UMN:CLI User Manual
SURPASS hiD 6615 S223/S323 R1.5
206 A50010-Y3-C150-2-7619
BPDU Policy
802.1d forwards BPDU following Hello-time installed in root switch and the other switch
except root switch its own BPDU only when receiving BPDU from 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 changed than the interval root switch exchanges, but with
802.1w it becomes faster to be master of the situation of 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. 8.15
Example of Receiving Low BPDU
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.
