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Regarding Traffic Flow of VPC of Nexus Switch


Hello Guys,

I need to know about the basic traffic flow of VPC of Nexus. We know in normal Port-channel the traffic can be load-shared between two physical link so that we can use maximum capacity of dual links.


But in case of VPC of Nexus we know we can create port-channel with two different switches. My question is how the traffic will flow in the below scenario? Will the traffic be load-balanced or it will choose primary and secondary link.








4 Replies 4

Christopher Hart
Cisco Employee
Cisco Employee

vPC-connected network devices will load-balance traffic across a vPC according to the network device's local load-balancing algorithm. This typically results in all members of the vPC being utilized (although this depends heavily on your traffic profile - just like a normal port-channel, it's possible for traffic to become polarized and only flow across one member of a port-channel). In other words, vPC-connected network devices will treat the vPC as if it is a normal port-channel - no special considerations are made by the vPC-connected network device simply because it is connected to a vPC (in fact, the network device has no knowledge that it's connected to a vPC - from the network device's perspective, it's a normal port-channel, and both Nexus switches appear as one logical unit.)
You did not specify what make and model router CORE-RT-01 and CORE-RT-02 are in your diagram, so I will assume they are ASR 1000 series routers. A great description of the port-channel load-balancing algorithm options on ASR 1000 series routers can be found here.
I hope this helps - thank you!

Hello Hart,

So any traffic can flown via both SW-1 and SW-2 by round-robin method, right?

It's not round robin. 

If it's layer2 only, then think about a normal port channel.

A server, or a device which is connected to both switches will egress hash the frames based on some algorithm over the two ports in the port channel.

If that algorithm chooses switch 1, and the destination is another port channel on switch 1, then switch 1 will prefer to send it to the locally connected device instead of through peer link.

If algorithm on device chooses switch 2, same thing happens.

So the load balancing is done on your device as it sends some frames to switch 1 and some to switch 2.  And then the VPC switches always choose the shortest path to destination. Will only use peer link if the destination MAC addr is NOT directly accessible from the switch.




Just try the following option in the config mode, you would notice list of load balancing algorithm supported.


(config) # port-channel load-balance ?


src-dst ip-l4port-vlan option is ideally preferred in most of the scenarios for efficient link utilization.


HTH !!



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