Cisco Switch Virtualization

Notes on switch virtualization, explaining how logical switches enable multi chassis EtherChannel and how Cisco StackWise elects a stack master.

With STP we have redundant links but we do not have them all in forwarding state. We do not even have redundancy in the Access layer.

Access and distribution joined by redundant links, where STP blocks one uplink and the access layer has no redundancy at all

One way of solving this problem is to create a logical switch which is made up of two or more physical switches:

Two physical distribution switches presented as one logical switch, which puts both access uplinks into forwarding

We can do the same thing in the distribution layer. You can guess the illustration.

Access layer virtualized as well, which removes the Layer 2 loop between the two layers entirely

The four links between the switch pairs can be combined into a single etherchannel since we have a single link between the two logical switches. This is called Multi-Chassis Etherchannel.

Before and after combining multiple switches

Before

Campus before virtualization, where every access switch is cross-linked to both distribution switches and both cores

After

Same campus after virtualization, where each pair becomes one logical switch and the cross-links collapse into a single etherchannel per block

Cisco StackWise

Cisco Stackwise allows us to turn multiple physical switches (up to 9 switches) into a single logical switch. One switch becomes master and others become members. If master fails, another member becomes the master.

Election process to choose the master

1. User priority: SW1(config)#switch 1 priority <1-15>
2. Hardware/Software priority (for example IP Service vs IP Base)
3. Default configuration: The switch which has already the configuration
4. Longest uptime
5. Lowest MAC address

Verification

SW1#show switch
SW1#show switch stack-ports
SW1#show switch stack-ring speed
SW3#show cdp neighbors

SW3 is another switch which is connected to our stack

Notes

• When a switch is added to a stack, the SDM template on the master will override the SDM template on the new switch