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.

The Stackwise cable is like an extension of the switching fabric of the switches. When an Ethernet frame has to be moved from one physical switch to another, the Stackwise “loop” is used. The advantage of using a cabled loop is that you can remove one switch from the stack, the loop will be broken but the stack will keep working.

One switch in the stack becomes the master that does all “management tasks” for the stack. All other switches are members. If the master fails, another member will become the new master. To select a master, Stackwise uses an election process that checks for the following criteria (in order of importance):

  1. User priority: we can configure a priority to decide which switch becomes the master.
  2. Hardware / software priority: The switch with the most extensive feature set has a higher priority than another switch (for example: IP Services vs IP base).
  3. Default Configuration: A switch that already has a configuration will take precedence over switches with no configuration.
  4. Uptime: The switch with the longest uptime.
  5. MAC address: The switch with the lowest MAC address.

It makes sense to choose the master ourselves so normally we use user priority to configure the master.

Once the stack has been created, the configuration of the switches is the same as if it were one single switch…they share the same management IP address, hostname, etc.

The Cisco Catalyst 4500 and 6500 also support something similar to Stackwise, it’s called VSS (Virtual Switching System) and I covered it in this lesson.

Here’s a picture of the stacking connectors, this is the rear of a Cisco Catalyst 3750 switch:

Cisco 3750 Rear

You can see the two connectors on the left side…Stack1 and Stack 2. Here’s what the Stackwise cable looks like:

Cisco 3750 Stackwise Cable
Cisco 3750 Stackwise Connector

Now you have an idea what Stackwise is about, let’s take a look at the configuration.

Configuration

I will use two Cisco Catalyst 3750 switches to configure Stackwise. Make sure they are all powered off and then cable them like this:

Cisco 3750 Stack Cables

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 stacki


First I will start the switch that I want to become the Master, i’ll use the one on top for this. Once you start it with the cables connected, you’ll see some Stackwise information during boot:

SM: Detected stack cables at PORT1 PORT2

Waiting for Stack Master Election...
SM: Waiting for other switches in stack to boot...
##################################################
SM: All possible switches in stack are booted up

Election Complete
Switch 1 booting as Master
Waiting for Port download...Complete

%STACKMGR-4-SWITCH_ADDED: Switch 1 has been ADDED to the stack
%STACKMGR-5-SWITCH_READY: Switch 1 is READY
%STACKMGR-4-STACK_LINK_CHANGE: Stack Port 1 Switch 1 has changed to state DOWN
%STACKMGR-4-STACK_LINK_CHANGE: Stack Port 2 Switch 1 has changed to state DOWN
%STACKMGR-5-MASTER_READY: Master Switch 1 is READY

Our switch detected the stacking cables and does an election to see who will become the master. By default each switch will think that it’s switch number 1 and the master. I’ll change the user priority to make sure that this switch will always be selected as the master:

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Forum Replies

  1. Hello Kesav!

    I’ll try to answer your questions one by one below:

    a) Why two virtual links need to be created? Is it that one virtual link is used for data traffic and one for management traffic? What is the benefit / use case of having two virtual links?

    Actually, Rene has created only one virtual link and one port channel. It just happens that he named the port channel on SW1-VSS Po1 and the port channel on SW2-VSS Po2. They’re just two ends of the same set of physical links that form the port channel. The same occurs for the Virtual link. The end of the

    ... Continue reading in our forum

  2. Hi,

    Rene If a switch is connected to VSS enable switch ( redundant connectivity switch 1 and switch 2 ) and RSTP is running in this case
    who will be the root bridge ?

    Thanks

  3. Hi Abhishek,

    If your VSL fails then the standby switch can’t tell if the active switch is still there or not. If the standby switch goes active while the active switch is still there, both will be active, forwarding packets and you’ll run into issues. This is called “dual active”.

    Dual-active detection is configured outside of the VSL link. There’s enhanced PAgp, BFD and Dual-Active Fast Hello Packets to use. You can use one or all three at the same time.

    Cisco has a pretty good document for this:

    http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst6500/i

    ... Continue reading in our forum

  4. Hello Helen

    The use of two or more links for the Virtual Switch Link (VSL) is only a recommendation by Cisco. It is not manditory. However, this is not recommended as VSS is designed to be a high availability system and by having only one link, this high availability is sacrificed. It should be implemented fully realising this. Ideally, you should purchase an additional module to implemente multiple links.

    I hope this has been helpful!

    Laz

  5. Hello Justin

    VSS requires that VSLs are configured on 10Gbps ports only. Not only this, but these ports must either be on the supervisor itself or on one of several switching modules. You can find more info about this at the following Cisco documentation:

    Look at the section titled “VSL Hardware Requirements”.

    Now the reason that only specific ports can be used is because the requirements of the VSL are very specific. A VSL link has the following characteristics:

    • The VSL gives control traffic
    ... Continue reading in our forum

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