Most business networks today use switches to connect computers, printers, and servers within a building or campus. Switches are the fundamental part of most networks; they enable several users to send information over a network. Users can send information at the same time without slowing each other down. Just as routers allow different networks to communicate with each other, switches allow different nodes of a network to communicate directly with each other. A node is a network connection point, typically a computer. Switches allow the nodes to communicate in a smooth and efficient manner. Effective switching is essential to handle the growing network traffic coming from video and other bandwidth-intensive applications, more user devices, and more packets headed to servers and storage in the cloud. Before you begin a cloud service for an organisation, don’t be surprised to see that you are pushing out 50 per cent more traffic to the Internet than you had before, so your network infrastructure must be ready for that, which means properly implemented switching. Cisco has approximately 70% of the market in routing and switching. Cisco provides normal Layer 2 LAN switches to high end Catalyst switches; if you want to know more about varieties of Cisco switches, visit this website; http://www.cisco.com/en/US/products/hw/switches/index.html#~all-prod. Network engineers choose different type of switches to meet their requirements.

CCNA Training – Resources (Intense)

Now the question is: How has Cisco built up so much trust in the market? Why have most organisations given preference to Cisco over the decades? In-depth answers can be found by learning about Cisco technology, so I am going to explore the technology behind Cisco switches in this article.

First of all, we will discuss the use of Cisco switch’s hierarchical architecture, which is shown in Fig. 1.

The main purpose of the access layer is to provide a means of connecting devices to the network and controlling which devices are allowed to communicate on the network.

The distribution layer aggregates the data received from the access layer switches before it is transmitted to the core layer for routing to its final destination. The distribution layer controls the flow of network traffic using policies and delineates broadcast domains by performing routing functions between VLANs defined at the access layer.

The core layer is critical for interconnectivity between distribution layer devices. It can also connect to Internet resources.

There are many benefits associated with hierarchical network designs:

  • Scalability—The modularity of the design allows you to replicate design elements as the network grows. Because each instance of the module is consistent, expansion is easy to plan and implement.
  • Redundancy—Access layer switches are connected to two different distribution layer switches. Distribution layer switches are connected to two or more core layer switches to ensure path availability if a core switch fails.
  • Performance—Data is sent through aggregated switch port links from the access layer to the distribution layer at near wire speed in most cases.
  • Security—You have the flexibility to use more advanced policies at the distribution layer. You may apply access control policies that define which communication protocols are deployed on your network and where they are permitted to go.
  • Manageability—Each layer of the hierarchical design performs specific functions that are consistent throughout that layer. Consistency between the switches in each layer allows for rapid recovery and simplified troubleshooting.
  • Maintainability—Because hierarchical networks are modular in nature and scale very easily, they are easy to maintain.

How does a Cisco switch work?

The switch maintains a MAC table that keeps a record of the host’s MAC address for making forwarding and filtering decisions. When a switch is powered on, it has nothing in its MAC table, but when the hosts start communicating, the switch places the source MAC address of each frame in the table along with the port that the frame’s source address corresponds to.

Let me explain with an example that shows how a MAC table is populated:

  1. Assume Host A sends a frame to Host B. Host A’s MAC address is 0000.0b01.0001; Host B’s MAC address is 0000.0b01.0001.
  2. The switch receives the frame on the Fa0/2 interface and places the source address is the MAC address table.
  3. Since the destination address is not in the MAC database, the frame is forwarded to all interfaces, except the source port.
  4. Host B receives the frame and responds to Host A. The switch receives this frame on Interface Fa0/4 and places the source hardware address in the MAC database.
  5. Host A and Host B can now make a point-to-point connection and only the two devices will receive the frames. Hosts C and D will not see the frames, nor are their MAC addresses found in the database because they haven’t yet sent a frame to the switch.

If Host A and Host B don’t communicate to the switch again within a certain amount of time (approx. 300 sec,), the switch will flush their entries from the database to keep it as current as possible.

We have talked about switch-to-host communication; what if we connected a switch to another switch? You may be wondering what is the purpose of connecting a switch to another switch. The answer is simple: A switch can have max 24 or 48 (depends on the model), hosts require 125. Now take three switches and connect the hosts as per requirement to all switches and also connect the switches with each other (as shown in Fig. 2). Ports connected to hosts should be access and ports connected to switches should be trunk.

To make a port to access/trunk, we have to write some commands in config mode:

(config)# int f0/1

(config-if)# switchport mode access / trunk

Switch-to-switch connectivity can make more hosts reachable but, as shown in Fig. 2, a loop is generated. By default, a switch will forward a broadcast or multicast to all ports, excluding the port the broadcast/multicast was sent from. If a host connected to SW-2 broadcasts a frame, first that frame is broadcast to SW-2, then to SW-1/SW-3, then again to SW-2 and so on. This can cause a problem because a highly destructive broadcast storm
can develop within seconds. Broadcast storms occur when broadcasts are endlessly switched through the loop, choking off all other traffic. So, STP (spanning tree protocol) is used to avoid loops in switching. Let’s take a brief overview of how STP works and the functions of STP.

