{"id":73,"date":"2013-04-25T21:17:02","date_gmt":"2013-04-25T21:17:02","guid":{"rendered":"https:\/\/learncisco.net\/index.php\/performance-with-spanning-tree\/"},"modified":"2023-01-11T08:59:35","modified_gmt":"2023-01-11T01:59:35","slug":"performance-with-spanning-tree","status":"publish","type":"page","link":"https:\/\/www.learncisco.net\/courses\/icnd-1\/medium-size-switched-network-construction\/performance-with-spanning-tree.html","title":{"rendered":"Performance with Spanning-Tree"},"content":{"rendered":"

We will first identify the problems associated with having redundant connections means extra links between switches. It creates some associated problems which we will go in details in the subsequent lessons and how through the use of spanning tree protocol, we will create a loop free environment, which will help us avoid all these associated problems in a switch environment that have multiple links. Finally, we will learn how to make spanning tree protocol more efficient and robust through the use of our introduction of rapid spanning tree protocol as far as through the use of configuring root switch and backup root switch in the switching environment.<\/p>\n

Interconnecting Technologies<\/h2>\n

When connecting our switching networks together, we can deploy different Ethernet technologies to provide the necessary bandwidth connectivity. So normally, for the end user connection we would use a fast Ethernet connection to the access switch. And the access switch connection to the central switch, called a distributions switch, we would choose to use gigabit Ethernet connection because many users’ fast Ethernet connection will be aggregating through this uplink. So we better ensure that we have ample bandwidth to accommodate this aggregation effect.<\/p>\n

Then we might use a 10-gig Ethernet connection for the switch to switch backbone connections as it is going to aggregate even more traffic in the backbone. We can also use etherchannel, which allows us to combine a few low-speed links into a logical high-speed link, which will allow us to increase the bandwidth capability through load balancing internally within the etherchannel link as well as providing redundancy because the etherchannel will load balance across available links. So, should there be an outage on one of the channelized link, the load balance mechanism simply load balance with the existing links.\u00a0To determine how much bandwidth we need for each link, we simply look at total amount of traffic that will flow through that link. That will give us an estimate of how much bandwidth we need.<\/p>\n

Redundant Topology<\/h2>\n

When connecting multiple switches together, we would like to use multiple links to connect the multiple switches together, thereby creating a redundant infrastructure, which eliminates a single point of failure. But this redundant topology causes a host of feedback loop-associated problems for the switch, ranging from broadcast storm to multiple framed copies as well as MAC address database instability problems.<\/p>\n

Here’s a quick review of how a switch handles broadcast frames.<\/p>\n

\"Broadcast<\/p>\n

When a switch receives a broadcast frame with a destination MAC address of FFFF.FFFF.FFFF, it will not find the destination MAC address in the MAC address database. As such, the broadcast frame is treated like an unknown MAC address, which are then flooded to all ports except the originating ports.<\/p>\n

Broadcast Storms<\/h2>\n

\"Broadcast<\/p>\n

So when host X sends a broadcast into the switch A, switch A floods the broadcast to all ports except the originating port, and the broadcast gets forwarded to switch B. Switch B would do the same and will then send the broadcast back to switch A. Switch A, upon receiving the broadcast, will continue to propagate the broadcast back to switch B resulting in an endless broadcast loop. There are many of these broadcast loops happening, and we call these broadcast storms.<\/p>\n

Multiple Frame Copies<\/h2>\n

In this example, host X sends a data frame to router Y. The data frame gets sent directly to router Y once.<\/p>\n

\"Multiple<\/p>\n

But then, switch A, upon receiving the data frame from host X, floods it to all the ports, as router Y is not a known MAC address in its MAC address database. And switch B upon receiving the data frame, floods to all the ports too. As a result, switch Y will receive an extra copy of the same data frame from the switching network. The receiver, router Y in this example, has to perform in-depth packet checking to ensure that it is not processing the same data frame.<\/p>\n

MAC Database Instability<\/h2>\n

In this example, host X sends a unicast frame to router Y and the router Y MAC address has not been learned by either switches.<\/p>\n

\"MAC<\/p>\n

So, switch A and switch B will learn the MAC address of host X is associated with port 1. And because the router Y MAC address is not learned yet, the switch will flood the frame to all ports. In this case, you’ll flood through port 2 except port 1. Switch A and switch B will then receive this data frame via port 2, and they will think that MAC address of host X is now associated with port 2. Now in short, the reason why we have MAC address database instability is because the switch does not know how to distinguish between a direct data frame versus an indirect data frames that came through another search. So, that’s why it gets confused.<\/p>\n

Loop Resolution with STP<\/h2>\n

Network administrator likes the idea of redundant network topology, but switches on the other hand, does not like the idea of redundant network topology because it causes a lot of feedback loop problems for the switches. So, the solution is to come up with a loop-free redundant network topology by making sure that only one link is active between switches. The extra link is temporarily disabled by temporarily blocking one of the ports so that there is no loop between the switches. The algorithm that was developed to ensure that the switching environment does not have any loops is achieved using IEEE 802.1d spanning tree protocol. PVST+ implementation will only be explained in the later slides.<\/p>\n

So how does spanning tree protocol ensure that we have a loop-free switching environment? Well, the basic idea of spanning tree protocol is very simple. We simply turn a multilink switched network with looping problems into a hub and spoke switching network using spanning tree protocol.<\/p>\n

How does STP works<\/h2>\n

The basic idea of spanning tree protocol is to transform a multilink switch network into a hub and spoke switch network whereby there is no loop between switches. To achieve this, the spanning tree protocol basically goes through these few steps:<\/p>\n