Cross-Stack EtherChannel Between C3750 and C2960

[ryanhoibm]

Hello experts,

Can cross-stack Etherchannel cannot be implemented between Cat3750 and Cat2960?

I recently have Forwarding loops issue in my customer's network.

The  setup is 3 Cat3750G-12S stack together using stackwise. they are  connected to 6 Cat2960G-24TC-L via cross-stack etherchannel. And I do  not know what happen, loops are being created. Can I confirm with you  once again that this setup is wrong? If its wrong, what should the  solution be to connect 6 Cat2960 to 3 Cat3750G stack switches?

Thanks a million in advance!!!

Regards

Ryan

[Nagaraja Thanthry]

Hello,

I see that PO4 and PO7 go to two different switches. Are they connected to

each other?

Regards,

NT

[ryanhoibm]

Hi NT,

Nope PO4 is at 24th flr and PO7 is at 26th flr if you look at the diagram which indicates all the POs.

I am thinking of implementing BPDU Guard on every user ports and Loop Guard globally. And I will convert the trunk and Port Channel links to access ports. Since its per floor Vlan, I will configure all uplinks on 24th flr to be on access vlan 24. Hope that will solve all these strange MAC flaps. And I will double check if the uplinks are not connected correctly to the patch panel.

Have you ever encounter this from your customer???

Regards

Ryan

[manish arora]

here's a link for you help , you can check for cross stack ether channel recommendations on this :-

http://www.cisco.com/en/US/products/hw/switches/ps5023/products_configuration_example09186a00806cb982.shtml#stack

Thanks

Manish

[ryanhoibm]

Hi Manish,

Thanks for the link. I have seen it before I post this discussion

Regards

Ryan

[Nagaraja Thanthry · Accepted solution]

Hello,

Yes, I have seen such issues and mostly it was due to physical loops. If

there is a server with dual NICs connected to two different switches, or if

there is a wireless client trying to associate with different AP's, we

witness such MAC flaps. In your case, I do not see any reason why a MAC be

flapping between PO4 and PO7 if the two switches are on different floors

unless there is some kind of physical loop created between these two floors.

Also, some of the error messages indicate issues between gi 1/0/5 and PO7

(but on a different vlan). So, yes, changing the ports to access mode could

help. But more than that, I would suggest you investigating the physical

loop part.

Regards,

NT

[ryanhoibm]

Hi NT,

I agreed. I will need to investigate on the physical connection. Thanks alot!!!

Regards

Ryan Ho

[ryanhoibm]

Hi NT,

I have a question.

If

there is a server with dual NICs connected to two different switches, or if

there is a wireless client trying to associate with different AP's, we

witness such MAC flaps

Servers with Dual NIC connecting to 2 different switches are for redundancy purposes. So what should be the correct way to connect servers with dual NIC? I thought Dual NIC has 2 different MAC addresses and since both the MAC addresses are different, why will it cause MAC flaps?

Regards

Ryan

[Nagaraja Thanthry]

Hello,

While it is true that each NIC will have its own MAC address, typically, when you configure NIC teaming, you will configure a virtual MAC that is shared by both NIC cards. Also, one another thing that people generally use is Bridging. If you are doing either one of these two and if the teaming broke, then there will be a physical loop. Since the MAC address is shared by both NIC cards (for redundancy), you will see it flapping between the interfaces.

Regards,

NT