- Here is the complete 802.1D standard document:
click here
- The tree-construction algorithm is on pages 77-124 in the PDF file) (or pages 59 - 106 in the document page numbering)
- I will only cover a
subset of the standard
(described in text books)
which contains the
gist of the protocol:
- how to set up a loop-free (tree) structure and
- how to recover from failures.
- Since the IEEE Standard
calls the
Ethernet switch a:
- Multiway Bridge
- Note:
- I will use the term
bridge in my
IEEE 802.1D Spanning Tree Protocol
lecture webpages
- The proper term should be: switch
- I will use the term
bridge in my
IEEE 802.1D Spanning Tree Protocol
lecture webpages
- Purpose of the
Spanning Tree Algorithm:
- The Impose a logical tree structure on top of the physical network (that may contain loops)
Example:
- The physical network:
- The logical network:
(A tree !!!)
- The logical network will be used in message forwarding operation
- State information:
- Each port has
a state associated with the
port
- Blocked port:
- If the state is
block,
the port
will not forward
frames !!!
(And the loop is broken)
- If the state is
block,
the port
will not forward
frames !!!
- Each port has
a state associated with the
port
- The 802.1D Spanning Tree Algorithm
is a distributed algorithm:
- Every node will
perform the
computation
on its own
- Each node
maintains some local information
- Local information is only accessible by the owner node
- Nodes will
exchange information to
help each other
perform the computation
- After receiving some
information, a node may
update its
local information
- If the node's local information is changed, the node will transmit its updated local information to other nodes !!!
- After receiving some
information, a node may
update its
local information
- Every node will
perform the
computation
on its own
- Distributed Algorithms:
- Distributed means:
- There is not a place (= node) in the entire system that has all (= complete) information
- Distributed Algorithms
always performs
their computation
by:
- Exchanging their local state informations (by sending your information to your neighboring nodes)
- Distributed means:
- Overview
of the Spanning Tree Algorithm:
- Each bridge will
first
determine (by itself)
the root bridge
- The root bridge is the bridge with the smallest node ID (= Ethernet Address)
- Each bridge then
uses the
root bridge ID to
determine (for itself)
its root port:
- Root port of a bridge = the port of the bridge that leads to the root bridge
- For each of the other ports
(the non-root ports)
a bridge will then
determine (for itself)
if the port is:
- A block port (status = blocked) or
- A unblock port (status = designated)
- Each bridge will
first
determine (by itself)
the root bridge