CCNA Cisco Certified Network Associate Study Guide - FTP Server

Spanning-Tree Protocol (STP) 83
If a switch determines that a blocked port should now be the designated
port, it will go to listening state. It will check all BPDUs heard to make sure
that it won’t create a loop once the port goes to forwarding state.
Convergence
Convergence occurs when bridges and switches have transitioned to either
the forwarding or blocking states. No data is forwarded during this time.
Convergence is important to make sure all devices have the same database.
Before data can be forwarded, all devices must be updated. The problem
with convergence is the time it takes for these devices to update. It usually
takes 50 seconds to go from blocking to forwarding state. It is not recommended
that you change the default STP timers, but the timers can be
adjusted if necessary. Forward delay is the time it takes to transition a port
from listening to learning state or from learning to forwarding state.
Spanning-Tree Example
It is important to see how spanning tree works in an internetwork, and this
section will give you a chance to observe it in a live network. In Figure 2.5,
the three switches all have the same priority of 32,768. However, notice the
MAC address of each switch. By looking at the priority and MAC addresses
of each switch, you should be able to determine the root bridge.
FIGURE 2.5 Spanning-tree example
1900B
Port 0
MAC 0c00c8111111
Default priority 32768
Port 1
Port 0
Root port (F)
100BaseT
Designated port (F)
10BaseT
1900A
MAC 0c00c8110000
Default priority 32768
Designated port (F)
Port 0
Port 1
Root port (F)
Root bridge Nonroot bridge
1900C
MAC 0c00c8222222
Default priority 32768
Nondesignated port (BLK)