Stp

Hi Everyone,

First topic of my ICND2 studies...STP! and it's kicking my ass. I'm going to keep hitting the material and hopefully I'll have it down no problem soon enough.

One question at the moment though. What exactly is the root bridge?

I understand that all switches think they are the root until they go through the election process and eventually the lowest BID is established. But what is the root exactly?

Would this be a root switch that connects in to the rest of the network, ie a router? I've read that you want the best switch in the network to be the root as most traffic will pass through but I'm just a bit confused.

thanks

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Comments

Todd Burrell

The main thing I understand about the root is that it will never have a port in a blocking state. So it should always have all ports "available". And it should also be the main switch that will carry the most traffic - from what I have determined with my limited studying. I also understand that you normally want to control which one is the root and not let it be determined by the MAC address - via the priority...

networker050184

The root is the switch from which the STP tree is built. So for example you have a three switch topology as below.

     A
   /    \
  /      \
 B ------ C

So imagine switch A is your beefier switch with an uplink out to the rest of the network, but switch C becomes the root. Your STP tree would be like below.

   A
     \
      \
B ---- C

So all traffic from B to the rest of the network would have to traverse the path B - C - A rather than B - A whch would be the optimal path. So what do you do? You make A the root and all traffic flows B - A or C - A. The less preferable path is then only used in the event of a failure.

This is a quite simplified diagram, but you can see how in a larger switched network you can end up with some undesired traffic paths if your do not properly place the root. Imagine switch A is a 6500 and switch C is an old 2900. You don't want all your traffic running through that little guy with a single power supply and a lot less connectivity.

alliasneo

This STP here though. It has blocked one port and I have read that no traffic will be forwarded out of the blocked port but it will accept frames and BPDU's. Surely if it accepts frames on this port then it could forward them back out of the other port and the loop would still exist?

urghh so confused!

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MrRyte

http://www.routeralley.com/ra/docs/stp.pdf

See if that link helps to explain things....

pham0329

The root is arbitrary - it's a point of reference for the STP algorithm. You can't calculate a loop free path if you don't know where it is that you're going to. It's like me going up to you, asking "what's the best way to get there?" without telling you where "there" is.

@alliasneo, I don't believe it will accept anything other than BPDUs. A port in blocking state will not record the source mac address of the frames it received in its mac address table.

advanex1

pham0329 wrote: »

The root is arbitrary - it's a point of reference for the STP algorithm. You can't calculate a loop free path if you don't know where it is that you're going to. It's like me going up to you, asking "what's the best way to get there?" without telling you where "there" is.

@alliasneo, I don't believe it will accept anything other than BPDUs. A port in blocking state will not record the source mac address of the frames it received in its mac address table.

Pham is correct, a port in blocking state will accept BPDU's and nothing else. In your studies you should see why it only accepts BPDU's and that's to be able to monitor if there is a change within the STP topology.

sthompson86

MrRyte wrote: »

http://www.routeralley.com/ra/docs/stp.pdf

See if that link helps to explain things....

Awesome site - Thanks for sharing.

ipSpace

Think of it as the King of the L2 topology. Everybody must have only one way to the King.

alliasneo

Thanks for the help everyone.

So ok I understand that every switch chooses one port to reach the root. How does this work in terms of the network structure? Do all frames go through the root? Or only frames that would be destined elsewhere in the network?

pham0329

No, not all frames have to go through the root. It will only go through the root if it has to.