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This is the second part in a two part post on Etherchannel on the
Nexus 7000.  In the first part I covered how to configure vPC on
the Nexus 7000, here I will cover what it takes to get a remote
switch to uplink to the Nexus 7000 core switches using
vPC/Multi-chassis etherchannel.
Here is a diagram depicting the layout that we are using.  For
this part of the post, we will focus on the blue line that is
connecting both Nexus switches to the 3750 Stack.

On the Cisco 3750 switches (they are in a stack configuration of
two switches) we need to configure the interface to be in a
channel-group - for this example Iam using Channel-Group 6
(the switch is actually named StackSwitch06). What you will
also notice is that you configure the 3750 Stack just like it
was only connected to one switch, just one single port-channel
that consists of all the ports connected to both Nexus switches.
For this example we are using ports G1/0/1, G1/0/24, G2/0/1,
and G2/0/24. One thing I want to mention, when you are thinking
about your uplinks to your core switches, be aware of the switch
ASIC layout.  I say this because I have seen many times when
companies use ports 23 and 24 to uplink to a core switch.
The problem with this is that:
 1) The same ASIC is probably controlling both ports, and if
    it goes bad your links to the switch are gone and your
    switch is also isolated.
 2) You have a better chance of oversubscribing the ASIC
    before the uplink when utilization is high on the channel.
Now, onto the configuration, first up the Cisco 3750s.
    interface GigabitEthernet 1/0/1
     description [----[ Uplink to N7K1 - E9/10 ]----]
     switchport trunk encapsulation dot1q
     switchport mode trunk
     channel-group 6 mode active
    interface GigabitEthernet1/0/24
     description [----[ Uplink to N7K2 - E9/10 ]----]
     switchport trunk encapsulation dot1q
     switchport mode trunk
     channel-group 6 mode active
    interface GigabitEthernet 2/0/1
     description [----[ Uplink to N7K1 - E10/10]----]
     switchport trunk encapsulation dot1q
     switchport mode trunk
     channel-group 6 mode active
    interface GigabitEthernet2/0/24
     description [----[ Uplink to N7K2 - E10/10]----]
     switchport trunk encapsulation dot1q
     switchport mode trunk
     channel-group 6 mode active
    
Once the interfaces are assigned to the channel-group, we
can configure the etherchannel on the Cisco 3750s. Notice
that there is no vPC info nor anything else that says this
is connected to two switches.
     interface Port-channel6
      switchport trunk encapsulation dot1q
      switchport mode trunk

Now, on the Nexus side we need to do some configurations
as well. Both Nexus switches are also configured the same,
so there are no differences in the switch configs.
     interface Ethernet9/10
       description [----[ StackSwitch6-1 ]----]
       switchport
       switchport mode trunk
       channel-group 6 mode active
       no shutdown


     interface Ethernet10/10
       description [----[ StackSwitch6-1 ]----]
       switchport
       switchport mode trunk
       channel-group 6 mode active
       no shutdown
Now, when it comes to configuring the etherchannel on the Nexus
switches, is is configured the same except for the addition of
a vPC identifier. I recommend using the same number that you used
for the port-channel for easy identification, but that is up to you.
   interface port-channel6
     description [----[ LACP EtherChannel for StackSwitch6 ]----]
     switchport
     switchport mode trunk
     vpc 6



Once you have it configured on the Nexus, make sure it is up and
in the vPC correctly.        

     N7K1# sh int port-channel 6  
     port-channel6 is up
     vPC Status: Up, vPC number: 6
     Hardware: Port-Channel, address: 5475.d04f.1165 (bia 5475.d04f.1165)  
     Description: [----[ LACP EtherChannel for RackSwitch6 ]----]   
     Members in this channel: Eth9/10, Eth10/10  
     N7K1#


Once you have confirmed that all is working correctly, you can
check out the StackSwitch spanning tree information: 

     StackSwitch06#sh spanning-tree interface port-channel 6 
     Vlan             Role Sts Cost      Prio.Nbr Type
     ---------------- ---- --- --------- -------- --------------------------------
     VLAN0001         Root FWD 3         128.656  P2p
     VLAN0002         Root FWD 3         128.656  P2p
     VLAN0003         Root FWD 3         128.656  P2p
     VLAN0004         Root FWD 3         128.656  P2p
     VLAN0005         Root FWD 3         128.656  P2p
     StackSwitch06#


You will see that even though you are connected to two switches,
the port-channel is seen as a single spanning-tree
path to the root.