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New Member

Disappearing packets

Hi all,

I'm currently quite baffled with the problem I'm encountering. I hope you can help me figuring this one out. My setup is as follows:

I've got four nexus 7k's as core switches. Two of them per datacenter. Connected to them are 4 5k's, again two per datacenter. The 5k's are redundantly connected with the 7k's on the same location. The 7k's are connected with each other and their namesakes in the other datacenter.

Connected to the 5k's is a UCS on which some VM's run. One of these VM's is a TFTP server. The other VM's attempt to boot using this TFTP server. The curious case is that some of these are able to boot from TFTP and other are not.

The server is in vlan 650 and the clients in vlan 251. Routing is done on the 7k's which are running hsrp. The current active hsrp node is called 1001. Its neighbor 7k is called 1002.

When tracing the traffic we see the TFTP requests leaving the UCS and arriving on one of the two 5k's, after this the traffic is spotted on one of the two 7k's. On 1001 the routing is performed and so the traffic leaves vlan 251 and enters 650. We can see the traffic on vlan interface 650's egress 'port'. However, it never arrives at the server. As far as I can see (using wireshark) it never even arrives on the 5k on its way back to the UCS.

So the traffic flows as follows:

VM --> N1000v --> interconnect --> N5k --> N7k, vlan 251 --> N7k, vlan 650 --> ???

We expected some deviation in the traffic that does not work in comparison with the traffic that does work. However, none is observed.

The interface counters on all switches remain at zero when looking at errors.

My suspicion is that the traffic is sent over the OTV links between the datacenters, even though the mac address table is correctly filled on the 7k's

I hope you can give me some pointers as to where I might look to figure out this one.

Thanks in advance.

New Member

Disappearing packets


If I understand it correctly, you are trying to have inter-vlan communication. that is clients in vlan 251 should be able to talk to servers in vlan 650 and vice versa.

can you pls check the following:

1. from the client and the server can you ping their respective default gateway ?

2. from client can you ping the default gateway of the server? and from the server can you ping the default gateway of the clinet?


New Member

Disappearing packets

Yes, that all works. They are also able to get DHCP from a server in vlan 650. It only appears to happen with TFTP.

What makes it even more confusing is that sometimes it does work and sometimes it doesn't. A system might have worked fine yesterday, but is unable to boot today.

When changing the mac address of the adapter it might suddenly work (but it might not as well) and using another tftp client might also work while it doesn't on another system.

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