Bridge Forward Delay Timer - 3Com PathBuilder S200 Series Owner's Manual

Bridge Forward Delay Timer

Forward Delay
For transparent bridges the bridge Forward Delay is used to allow the spanning tree
algorithm to converge to a stable topology before the bridging process is allowed to
proceed. Spanning tree topology determination is an iterative process and requires
time to converge. The bridge should not forward packets during this time because
temporary loops might cause forwarded packets to be exponentially duplicated and
disable the network. Once the topology has stabilized, the bridge should not forward
packets immediately since, initially, it will not have learned station locations and will
have to broadcast packets when it does not find the entry in its local station cache.
Once the bridge has built up its cache by listening for a short period of time, it can
forward packets directly, rather than using high overhead broadcasts.
The Forward Delay is used twice: once to allow the topology to stabilize and during
this time to process only spanning tree protocol messages; and then again to allow
the bridge to learn station locations, during which time data packets are received but
not forwarded.
To understand how long it takes to converge a spanning tree, consider the following
simplified network shown in Figure 47, together with the message events shown as a
timed sequence below the network:
Bridge 1
t=0:
t= Hello Time:
t=2x Hello Time:
Figure 47. Message Events in Network
Bridging
T0008-16F
Bridge IDs
01 03
Bridge 3
5
Link Numbers
01,3536
03,3536
01,3536
01,3536
Spanning Tree Protocol Entity (STPE)
02
Bridge 2
1 1
02,25 02,3536
03,25
02,25
02,3536
01,3561
01,7097
01,3561
04
Bridge 4
6
04,3536
97
Release 5.2M