Cisco 6500 VSS with Daisy Chained Access Switches

Hi,

We are about to refresh our 6500s and we are moving to VSS.

Our access switches are daisy-chained.  How will MECs work on daisy chains?

6500 Switch 1 connects to Access 1
6500 Switch 2 connects to Access 2
Access 1 connects to Access 2

Thanks!
svillardiAsked:
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Nayyar HH (CCIE RS)Network ArchitectCommented:
In a VSS design the daisy chain can be broken as you'll only be creating a loop in the topology which means you'll be dependent on Layer-2 protocols for failover. The happened to be one of the things of VSS elimates. There's really no need to introduce Layer-2 protocols into your VSS topology it defeats the purpose. See link below...

http://www.cisco.com/en/US/docs/solutions/Enterprise/Campus/VSS30dg/VSS-dg_ch1.html#wpxref71787
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svillardiAuthor Commented:
The problem is, if I disconnect the access switches from each other there is a single point of failure.  In our current config 2 3560g switches are connected together, with each 3560 also connected back to each 6509.

I don't understand how we can break the link between the 3560g switches?  Where will the redundancy by?  I understandt that VSS gives us redundancy at the core/distribution layer, but the access layer is different.

Please elaboraate.
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Nayyar HH (CCIE RS)Network ArchitectCommented:
You could acheive redundancy at the Access layer by dual-homing the Servers onto both switches (even with no link between them). In this configuration you would span VLANs hosting these servers across the three switches. You would also implement MEC between Access and Core. Effectively creating a loop-free yet highly resilient topology. Implement link state tracking on access switches to enable servers detect uplink failures.

As an example, say we create VLAN 100-105 across all switches and create trunks (MEC) between VSS and access switches (no link between access) with trunk ports carrying VLAN 100-105. This creates the loop-free logical topology below;


   Access1==VSS==Access2
       \                              /
         \                          /
           \                      /
             \                  /
                SERVER1

Now to consider the effect of some possible failure scenarios on convergence :

Core Switch: An active L2 path still existing between Server1 and Core - No Convergence
Access Switch: Server should be able to detect this and switch to backup NIC
Uplink path Access to Core: This type of failure could create a blackhole but with link state tracking configured on the access, the switch whos uplink failed would drop its downstream ports to the server to enable the server switch to its backup NIC.

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svillardiAuthor Commented:
Nazsky,

Our config is more like this:

Client1                     Client4
Client2                     Client5
Client3                     Client6
|                                     |
|                                     |
Access1======Access2
|                                    |
\                                    |
 6506====VSS===6506

What happens?  I can't disconnect my Access switches from each other.
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svillardiAuthor Commented:
And we aren't planning to change the 3560g switches.
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Nayyar HH (CCIE RS)Network ArchitectCommented:
I might be missing something here, in your new setup wouldnt the 6506s be part of the same VSS domain? If so, Access1 and 2 would be physically connected to both members in the VSS domain with MEC.

Excuse my diagram, new topology would look someting like this ...

Client1                     Client4
Client2                     Client5
Client3                     Client6
|                                     |
|                                     |
Access1               Access2
|              \         /          |
|                \     /            |
[6506A===VSS===6506]


In you new topology, what is the reason you feel disconnecting access switch links would not be possible? traffic loads?


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svillardiAuthor Commented:
If one uplink will logically connect each access switch to the VSS domain (both 6506s) we would be good, EXCEPT if that if the phyiscal link goes down, I don't have a connection to anything, because theres only ONE uplink to each access switch.  :(

The reason that the 3560g's are daisy chained is in many cases they aren't near each other.  They did this to save money on fiber.

Room 1                     Room 2
______                     ______
Client1                     Client4
Client2                     Client5
Client3                     Client6
|                                     |
|                                     |
Access1======Access2
|                                    |
\                                    |
 6506====VSS===6506

So the question is, can you create a MEC with only one connection to each 3560g?  What actually needs to be done on the 3560g's?

Thanks for the assistance.
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Nayyar HH (CCIE RS)Network ArchitectCommented:
I'm afraid not that is not possible, it has to be the SAME node.

See Cisco FAQ below

Q. What is multichassis EtherChannel?
A. Multichassis EtherChannel (MEC) is a Layer 2 multipathing technology. This form of EtherChannel allows a connected node to terminate the EtherChannel across the two physical Cisco Catalyst 6500 Series Switches that make up the VSS leading to creating simplified loop-free Layer 2 topology. Using MEC in VSS topology results in all links being active and at the same time provides for a highly available topology without the dependency of Spanning Tree Protocol. With the introduction of 12.2(33)SXI, the virtual switching system supports a maximum number of 512 MECs.

http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps9336/prod_qas0900aecd806ed74b.html


If Access1 and Access2 were stacked switches i believe this would have been possible.
 
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svillardiAuthor Commented:
So, what you are saying is any access switch that only has one uplink will not be able to take advantage of MEC.
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Nayyar HH (CCIE RS)Network ArchitectCommented:
Certainly, you would need at least two physical uplinks to take advantage of MEC.
Multi Chassis Etherchannel is from the core's perspective.
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svillardiAuthor Commented:
Thank you!
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