​​​Here are the basic rules of STP

.

1. Lowest bridge ID (Priority:MAC Address) switch becomes the Root-Bridge
2. Each non-root bridge should have ONE root port (RP) which is the port having lowest path-cost to Root Bridge.

If the path cost equal, then here are the criteria:

  • 1. Lowest Sending Bridge ID
  • 2. Lowest Port Priority (of sender)
  • 3. Lowest Interface number (of sender)​

3. All ports in Root Bridge become Designated Ports (DP)
4. Each segment should have one Designated Port (DP)
5. All RP/DPs will be in FORWARDING state & all other ports will be in BLOCKING state.

You may wish to look at this animation:
http://www.cisco.com/image/gif/paws/10556/spanning_tree1.swf

You can see some detail here also:

https://mail.google.com/mail/u/0/#inbox?compose=1538d1de7f4a35d7

By vichhaiy Posted in Cisco

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s