User's Manual

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Users Manual of CS-6306R
Figure 1.4 Backbone Fast
Suppose the bridge priority of switch C is higher than that of switch B. When L1 is disconnected, switch B is
selected to send BPDU to switch C because the bridge priority is used as root priority. To switch C, the
information contained by BPDU is not prior to information contained by its own. When Backbone Fast is not
enabled, the port between switch C and switch B ages when awaiting the bridge information and then turns to
be the designated port. The aging normally takes a few seconds. After the function is configured in global
configuration mode by running the command spanning-tree backbonefast, when the Alternate port of switch
C receives a BPDU with lower priority, switch C thinks that an indirect-link and root-switch-reachable
connection on the port is disconnected. Switch C then promptly update the port as the designated port without
waiting the aging information.
After the Backbone Fast function is enabled, if BPDU with low priority is received at different ports, the switch
will perform different actions. If the Alternate port receives the message, the port is updated to the designated
port. If the root port receives the low-priority message and there is no other standby port, the switch turns to
be the root switch.
Note that the Backbone Fast feature just omits the time of information aging. New designated port still needs
to follow the state change order: the listening state, then the learning state and finally the forwarding state.
Similar to Uplink Fast, the Backbone Fast feature is effective in SSTP and PVST
modes.