EXP Computer S200 manual Bridge Forward Delay Timer, Message Events in Network

Spanning Tree Protocol Entity (STPE)

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 IDs

01 03 02 04

Bridge 1	Bridge 3		Bridge 2	Bridge 4
	5	1	1	6
	Link Numbers

t=0:	01,3536		02,25			02,3536
		03,3536		03,25			04,3536
t= Hello Time:	01,3536				02,25	02,3536
				01,3561

t=2x Hello Time:

01,3536

01,3561

01,7097

Figure 47. Message Events in Network

Bridging 97

T0008-16F Release 5.2M