VSS (distribution) connects to 3750 stacks

Routing & Switching Switching
1

Rene,
I have two 6509. They are VSS.
I have 4 3750 with stackwise.

3750 connects to 6509 with two cables.

On 6509 end, it is configured as Port-channel.
On 3750 end, it is not configured as Port-channel.

This seems very odd to me.

Should it be port-channel as well on 3750 side so that there is no spanning-tree?

2

Hello Hong.

In order for the port channel to function correctly, both ends of the link must be configured such that a port channel is successfully created. In order for it to function in the topology you described, the following prerequisites must be adhered to:

  1. A port channel can be created on multiple ports of the SAME PHYSICAL SWITCH in order for it to function correctly on both ends. There are exceptions to this rule, and these are described below.
  2. On a set of VSS switches, if the port channel is configured as a multi-chassis port channel, that is, the port channel is composed of links that connect to more than one chassis, the appropriate IOS versions must be present and modules must be installed that support this feature. For more information about compatible modules and IOS versions, take a look at this Cisco link.
  3. On a set of stacked 3750 switches, it is possible to create a cross-stack EtherChannel, but there are certain prerequisites for this. These can be found in the following Cisco documentation.

Now there are circumstances under which a configuration you describe above would provide some functionality, but this would be far from ideal. Check to see which protocol is being used and if the EtherChannel has been hardwired or if a negotiation is occuring. There could be a misconfiguration concerning negotiation that is not allowing the EtherChannel to form.

Ideally, if you make sure that multi-chassis EtherChannel is available on VSS and cross-stack EtherChannel is available on the stack, then you should configure both ends of the EtherChannel to function as EtherChannel without the use of negotiation. It is a shame to have two connections between the switches and to use the bandwidth of only one.

So, I would suggest you configure port channel on the 3750 as well so that you can take advantage of the full bandwidth available to you.

I hope this has been helpful!

Laz

3

dang this website is awesome.
Thank you very much.

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