Switch stacking Vs Chassis Aggregation
A stackable switch is a network switch that is fully functional operating standalone but which can also be set up to operate together with one or more other network switches, with this group of switches showing the characteristics of a single switch but having the port capacity of the sum of the combined switches.
The term “stack” refers to the group of switches that have been set up in this way.
The common characteristic of a stack acting as a single switch is that there is a single IP address for remote administration of the stack as a whole, not an IP address for the administration of each unit in the stack.
Stackable switches are customarily Ethernet, rack-mounted, managed switches of 1–2 rack unit (RU) in size, with a fixed set of data ports on the front. Some models have slots for optional slide-in modules to add ports or features to the base stackable unit. The most common configurations are 24-port and 48-port models.
Benefits of Switch Stacking
- Simplified network administration: Whether a stackable switch operates alone or “stacked” with other units, there is always just a single management interface for the network administrator to deal with. This simplifies the setup and operation of the network.
- Scalability: A small network can be formed around a single stackable unit, and then the network can grow with additional units over time if and when needed, with little added management complexity.
- Deployment flexibility: Stackable switches can operate together with other stackable switches or can operate independently. Units one day can be combined as a stack in a single site, and later can be run in different locations as independent switches.
- Resilient connections: In some vendor architectures, active connections can be spread across multiple units so that should one unit in a stack be removed or fail, data will continue to flow through other units that remain functional.
- Improving backplane: A series of switches, when stacked together, improves the backplane of the switches in stack.
Drawbacks of Switch Stacking
- For locations needing numerous ports, a modular chassis may cost less. With stackable switching, each unit in a stack has its own enclosure and at minimum a single power supply. With modular switching, there is one enclosure and one set of power supplies.
- High-end modular switches have high-resiliency / high-redundancy features not available in all stackable architectures.
Chassis aggregation is a Cisco technology to make two switches operate as a single logical switch. It is similar to stacking but meant for chassis switches like the 6500 and 6800 series switches. It is often used in the core layer and sometimes in the distribution layer.
Let’s start by taking a closer look at a chassis
These chassis switches have one or two supervisors, this is the brain of the switch which has the CPU, memory, flash, etc. The supervisor is responsible for packet forwarding. In case one supervisor goes down, the second one will take over. At the bottom of the switch, we have two power supplies with redundant feeds. In the remaining slots, we can insert line cards for interfaces or some other modules. Here’s an illustration of the chassis:
Above we see a chassis with some line cards, two supervisors, and two power supplies. Even though we have a lot of redundancy in a single chassis, it is a good idea to add a second chassis for extra redundancy. To connect the two switches to each other, we use regular Ethernet interfaces, and in case a single line card fails, multiple line cards:
To create a single logical link out of these physical links, we can create an EtherChannel:
We now have a redundant core layer, but still two physical switches without switch aggregation. Let’s look at the “bigger picture” and connect some distribution layer switches to our two core switches:
Without switch aggregation, we do have some redundant uplinks from the distribution layer to the core layer. If you use VLANs on these interfaces, spanning-tree will block some of the links to create a loop-free topology.
Unlike StackWise, switch aggregation can be configured with regular Ethernet interfaces and EtherChannel. When we enable switch aggregation, our two switches on the core layer become one logical switch:
The two core switches will act as a single switch to the outside world when it comes to control plane protocols like ARP, spanning-tree, EtherChannel and routing protocols. Information like MAC addresses in the MAC address table are synchronized between the two switches.
From the perspective of the distribution layer switches, they are connected to a single core switch. We can create EtherChannels on the distribution layer switches even though the physical links connect to two different physical core switches. This is called MEC (Multichassis EtherChannel). All links will be in forwarding mode, there is no loop unless I add a link in between the two distribution layer switches.
We can take this one step further by using StackWise on the distribution layer switches, creating a single logical distribution layer switch. Our picture then looks like this:
We now have a single EtherChannel. Even though we four physical switches, our logical topology only has two switches. All links will be in forwarding and nothing will be blocked by spanning-tree.
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