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Fabric Extenders (FEX) Overview

posted Mar 20, 2014, 7:45 AM by Rick McGee   [ updated Mar 20, 2014, 7:41 PM ]
N2K Fabric Extender (FEX) acts as a remote line card of a 7K or 5K switch
    Containing cable run's within the Data Center
    Simplify DC design
        Don't have multiple NX-OS images to manage 
        Centralized Management 
            Parent Switch (N5K or N7K)
        Saving on Cabling runs
    All management performed on the Parent Switch
        No Console or VTY ports on the FEX
        Automatically downloads NX-OS image from parent switch

No Intelligence in the FEX
    You cannot switch between N2K port 1/1 to N2K 1/2 locally from the same N2K FEX. Would have to go to Parent switch for forwarding decision 
    This will give you a North - South traffic flow 
    Think about DB replication between Servers you would have to take this traffic flow into consideration
    They try to through multiple 4 10GE port on the newer N2K ports

N5K vs. N7K Parent Switch

    N5K as FEX's Parent
        - Supports Static Pinning and vPC Topologies 

N7K as FEX Parent 
       - Not all line cards support FEX (this has changed with F2 and now with F3 both support FEX's)
       - Static Pinning only NO!!!! support for vPC up to two different line cards/modules (this is sometimes referred to EvPC (Enhanced vPC)
            - Fex link must be a port-channel
FEX ports are L2 switch ports or native L3 routed interfaces

Fex Configuration 
    - Nexus 5K
          -Initialize FEX feature set
            - "feature fex" disabled by default!
    - Show Fex
        - This will show you discovered FEX models that it learned about but not the port it's one
    - Nexus 7K 
        - "install feature-set fex" and "feature-set fex"
    -Configure Downlinks
    - For Nexus 5K
        - "int e1/x - x"
        - "switchport mode fex"
        - "fex associate 10X"
    -Nexus 7K
        - "int e1/x - x"
        - "configure channel-group 101 mode on"
        -" interface po101"
        -"switchport mode fex"
        -"fex associate 101"

    - With vPC you have to be careful on how you configure your Nexus switches
        - For instance vPC doesn't create a single control plane like VSS does
            - You would have to configure both northbound switches in a vPC pair with the same 

    - Fex should be discovered automatically 
        - "show fex detail"
        - NX-OS downloads to the FEX from parent switch which will take a few minutes 
        - Once complete FEX appears as just normal port on the Parent switch

    - Interesting Caveats 
        - Nexus 7K on the modules to N2K FEX's doesn't support CDP
            -CDP is supported on the N5K's 
        - Layer 2 FEX ports are STP "edge" ports
            - Are not subject to STP listening and learning delay, go straight to forwarding
            - If FEX ports are sending BPDU's it will shut down the port via BPDUGuard which is on by default
        - Nexus 2232
            - Even though it says it's a 10Gbps port FEX you can configure the FEX (non uplink ports) as 1Gbps
                - Shutdown the port and change speed 1000
        - On the Nexus 5K's instead of using the max-links ( 1 is the default) control under the FEX commands, use a Port-Channel and have that do
        the port-channel hashing instead.

FEX Design's 

Standard Design No vPC's

Multiple Parents in this design with vPC's

EvPC design with NIC teaming 802.3ad on the host. The host will determine what kind of NIC teaming protocol is supported

vPC with the Nexus 7K's Port-channel down to the FEX's  No EvPC, no dual vPC topology