Wednesday, April 18, 2012

Spanning Tree - STP - Love it & Hate it

So every network engineer I talk to seems to have the same relationship with Spanning Tree. The love it when it works and hate it when it works. Spanning tree is used to prevent loops in the network. Link to Read more, as I will not explain the whole thing here.

When I posted the following picture on twitter, it generated some chatter.
What??? That could be a problem. Especially since those are both loops. Now if these are both cabled to two ports on the same switch and STP is on, one of the ports will go into blocking mode. If STP is off we have a loop and then we have lost all control. Which should be a lesson for these younglings in which they have to learn. This lesson is best taught in multi-vendor networks as there are different default settings for STP between vendors.

This picture was taken in the Computer Science Networking Lab. Two things about the lab from my prospective:

  1. I give them a single feed to the rest of the network, one vlan and a small dhcp scope in which to make things easy on them (There is more address space for them to use outside the scope, use at their own risk) 
  2.  Any problems I have in which the rest of my network is threatened, shutdown their feed.

Now back to the photo, is STP in play, is there a loop? 

No loop! except in the cable. Each jack in this case represents a different rack, so the wall plates are "stations" to work at. This keeps the younglings from stacking onto of each other doing labs.

This is the way they chose to bring production network to the other side of the room to the lab rack. This was done by our Cyber-Defense team, last year's team can be found here.  

No worries on this one but it is right inside the door and makes me double-take every time I see it.

Thanks to all that responded to the post on twitter. 

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