inaki

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When the spanning tree can be used. How spanning tree can effect to the network. I understand that spanning tree purposely for media access control bridges. So how can we relate with using of switch or router.

dougz

The Spanning Tree Protocol helps to prevent switching loops in a network.

For instance, let's suppose that you have 10 data distribution switches (your choice of brand/model). Five of them are in closet A and five are in closet B. Each set of five are daisy-chained together and closet A & B are connected via an Ethernet cable.

Suppose I come along and decide that closet A & B need to be connected not knowing of the existing connection. I make a second connection and now we have a switching loop.

Now suppose that I try and ping a host that is not accessible to our network. Since the host is not accessible, an ARP broadcast is created and sent throughout the network to look for the host.

With Spanning Tree, one of the four interfaces within our two connections gets shut down and our ARP broadcast gets propagated once and dies. Without Spanning Tree, our ARP broadcast cycles through the entire network then starts again thinking it found new hosts to query.

Now take this scenario and multiply it by 500 ARP broadcasts per minute that a moderately busy network would create and you have a recipe for disaster.

The moral of the story is that Spanning Tree is your friend and needs to be used wisely whenever possible.

Doug.

enthusiast_abdul

Spanning Tree Protocol, IEEE 802.1D prevents loops from being formed in a bridged LAN. Even if a loop exists due to cable connection, one of the ports connecting the cable is disabled by the STP. Loops are a potential disrupting force in a LAN. They lead to broadcast storms, frame duplication & frame misordering. STP achieves a loop free LAN topology by periodic exchange of BPDUs(Bridge Protocol Data Units), with a destination Multicast Address 01:80:c2:00:00:00. Each Bridge transmits a BPDU every hello time(2 sec).

firefox2501

Only thing that I have to add is that you should make sure that you configure one of your centrally located and preferably fasted connected switches (centered logically, not necessarily physically) as your root switch. Ideally, this would be one of your core switches or one of your primary distribution switches. In many case, you should setup a second switch to be the alternate root switch in the event that the primary goes down.

Unfortunately, I have seen networks with a GigE core switch, several 100mb/s switches, and one switch with a T-1 connection, and due to the way that STP root switches are determined, the switch hanging off of the T-1 was elected the root. This played havoc with the fail over on the rest of the network and was only corrected once the GigE core switch was configured to be the root.

Keep in mind, most consumer level switches do not support STP, and those that do do not let you configure the STP settings. Most enterprise level switches do let you configure these settings, and in many cases these settings are as important as the subnets and route table settings on the routers, so be sure to plan accordingly.

FYI, here is a link to a pretty good article called " Understanding Spanning-Tree Protocol" on the Cisco UniverCD site.
http://www.cisco.com/univercd/cc/td/...an2/stpapp.htm


Hope this helps