STP-enabled switches communicate to form a topology of the entire switching network, and they shut down (or block) a port if a loop exists. The blocked port can be reactivated if another link on the switching network goes down, thus preserving fault tolerance. Once all switches agree on the topology database, the switches are considered to be converged. STP switches send BPDU’s (bridge protocol data units)
to each other to form their topology databases. BPDU’s are sent out all ports every two seconds and are forwarded to a specific MAC multicast address: 0180.c200.0000.

STP operations

  1. Selecting a root bridge—A switch with lowest bridge I becomes the root bridge
  2. Determine the least cost paths to the root bridge—Lowest cost to the root bridge( switch with the least cost to root)
  3. Lowest sender bridge ID—Serves as a tie-breaker if multiple upstream switches have equal cost to root
  4. Lowest sender port ID—Serves as a tie-breaker if a switch has multiple (non-ether-channel) links to a single upstream switch, where:
  • Bridge ID = priority (16 bits) + ID [MAC address] (48 bits); the default bridge priority is 32768, and
  • Port ID = priority (4 bits) + ID [Interface number] (12 bits); the default port priority is 128.

STP is a big concept, so we will explain it in detail in another article. For now, switch-to-switch connectivity is not an issue but, if you are working with a large number of switches, you need to do all VLAN configurations to each switch. For example, you need to configure VLANs on all switches. VTP (VLAN trunking protocol) is the solution; just make a switch a VTP server and remaining as VTP client/transparent with a config mode command.

(config)# vtp mode server/client/transparent

Now it’s time to tell you little more about VTP; (VLAN Trunking Protocol) is a Cisco Layer 2 messaging protocol that manages the addition, deletion, and renaming of VLANs on a network-wide basis. VTP reduces administration in a switched network. When you configure a new VLAN on one VTP server, the VLAN is distributed through all switches in the domain. This reduces the need to configure the same VLAN everywhere. VTP is a Cisco-proprietary protocol that is available on most of the Cisco Catalyst family switches.

VTP ensures that all switches in the VTP domain are aware of all VLANs. There are occasions, however, when VTP can create unnecessary traffic. All unknown unicasts and broadcasts in a VLAN are flooded over the entire VLAN. All switches in the network receive all broadcasts, even in situations where few users are connected in that VLAN. VTP pruning is a feature used to eliminate (or prune) this unnecessary traffic.

By default, all Cisco Catalyst switches are configured to be VTP servers. This is suitable for small-scale networks where the size of the VLAN information is small and easily stored in all switches (in NVRAM). In a large network, a judgment call must be made at some point when the NVRAM storage needed is wasted, because it is duplicated on every switch. At this point, the network administrator should choose a few well-equipped switches and keep them as VTP servers. Everything else participating in VTP can be turned into a client. The number of VTP servers should be chosen so as to provide the degree of redundancy desired in the network.

Modes of Operation

Server

In VTP server mode, you can create, modify, and delete VLANs and specify other configuration parameters (such as VTP version and VTP pruning) for the entire VTP domain. VTP servers advertise their VLAN configuration to other switches in the same VTP domain and synchronize their VLAN configuration with other switches based on advertisements received over trunk links. VTP server is the default mode.

Transparent

VTP transparent switches do not participate in VTP. A VTP-transparent switch does not advertise its VLAN configuration and does not synchronize its VLAN configuration based on received advertisements. However, in VTP version 2, transparent switches do forward VTP advertisements out over their trunk ports.

Client

VTP clients behave the same way as VTP servers, but you cannot create, change, or delete VLANs on a VTP client.

Conclusion & Tips for Exams:

I hope with this article along with my previous article on LAN switching will help you to get started in switching technology. Cisco switches are great devices to work with; I personally recommend that you have hands-on experience with Cisco Catalyst switches. If you don’t have any access to them, you can use packet tracer or other simulation software to get familiar with. Hope you enjoyed this article and that it was informative for you. If you have any questions about it, use the comment box below to get in touch with me.

References:

  1. Cisco LAN Switching Configuration Handbook, Second Edition, written by Steve McCurry, David Jansen, David Hucaby, is a quick and portable reference guide to the most commonly used features that can be configured on Cisco switches.
  2. Cisco Switching Black Book: A Practical In-Depth Guide to Configuring, Operating and Managing Cisco LAN Switches written by Sean Odem & Hanson Nottingham.
  3. Cisco LAN Switching Configuration Handbook by Stephen McQuerry, David Jansen & David Hucaby, is a quick and portable reference guide to the most commonly used features that can be configured on Cisco® Catalyst® switches.
  4. Guide to Cisco Certified Network Associate certification by Todd Lamlee, Sybex Press.
  5. Guide to Cisco Certified Network Associate by Richard Deal.
  6. Cisco Certified Internetwork Expert by Wendell Odom and others, Ciscopress.com
  7. Cisco Certified Internetwork Expert Quick reference by Brad Ellis, Ciscopress.com