- Each bridge maintains the following
state variables:
-
MyRoot
=
the ID
of the root bridge
that the bridge
has discovered so far
- Root Path Cost = the
cost (= distance),
(in number of hops) to the
root bridge
- Port Status =
the status of a
port
Possible states:
- R = the port is a root port
- D = the port is a designated port
- B = the port is a blocked port
Example:
- Sample state information
of bridges:
Note:
- The root bridge is
bridge 0
- The Distance variable
is equal to the
number of hops to
the root bridge
- The root ports of each bridge is the direction of the shortest path to get to the root bridge
- The root bridge is
bridge 0
-
MyRoot
=
the ID
of the root bridge
that the bridge
has discovered so far
- System initialization:
- When a bridge
starts up, the
bridge will
initializes its
state variables to:
RootID = myID; // Bridge assumes it is the root bridge Distance = 0; // Distance to the root bridge = 0 for ( each port p ) { status[p] = D; // Each port is "designated" }I.e.:
- The bridge assumes that it is the root bridge !!!
Example:
- When a bridge
starts up, the
bridge will
initializes its
state variables to:
- Overview
of the Spanning Tree
algorithm:
- Bridges in the
network
transmits
configuration control messages
(only) to its
neighbor nodes
- When a bridge receives a
configuration control messages
- The bridge will process the configuration data and may update some of its state variables
- If the
node has
updated some of its
state variables:
- The bridge
will
transmit its
new configuration
in a configuration message
to all its neighbors
(Except to the neighbor that sent the configuration message)
- The bridge
will
transmit its
new configuration
in a configuration message
to all its neighbors
- Bridges in the
network
transmits
configuration control messages
(only) to its
neighbor nodes
- The structure
of the
Configuration Control Message
is as follows:
Example:
Explanation:
- MyID
= the ID
of the bridge that sends the
configuration message
- MyRootBridge
= the ID
of the root bridge
that the bridge
has discovered so far
- MyDistanceToRoot = the distance (# hops) of the bridge to the root bridge
- MyID
= the ID
of the bridge that sends the
configuration message
-
Purpose
of Configuration Control Message:
- Enables a neighbor bridge to
identify the
root bridge
- Enables a neighbor bridge to
compute the
shortest distance
from itself to the root bridge
- Enables a neighbor bridge to
determine its
root port
- Enables a neighbor bridge to
determine all its
designated ports
- Enables a neighbor bridge to
determine all its
blocked ports
- Enables a neighbor bridge to
identify the
root bridge
- When does a node
send a configuration control message:
- A root node
(i.e.: a node with
RootID == myID) will:
- periodically transmit a configuration control message
Example:
- A non-root node
(i.e.: a node with
RootID ≠ myID) will
transmit a
configuration message:
- When some of its
state variables:
- RootID
- DistanceToRoot
- status[port])
has changed its value
Example:
- Suppose the bridge 4
just updated
(some of) its
state variables to:
MyRoot = 0 Distance = 3Its new state is:
- Then bridge 4 will transmit
the following
configuration message
to
all its neighbors ---
except to the
neighbor node who
sent the
configuration message:
- When some of its
state variables:
- A root node
(i.e.: a node with
RootID == myID) will: