SMC Networks 7724M/VSW - annexe 1, Manual

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Management Guide CLI
TigerAccess™ EE
SMC7824M/VSW 263
The difference of between alternate port and backup port is that an alternate port can al-
ternate the path of packet when there is a problem between Root switch and SWITCH C
but Backup port cannot provide stable connection in that case.
8.3.2.2 BPDU Policy
In 802.1d, only root switch can generate BPDU every hello time and other swithches can-
not. They can create BPDU when receiving BPDU from the root switch. However, in
802.1w not only root switch but also all the other switches forward BPDU following Hello-
time. BPDU is more frequently issued than the interval the root switch exchanges, but
with 802.1w conversion to the forwarding state become faster to keep up with changing
network.
By the way, when low BPDU is received from root switch or designated switch, it is im-
mediately accepted. For example, suppose that root switch is disconnected to SWITCH B.
Then, SWITCH B is considered to be root because of the disconnection and forwards
BPDU.
However, SWITCH C recognizes root existing, so it transmits BPDU including information
of root to Bridge B. Thus, SWITCH B configures a port connected to SWITCH C as new
root port.
Switch B
Switch A
Switch C
BPDU including
Root information
ROOT
New Root
Port
Low BPDU
Fig. 8.16 Example of Receiving Low BPDU
8.3.2.3 Rapid Network Convergence
A new link is connected between SWITCH A and root. Root and SWITCH A is not directly
connected, but indirectly through SWITCH D. After SWITCH A is newly connected to root,
packet cannot be transmitted between the ports because state of two switches becomes
listening, and no loop is created.
In this state, if root transmits BPDU to SWITCH A, SWITCH A transmits new BPDU to
SWITCH A and SWITCH C, switch C transmits new BPDU to SWITCH D. SWITCH D,
which received BPDU from SWITCH C makes port connected to SWITCH C Blocking
state to prevent loop after new link.