabricPath interfaces.
The interface on the edge of the FabricPath network encapsulates the original frame inside the FabricPath header. Once the frame reaches the last, or directly connected, FabricPath switch, the egress interface strips the FabricPath header and forwards the frame as a normal CE frame.
The ports on an F Series module at the edge of a FabricPath network can use conversational learning to learn only those MAC addresses that the specified edge port is having a bidirectional conversation with. Every edge interface does not have to learn the MAC address of every other edge interface; it just learns the MAC addresses of the speakers.
As the frame traverses the FabricPath network, all the devices work only with the FabricPath header. So, the FabricPath interfaces work only with the ODAs and OSAs; they do not need to learn the MAC address for any of the CE hosts or other devices attached to the network. The hierarchical MAC addressing provided by the FabricPath headers results in much smaller MAC tables in the FabricPath network, which are proportional to the number of devices in that network. The interfaces in the FabricPath network only need to know how to forward frames to another FabricPath switch so they can forward traffic without requiring large MAC address lookup tables in the core of the network.
The switches in the FabricPath network decrement the TTL in the FabricPath header by 1 at each hop. When the TTL reaches 0, the packet is dropped. This process prevents the continuation of any loops that might form in the network.
