I can see transparently now!

[k0mbachi]

The wording of this question bothers me:

What is the main function the spanning-tree protocol performs?
A. Transparently reconfigures bridges and switches to avoid the creation of loops
B. provides routing information to the connected neighboring hosts
C. manages the entire topology information
D. none of the above

Answer: A

I have been at a client's site with some guy fiddling around near a rack where he accidently unplugs a component and initiates a spanning tree. With about 800 users on a flat network, of course the phone lights up like a Christmas tree. This is "transparent"? What is transparent about blocking all ports while the switches relearn their status?

Debateably interesting aside with embedded lesson: The guy who unplugged the component saw fit to blame the two outsiders in the room, performing a network performance evaluation and audit, who just happened to be sniffing the network and were able to replay exactly what happened. I just got word that he is no longer employed at that site.

 

[wybnormal]

heheheh.. I like the part about replaying back the sniffer trace

The lesson here is the network is architected poorly. In order for the switches to go into learn mode, there had to be redudent links between them. Spanning tree is not intended make failover links, it's designed to prevent data loops that can accidently form. If there has to be redudent links between switches, then other alternatives need to be looked at OR it needs to be accepted that if a link drops, the whole network will unvailable for 30-50 seconds while the switches decide who is in charge and where the forwarding ports need to be and which ones are to be blocked. THere are some parameters that can be adjusted to speed up the process but you still lose the connection for a brief period. Welcome to the wonderful world of switches

MikeS
Find me at

http://www.packetattack.com

"Take advantage of the enemy's unreadiness, make your way by unexpected routes, and attack unguarded spots."
Sun Tzu

 

[k0mbachi]

Yes, there are multiple lessons to be learned from this case study.

Two alternatives that I can think of would be introduction of at least one router with multiple ports and VLANs. Are there others?

The query about "transparently" still stands.

By the way, MIkeS, I've visited

http://www.packetattack.com

and was quite impressed with the quantity and quality of data and links there. I was familiar with some, but certainly not all. Very good site which I'm still exploring.

 

[wybnormal]

transparent referres to the fact that the bridge/switches does nothing to the frame in transit through the network. Unlike a router that can repackage the packet or change the IP, the switch/bridge just either forwards the frame or drops it.

This is grossly simplified but you get the idea

MikeS
Find me at

http://www.packetattack.com

"Take advantage of the enemy's unreadiness, make your way by unexpected routes, and attack unguarded spots."
Sun Tzu