Introduction to Switch Virtualization

In a previous blog we discussed about campus network design and how we use different layers and “switch blocks” to create a hierarchical design that has redundant links.

We also learned in the spanning-tree how spanning-tree creates a loop-free topology by blocking some of the redundant links. The “thing” with spanning-tree is that we have a loop-free topology, we have redundancy but we can’t use all the redundant links for forwarding. Here’s an illustration to visualize this:

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Cisco Stackwise

Cisco’s access layer switches used to be all separate physical switches where we use Ethernet cables for connectivity between the switches. Cisco Stackwise changed this, it allows us to turn multiple physical switches into a single logical switch.

Switches that support Stackwise use a special stacking cable to connect the switches to each other. Each switch has two stacking connectors that are used to “daisy-chain” (loop) the switches together. Each switch is connected to the one below it and the bottom switch will be connected to the one on top.

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Cisco 6500 VSS Configuration Example

The Virtual Switching System (VSS) allows two Cisco Catalyst 6500 or 4500 chassis to bond together so that is seen as a single virtual swich to the rest of the network. Other devices will see the VSS configured 6500 as a single device which means it’s possible to use multi chassis etherchannel and protocols like spanning-tree will only see a single switch.

Some other features are NSF (Non Stop Forwarding) / SSO (Stateful Switchover) which means that when a single chassis fails the other one will take over without any downtime since the routing table / CEF table etc. are stored in both chassis’ supervisors.

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