CS485 Sylabus

### Processing a configuration control message

• Recall: purpose of the configuration messages

• Recall:

• Bridges exchange configuration messages in order to:

 Find the tree that is rooted at the bridge with the smallest ID

• Processing a configuration message

• I will first illustrate the processing with a concrete example

• Suppose that:

• bridge 1 sends the configuration message:

 ``` (ID=1 , Root=1 , Dist=0) ```

to all its neighboring nodes:

• How the bridge 2 processes the configuration message from bridge 1:

• Bridge 2 detects that the information in the configuration message can improve its current state (= assumption):

Explanation:

 ``` Bridge 2: Root ID = 2 Distance = 0 Configuration messages: Root ID = 1 <---- Smaller !! Distance = 0 Bridge 2 has discovered a better candidate for the root bridge !!! Bridge 2 will update its state variables to: Root ID = 1 Distance = 1 (= 0 + 1) ```

Result:

• How the bridge 0 processes the configuration message from bridge 1:

• Bridge 0 detects that the information in the configuration message can not improve its current state (= assumption):

Explanation:

 ``` Bridge 0: Root ID = 0 Distance = 0 Configuration messages: Root ID = 1 <---- Smaller !! Distance = 0 Bridge 0 itself has the better candidate for the root bridge !!! Bridge 2 will NOT update its state variables ```

• An more abstract example

• A more abstract example:

• Situation:

 Bridge 4 has learned that the root bridge is bridge 0 Bridge 4 is 3 hops away from the root bridge 0

Graphically:

• Suppose bridge 4 sends the following control message to bridge 1:

• How the bridge 1 processes the control message:

• Bridge 1 will discover a better root bridge:

 Bridge 0 !!! (because bridge 0 has a smaller ID than the its current root bridge (= 1)

• Bridge 1's distance to the new root bridge is:

 Distance to new root = 3 + 1 = 4 hops (Because it's neighbor was 3 hops away)

• The root port (= the direction to the root bridge) is:

 the port on which the control message was received !!!!

• The new state of bridge 1 after updating the state variables:

• Note: (recall)

• Since the bridge 1 has updated some of its state variables:

 Bridge 1 will transmit a configurate message on all its port (Except on the port on which the better configuration message was received !!!)

Example: bridge 1 will send the following configurate message out:

• Next:

 How to tell if a configuration message contains a better configuration (than what a bridge already has)