3Com 3C13636 Configuration Manual page 358

Router 3000 ethernet family

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3Com Router 3000 Ethernet Family

Configuration Guide

Figure 3-10 Spanning tree topology

II. BPDU forwarding mechanism

Upon the initiation of STP, all the bridges assume themselves as the root bridge. The

designated interface of the bridge regularly sends its BPDU once each Hello Time. If it

is the root port that receives the BPDU, it will increase the Message Age carried in the

BPDU and enable the timer to time this BPDU. If a path fails, the root port on this path

will not receive new BPDUs any more and old BPDUs will be discarded due to timeout,

which will result in the spanning tree recompilation. A new path will thus be generated

to replace the failed one.

However, the recomputed new BPDU will not be propagated throughout the network

right away, so the old root port and designated ports that have not detected the

topology changes will still forward the data through the old path. If the newly elected

root port and designated ports begin to forward data immediately, a temporary loop

may be introduced. In STP, a transitional state mechanism is thus adopted.

Specifically, the root port and the designated ports will undergo a transitional state for

an interval of forwarding delay to enter the forwarding state to resume the data

forwarding. Such a delay ensures that the new BPDU has already been propagated

throughout the network before the data frames are forwarded according to the latest

Root Bridge/

Designed Bridge

3Com Corporation

Chapter 3 Bridge Configuration

RP = Root Port

DP= Designated Port

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3c13636-us - router 3036 3000 series