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Nexus 5k with NLB & vPC

We have a customer with NLB problem.

2 servers, each connecting to a seperate N2K, which in turn connect to a seperate N5K


   I                                                  I

   I                                                  I

   I                                                  I

N2K                                            N2K

   I                                                  I

   I                                                  I

SERVER1                                  SERVER2

NIC1                                             NIC1


So we have a vPC configured between the 2 interfaces on the N2K'S & it is up

sh vpc 14

vPC status


id     Port        Status Consistency Reason                     Active vlans

------ ----------- ------ ----------- -------------------------- -----------

14     Po38        up     success     success                    190       

Is this supported on the Nexus switches as both ports are up, but you can only get a reply form one of the servers




Nexus 5k with NLB & vPC

Hi Colin,

I have a lot of problems with microsoft NLB but with switches cisco not Nexus.

NLB use a special mac-address that the switches don't forward, I don't know if in nexus the probles ot's the same. For solve the problem in Cisco Switches I configured a Static mac address entry with the NLB mac address, also you customer can change the NLB configuration to unicast.

Recomend to your customer to remove NLB and replace with a hardware load balance.

Hope this help,



Cisco Employee

Nexus 5k with NLB & vPC

Hi Colin,

From the diagram I don't think you have this setup in a supported way.  MS NLB isn't designed to vituralize two devices as 1 at a hardware level like this.

The vPC from your Nexus 5000 pair  should to to 1 server...not 2 running NLB.

So the way NLB works is to get identical frame to each server.  This is accomplished a few ways.  Could be with multicast or using unknown unicast addresses to cause flooding.  vPC will not allow a frame to go to both devices because from the 5k perspective the device at the only end of the vPC is 1 device, and only should recieve 1 packet.

Hopefully that makes sense.  We can talk more about vPC or NLB if you want, but you don't want to set it up the way you have it there. 



Nexus 5k with NLB & vPC

What is the best way to setup NLB on the Nexus 5000?




Nexus 5k with NLB & vPC

Not sure if this helps any...

Our setup is ...

We use VPC and (painfully) WNLB.

We added the static ARP and MAC ADDRESS-TABLE entries on the WS-C4900M's (L3 - where our default gateway IP's are configured) ... and NOT on the N5K's.

NLB works.

I applied this command on both core switches (4900) -

arp 10.x.x.x 03bf.0a85.0164 arpa               (where 10.x.x.x is actually your NLB address)

mac address-table static 03bf.0a85.0164 vlan 101 interface po1          (our core is connected to the N5K's by 2 x 10Gb interfaces configured as PortChannel).

Some handy info...

The first number in the mac address is the type of NLB configuration:

01=IGMP, 02=Unicast, 03=Multicast

I know that you can statically assign arp entries on the N5K's - but it is done in the interface configuration and must be an L3 port ("no switchport" command needs to be applied - which defeats the point for us in our config - hence we applied only on the 4900's and it just worked).

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