Today we will discuss the advantages of two types of aggregation switches: Switch Stacking, or stacks of switches, and Chassis Aggregation, or aggregation chassis switch. This section 1.6 exam topics ICND2.

When designing the company network, you will need to provide for the placement of access switches Access Switches connect multiple computers, users, and switches switchboards Distribution Switches that connect to these switches access.
The diagram shows a model of the Cisco 3-rd level of OSI, where switch access is marked with the letter a, And switches the allocation by the letter D. On each floor of the building company you can have hundreds of devices, so you need to make a choice between the two methods of determining the location of the switches.

Each Access level switch has 24 ports, and if you need 100 ports, it is about 5 of these switches. So there are 2 ways: to increase the number of small switches or a single large switch with hundreds of ports. The topic CCNA are not considered the model of the switch 100 port, but you can get such a switch, it is quite possible. So, you have to make a decision that suits you – several small or one large switch.

Each option has its advantages. You can configure only 1 big switch instead of setting up several small, but there is a drawback – only one point of connection to the network. If such a large switch fails, it will collapse the whole network.
On the other hand, if you have five 24-port switches and one of them will break, you will agree that the chance of failure of one switch much more than the chance of simultaneous failure of all five devices, so 4 of the remaining switch will continue to ensure the existence of the network. The disadvantage of this solution is the need to control five different switches.

Our diagram shows 4 access switch connected to two distribution switches. According to 3 level of the OSI model and the requirements of the Cisco network architecture, each of these 4 switches should be connected to both distribution switches. When the Protocol is STP one of the 2 ports of each Access switch connected to Distribution-switch will be blocked. Technically you can’t use the full bandwidth of the switch because one of the two communication lines are always disabled.

Usually all 4 switch are located on the same floor in the common rack – shown in the photo 8 of installed switches. In total, the rack has 192 port. Thus firstly, you must manually configure the IP addresses for each of those switches, and second, to configure the VLAN everywhere and it’s a serious headache for the network administrator.

There is a thing that can ease your task – Switch Stack. In our case, this thing will try to combine all 8 switches into one logical switch.

While one of the switches will play the role of Master switch, or the host stack. The network administrator can connect to the switch and to perform all the necessary settings that will automatically apply to all switches in the stack. After that all 8 switches will work as one device.

Cisco uses various technologies for interconnection of switches in stacks, in this case, the external device is called “the FlexStack module”. On the rear panel of the switch has a port, which is inserted into the module.

FlexStack has two ports, where are inserted the connecting cable: the lower port of the first switch in the rack is connected with the second upper port, the lower port of the second or upper port of the third and so on until the eighth switch, the lower port which connects with the upper port of the first switch. In fact, we have formed the annular connection of the switches in one stack.

While one of the switches selects the leading (Master) and the rest slaves (Slave). After using all 4 modules FlexStack switch our scheme will act as 1 logical switch.

If you fail the Master switch A1, all the other switches in the stack will stop working. But if you break the switch A3, the three remaining switch will continue to operate as 1 logical switch.

In the original scheme we present 6 physical devices, but after the organization of the Switch Stack of them was only 3: 2 physical and 1 logical switch. The first option you would have to customize 6 different switches that are already quite troublesome, so you can imagine how time consuming the process of hand setting hundreds of switches. After the unification of the switches in the stack we came up with one logical access switch that is connected to each of the distribution switches D1 and D2 to four lines of communication, combined in an EtherChannel. Because we have 3 devices, to prevent the formation of loops of one EtherChannel traffic will be blocked by the STP Protocol.

So, the advantage of the stack of switches is the ability to control one logical switch instead of multiple physical devices, which facilitates the process of setting up the network.
There is another technology enterprises switches called Chassis Aggregation. The difference between these technologies lies in the fact that the Switch Stack need special external hardware module that is inserted into the switch.

In the second case, just the combining of multiple units on a single chassis, resulting in you a so-called chassis switch aggregation. In the picture you see the chassis for switches Cisco 6500 series. It combines the 4 NICs on 24 ports, so this unit has 96 ports.

If necessary, add more interface modules – network cards, and they will all be controlled by one module — the supervisor, which is the “brain” of the entire chassis. This chassis has two supervisor module in case one of them fails, which creates some redundancy, but increases network reliability. Usually these expensive chassis used at the kernel level of the system. This chassis has two power supplies, each of which can be powered from different power source, which also increases the reliability of the network in case of outage on one of the power substations.

Back to our original scheme, where between D1 and D2 is also a EtherChannel. Usually in the organization of such connections use Ethernet ports. When using chassis switches do not need any external modules to merge the switchs Ethernet ports can be used directly. You just connect the first interface module D1 with the same module D2, and D1 the second module with the second module D2, and everything works together, forming a single logical switch distribution Switch Distribution Layer.

If you look at the first version of the schema for aggregation 4 access switches and the distribution Suite, you need to use the program Multi–sa EtherChannel, which organizes the channels an EtherChannel to each switch of access. You can see that in this case the p2p connection is “point to point”, prevents the formation of loops of traffic, and in this case, use all available communication lines, and we have no reduction in throughput.

Chassis are usually used for high-performance Aggregation switches, but not for a less powerful switch access. The Cisco architecture allows simultaneous usage of both solutions and Aggregation Chassis and the Switch Stack.

In this case, a single shared logical switch distribution and a single logical switch access. In our scheme will be created 8 channels EtherChannel that will work as one line of communication, that is, as if we connected one distribution switch with a single switch access a single cable. The “ports” of both devices will be in the forwarding state and the network will operate with maximum performance using throughput of all 8 channels.

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