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Nexus 5K to 2K fex link level loadbalancing


I have created a setup as attached in the diagram. N2K is dual uplinked to N5K using 10G FEX uplinks. We are using Enhanced VPC design for the connectivity.

Note I have connected HostA and HostB & C on single N2K. I would like to know the traffic flow for HostA to HostB & Host A to C on the FEX link level.

I am just summarizing my understanding below

1. N2k doesn't have any intelligence, it depends on parent N5K switch even for local switching.

2. N5k switch uses source and destination Mac address as a criteria for Load balancing algorithm on the port channel for Layer2 frames and source, destination mac and source, destination IP for Layer 3 frames.

3. In our example host A and B & C are in the same VLAN, hence Layer 2 algorithm is applicable.

4. Nexus 5K switch is having some intelligence where the frames from N2K-1 coming to N5K-1 will be sent back from N5K-1 to N2K-1 instead of sending the Frames crossing the Peer link.



1. Here our doubt is, if Frames from HOST A flows to N5K-1 for a local switching does it takes Port-1 for N2K-1 to N5k-1 traffic flow and takes Port-2 for return traffic from N5K-1 to N2K-1 (or does it use same Port for both the traffic flow for example PORT-1 for both).

2. Traffic between Host A and C --  does the frames travels from N2K-1 to N5K-2 using PORT-3 or Port-4 / Does it use between N2K-1 to N5K-1 port-2.

Can anyone help us in clarifying our doubts?

Thanks in advance.


The N5K and N2K use a a hash to determine the link that is used for traffic.

With VPC, the device always uses a local link if its available.  So, it will never use the VPC Peer-link in your design.

So, any traffic from any host can select any of the 4 links north bound to the N5K's.  Then, the return traffic from the N5K will choose only one of the 2 local links.

You can use "show port-channel load-balance ?" to help determine the link that will be chosen.

I hope this helps.


A little late to the party. A little background information on why I am seeking this information. We have multiple fex's dual homed to two 5k's, and sporadically if both links from one 2232 to the 5k's are up, we will blackhole traffic. We are unaware if the traffic is being blackholed on the VM/Host (drivers), fex (asic), or even making it to the 5k. Hosts are dual homed to two 2ks as well, so the host will determine which fex to utilize. Is there a way to correlate which northbound link the traffic is flowing from the 2k to the 5k? Is there an excel spreadsheet out there to be able to determine based on number of links/vlan/vpc/mac/IP, etc, which path the algorithm will chose?