CS485 Sylabus

### Intro to the IEEE 802.1D Spanning Tree Protocol

• IEEE 802.1D documentation

 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.

• Nomenclature comment

• 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

• Goal of the Spanning Tree Algorithm

• 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

• How to create a logical network on top of the physical network

• 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)

• Intro to the 802.1D Spanning Tree Algorithm

• 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 !!!

• 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)

• Overview of the Spanning Tree Algorithm

• Overview of the Spanning Tree Algorithm:

1. Each bridge will first determine (by itself) the root bridge

 The root bridge is the bridge with the smallest node ID (= Ethernet Address)

2. 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

3. 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)