CTC Union FRM220-NMC, User Manual, Page: 161 / 182

Appendix A
159
6. Select
<S> . to bring up the Spanning Tree Status and Configuration menu.
The root bridge of the spanning tree is the bridge with the lowest bridge ID. Each bridge has a unique identifier
(ID) and a configurable priority number; the Bridge ID contains both numbers. To compare two bridge IDs, the
priority is compared first. If two bridges have equal priority, then the MAC addresses are compared. For example, if
switches A (MAC=0002abfffe02) and B (MAC=0002abfffe03) both have a priority of 8000, then switch A will be
selected as the root bridge (it has the lower value MAC address). If the network administrators would like switch B
to become the root bridge, they must set its priority to be less than 8000.
Spanning tree computes its path by following the path of least cost, i.e., a path from the device to the root that
has minimum cost among all paths from the device to the root. The cost of traversing a path is the sum of the costs
of the segments on the path. The port connecting to that path becomes the root port (RP) of the bridge. Different
technologies have different default costs for network segments. An administrator can configure the cost of
traversing a particular network segment. The default
root path cost in the 10/100A is 0.
The switch sends a BPDU (Bridge Protocol Data Units) frame using the unique MAC address of the port itself
as a source address, and a destination address of the STP multicast address 01:80:C2:00:00:00.
There are three types of BPDUs:
1. Configuration BPDU (CBPDU), used for Spanning Tree computation
2. Topology Change Notification (TCN) BPDU, used to announce changes in the network topology
3. Topology Change Notification Acknowledgment (TCA)
Hello Time is the time interval (number of seconds between) at which the root bridge transmits configuration
BPDUs.
When a device is first attached to a switch port, it will not immediately start to forward data. It will instead go
through a number of states while it processes BPDUs and determines the topology of the network. The time spent
in the listening and learning states is determined by a value known as the
forward delay (default 15 seconds and
set by the root bridge).
Maximum Age , referred to by the switch as Max Age , is the amount of time a switch will retain a BPDU's
contents before discarding it. The default is 20 seconds. It also influences the bridge table aging timer during the
Topology Change Notification process.
Remember that these timers should not be changed lightly. If you decide to change any or all of these timers,
that change must be configured on the root bridge! The root bridge will inform the nonroot switches of the change
via BPDUs.
*************************************************
*** CTC UNION TECHNOLOGIES CO., LTD. ***
*** FRM220-10/100AS-2 ver:3.016 ***
*************************************************
<< Local Spanning Tree Status >>
Bridge Status:
STP Enable [ON ] Bridge ID [8000.0002abfffe02]
Designated Root [8000.0002abfffe02] Root Port [0 ]
Root Path Cost [0 ] Max Age [20 ]
Hello Time [2 ] Forward Delay [15 ]
Port 1 Status:
Bridge Port [Yes ] Port ID [8001 ]
Port State [Forwarding ] Path Cost [4 ]
Designated Bridge[8000.0002abfffe02] Designated Port[8001 ]
<P><LeftArrow> :Previous Port.
<N><RightArrow>:Next Port.
<G> :Go to STP Configuration Menu.
<ESC>:Previous Menu.