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Bridging, Switching, and Spanning Tree 165
Noticing and Reacting to Changes in Network Topology
A periodic notice is sent to tell all bridges that nothing has changed in the network. The protocol
mechanism begins when the root sends CBPDUs on all its interfaces with the same information
in it as before: its bridge ID, priority, cost (0), and the root bridge ID, which is itself. As seen in
Figure 4-17, the bridges receive the CBPDUs, adjust the cost, and send the CBPDUs on all
interfaces except their root ports.
The CBPDU created by the root also includes some important timers:
·
Hello time--The time that the root waits before resending the periodic CBPDUs, which
are then forwarded by successive bridges.
·
MaxAge--The time any bridge should wait before deciding that the topology has
changed.
·
Forward Delay--Delay that affects the time involved when an interface changes from a
blocking state to a forwarding state; this timer will be covered in more depth shortly.
The MaxAge timer is typically a multiple of Hello. This allows some CBPDUs to be lost,
without the bridges reacting and changing the Spanning Tree. The MaxAge setting should also
consider the variations in how long it takes the CBPDUs to traverse the network. In a local
environment, these variations should be minimal unless severe congestion causes a large
number of frames to be discarded.
Table 4-11
The State of Each Interface
Bridge Interface
State
Reason Interface Is in
Forwarding State
Bridge 1, E0
Forwarding
Interface is on root bridge
Bridge 1, E1
Forwarding
Interface is on root bridge
Bridge 2, E0
Forwarding
Root port
Bridge 2, E1
Forwarding
Designated bridge
Bridge 3, E0
Forwarding
Root port
Bridge 3, E1
Blocking
Not root bridge, not root port, no designated
bridge
Bridge 4, E0
Forwarding
Designated bridge
Bridge 4, E1
Forwarding
Root port
Bridge 5, E0
Forwarding
Root port
Bridge 5, E1
Forwarding
Designated bridge
ch04.fm Page 165 Monday, March 20, 2000 5:02 PM