EXP Computer PathBuilder S200 - Manual (Page 102)

98 Bridging
Spanning Tree Protocol Entity (STPE)
Suppose all bridges come up at the same time. Initially, at t=0, all bridges think they
are the root and they issue the messages shown on the t=0 line. For simplicity, the
bridge ID is a two digit number and the couplet such as 01,3561 should be
interpreted as: root_ID, cost_to_root. The messages sent to the LAN attached to
B1 and B4 are irrelevant to this discussion and are not shown.
At the end of this first iteration, after the bridges have compared messages sent and
received on the various links, they conclude the following:
• B1 will continue to see itself as the root bridge. Its link 5 and link 1 (to the
LAN, not shown) will be set as designated links.
• B2 will continue to see itself as the root bridge temporarily. Its link 1 and link
6 will be set as designated links.
• B3 will determine B1 as the root bridge. Its link 5 is its root link and its link 1
is a designated link.
• B4 will temporarily see B2 as the root bridge. Its link 6 is its root link and its
link 1 (not shown) is a designated link.
At t = Hello Time, the root bridges issue the spanning tree hello message. After the
bridges have compared the new messages, they form the following conclusion:
• B1 will continue to see itself as the root bridge. Its link 5 and link 1 will be set
as designated links.
• B2 will determine B1 as the root bridge. Its link 1 will be the root link and
link 6 will be set as a designated link.
• B3 will determine B1 as the root bridge. Its link 5 is its root link and its link 1
is a designated link.
• B4 will temporarily still see B2 as the root bridge. Its link 6 is its root link and
its link 1 is a designated link.
At t = 2 x Hello Time, there is only one root bridge B1 to issue the spanning tree
hello message. This message is passed along the tree and, at this iteration, B4 finally
sees B1 as the root bridge, and the spanning tree has converged.
From this example, it is seen that the convergence time is dependent on how many
link hops the farthest bridge is from the root bridge and how frequently the hello
message is sent. Until the bridges converge, no bridge should learn station locations
(nor forward data frames); therefore, the forward delay (the time the bridge should
wait before learning), should be set to a value that might be at least as high as the
hello time multiplied by the network diameter. With special consideration this time
might be considerably reduced. For example, in the preceding network, if B2 and B3
are interchanged, the spanning tree will converge at t = Hello Time (half the time of
the original network).
Note
The Forward Delay time is used a second time to allow the bridge to learn station
locations before allowing the bridge to forward frames.
When the spanning tree is configured for manual operation, no spanning tree
protocol needs to converge, but the station locations still must be learned.
For manual operation, a different timer is used only for setting the Learn Only
Period. This timer is used by the bridge to set the time it will learn only after the
bridge has booted.