let’s look at some key technologies within LAN switching.
- – 802.1d Spanning-Tree Protocol
- – Multicasting
The Need for Spanning Tree
Specifically we’ll look at the Spanning Tree Protocol, and also some multicasting controls that we have in our network.As we build out large networks, one of the problems we have at Layer 2 in the OSI model, is if we’re just making forwarding decisions at Layer 2, that means that we cannot have any Physical Layer loops in our network.
So if we have a simple network, as we see in the diagram here, what these switches are going to do is that anytime they have any multicast, broadcast traffic, or any unknown traffic, that’s going to create storms of traffic that are going to get looped endlessly through our network.So in order to prevent that situation we need to cut out any of the loops.
802.1d Spanning-Tree Protocol (STP)
Spanning Tree Protocol, or STP. This is actually an industry standard that’s defined by the IEEE standards committee, it’s known as the 802.1d Spanning Tree Protocol.This allows us to have physical redundancy in the network, but it logically disconnects those loops.
It’s important to understand that we logically disconnect the loops because that allows us to dynamically re-establish a connection if we need to, in the event of a failure within our network.The way that the switches do this, and actually bridges can do this as well, is that they simply communicate by way of a protocol, back and forth. The basically exchange these little hello messages.
If they stop hearing a given communication from a certain device on the network, we know that a network device has failed. And when a network failure occurs we have to re-establish a link in order to maintain that redundancy.technically, these little exchanges are known as BPDUs or Bridge Protocol Data Units.
Now, Spanning Tree protocol works just fine, but one of the issues with Spanning Tree is that it can take anywhere from half a minute to a full minute in order for the network to fully converge, or in order for all devices to know the status of the network.So in order to improve on this, there are some refinements that Cisco has introduced, such as PortFast and UplinkFast, and this allows your Spanning Tree protocol to converge even faster.
Now, another issue that we have in Layer 2 networks, or switched networks, is control of our multicast traffic. There’s a lot of new applications that are emerging today such as video based applications, desktop conferencing, and so on, that take advantage of multicasting
But without special controls in the network, multicasting is going to quickly congest our network. Okay, so what we need is to add intelligent multicasting in the network.
Now, again, let’s understand that there are a few fundamental ways that we have in order to achieve multipoint communications, because effectively, that’s what we’re trying to do with our video based applications or any of our multimedia type applications that use this mechanism.
One way is to broadcast our traffic. And what that does is it effectively sends our messages everywhere. The problem, and the obvious down side there is that not everybody necessarily needs to hear these communications.So while it will get the job done, it’s not the most efficient way to get the job done. So the better way to do this is by way of multicasting.
And that is, the applications will use a special group address to communicate to only those stations or group of stations that need to receive these transmissions.And that’s what we mean by multipint communications. That’s going to be the more effective way to do that.