Spanning Tree Operation

Spanning Tree Operation
In layer 3 devices, which are typically routers, the routing protocols are responsible for making
sure routing loops do not occur in the network. What is used to make sure network loops do
not occur in layer 2 switched networks? That is the job of the
Spanning Tree Protocol (STP)
.
Digital Equipment Corporation (DEC), which was purchased by Compaq before the merger
with Hewlett-Packard, was the original creator of STP. Actually, Radia Perlman is credited with
the main development of STP and should get the credit. The IEEE created its version of STP,
called 802.1D, using the DEC version as the basis. By default, all Cisco switches run the IEEE
802.1D version of STP, which is not compatible with the DEC version.
The big difference between the two types of STP from an administrative point of
view is the range of values that can be set for the priority. A bridge using DEC STP
can be set as high as 255, and a switch using IEEE STP can be set as high as
65535. If the two could be used together, a bridge set as a very low priority on
DEC would stand a good chance of becoming the root in an IEEE STP network.
Segment 1
Segment 2
Unicast
Unicast Unicast
Router C
Switch B Switch A
488
Chapter 15 
Layer 2 Switching and the Spanning Tree Protocol (STP)
The big picture is that STP stops network loops from occurring on your layer 2 network
(bridges or switches). STP switches constantly monitor the network to find all links and to make
sure loops do not occur by shutting down redundant links.
The Spanning Tree Protocol executes an algorithm called the spanning tree algorithm. Switches
choose a reference point in the network and calculate the redundant paths to that reference point.
After a loop in the network is discovered, the spanning tree algorithm chooses one path on which
to forward frames and shuts down the other redundant links to stop any frames from being forwarded
along looped paths. The reference point is called the root bridge.
There can be only one
root bridge
in any given network. The root bridge ports are called designated
ports, and designated ports operate in what is called forwarding state. Forwarding state
ports send and receive traffic.
If you have other switches in your network, as shown in Figure 15.4, they are called non-root
bridges. However, the port that has the lowest cumulative cost to the root bridge is called a root port,
and it sends and receives traffic. The cost is determined by the bandwidth of a link.
FIGURE 1 5 . 4
Spanning tree operations
Ports that forward traffic away from the root bridge are called the
designated ports
. Because
the root can forward traffic only away from itself, all its ports are designated ports. The other port
or ports on the bridge are considered
nondesignated ports
and will not send or receive traffic. This
is called blocking mode.
This section covers exactly how a group of switches determines the best path throughout the
network and how you can modify the results. This section covers port selection and link cost
values as well as the different spanning tree states that a particular port might be in.
Selecting the Best Path
Using spanning tree, a group of switches determines the best path from any point A to any point B.
To do this, all the switches need to communicate, and each switch needs to know what the network
looks like. In order to know what links should be dynamically disabled, a root bridge must be
selected and each switch needs to determine the type of each port.
100BaseT
10BaseT
Designated port (F)
Designated port (F)
Root port (F)
Nondesignated port (B)
Non-root bridge SYST RPS
STRT UTIL DUPLXSPEED
MODE
1 2 3 4 5 6 7 8 9 10 11 12
10Base-T/100Base-TX Catalyst 2950 SERIES Root bridge
2950 A
SYST RPS
STRT UTIL DUPLXSPEED
MODE
1 2