In a topology like this, we had to configure spanning tree, HSRP link aggregation and other technologies
to try and optimize the way that the topology works.
As an example, we made a switch 1 route for some VLANs, and then we made a switch 2 route
for other VLANs.
So we managed to optimize Layer 2 forwarding by optimizing spanning tree, but then to optimize the routing
in this topology. We had to configure HSRP and then configure switch 1 as the HSRP primary for the
same VLANs as the as the route switch.
In other words, if switch 1 is the spanning tree route for VLAN 10, then we had to make it to the
HSRP primary router for VLAN 10.
So in other words, we have to optimize both spanning tree as well as HSRP to make sure that they are
aligned.
In other words, if switch 1 is the spanning tree route for VLAN 10, we don't want to make switch 2
the HSRP primary router for VLAN 10. We want to match them up to optimize the forwarding.
So there's a lot of additional work that you need to do here.
You need to configure spanning tree.
You need to configure HSRP, you need to configure link aggregation, you've got to match it all up.
It's not very efficient.
Is there a better way of doing this?
And fortunately the answer is yes, there is a better way of doing this.
I've done a search in Google for Cisco stacking images and you'll find many images such as the following
showing you examples of Cisco switches being stacked.
Now, there are different technologies for doing this.
In other words, there are different stacking technologies, one of the oldest is stackwise, which
was used on the 3750 switches.
But again, if you do a search in Google where you have a look at some of the Cisco documentation,
you'll see a lot of examples of different stacking technologies that can be used to stack Cisco switches.
So what's the advantage of stacking switches in this way?
In brief, when you stack switches, they appear to be a single switch to the rest of the network,
you configure it as a single switch.
They act as a single switch.
Protocols such as spanning tree and CDP, see that switch as a single switch.
So, again, Cisco has various technologies that you can use to stack switches or aggregate Chassis.
So we have these terms, switch stacking and Chassis aggregation, separate physical switches, work
and cooperate together to act and appear as a single switch rather than multiple discrete switches.
As an analogy, it's as if the switches are acting as blades in a chassis-based switch.
Switch stacking is often used at the access layer and Chessie aggregation is often used at the distribution
and core layers of a network.
So if you've got multiple layer access switches or multiple distribution or Core layer switches, rather than
you having to configure each switch individually and configure protocols such as spanning tree, VTP,
CDP and so forth, you configure multiple physical switches as they're one switch.
They have a Mac address table.
They run protocols as if they were a single switch and share a Mac address table.
So when you stack switches at the access layer, you're literally creating a stack of switches joined
together via special cables.
So a stack of physical switches in a wiring closet as an example can act as a single switch.
You would manage the stack with a single management IP address, you would telnet or SSH to one switch,
the one that has the management IP address and not have to telnet or SSH to multiple switches.
There's one configuration file which is included across all the physical switches spanning tree, CDP,
VTP or running on one switch, not on multiple switches.
The ports on each physical switch appear to be part of the same logical switch.
In other words, you may have four physical switches, each having their own physical ports.
But logically you've got one switch with all of those ports.
There's one Mac address table that references all the ports on all physical switches.
There are some additional advantages, but the moral of the story is you have a single virtual switch
that you managing rather than four separate, discrete switches.
Now, to connect the switches together, you use special hardware ports called stacking ports.
Again, there are different
Cisco Technologies, we've got, as an example, Cisco FlexStack and FlexStackPlus stacking technology.
You would, as an example, insert a stacking module in each switch and then connect them with a stacking
cable.
Different stacking technologies work on different switches, so FlexStack and FlexStackPlus all supported
by 2960 switches such as the 2960S, 2960R, 2960X and 2960XR switch families.
3750 switches, which are all the switches support Stackwise and what are the stacking
cables do is they form a ring between the switches.
In other words, the switches are connected in a series with a last switch connected back to the first
switch as shown in this topology, by using full duplex on each link, the stacking modules and cables
create two paths to forward data between the physical switches in the stack.
The switches use these connections to communicate between the switches in the stack, as well as to
forward frames and perform other overhead functions.